Scheme of a pulse generator with a transformer. Schemes of pulse generators. Unusual transistor operation

Rectangular pulse generators are widely used in radio engineering, television, automatic control systems and computer technology.

To obtain rectangular pulses with steep fronts, devices are widely used, the principle of operation of which is based on the use of electronic amplifiers with positive feedback. These devices include the so-called relaxation generators - multivibrators, blocking generators. These generators can operate in one of the following modes: standby, self-oscillating, synchronization and frequency division.

In standby mode, the generator has one stable equilibrium state. An external trigger pulse causes the waiting generator to jump to a new state that is not stable. In this state, called quasi-equilibrium, or temporarily stable, relatively slow processes occur in the generator circuit, which eventually lead to a reverse jump, after which a stable initial state is established. The duration of the quasi-equilibrium state, which determines the duration of the generated rectangular pulse, depends on the parameters of the generator circuit. The main requirements for waiting generators are the stability of the duration of the generated pulse and the stability of its initial state. Waiting generators are used primarily to obtain a certain time interval, the beginning and end of which are fixed, respectively, by the front and fall of the generated rectangular pulse, as well as for expanding pulses, for dividing the pulse repetition rate, and for other purposes.

In the self-oscillatory mode, the generator has two states of quasi-equilibrium and does not have a single stable state. In this mode, without any external influence, the generator sequentially jumps from one state of quasi-equilibrium to another. In this case, pulses are generated, the amplitude, duration and repetition frequency of which are determined mainly only by the parameters of the generator. The main requirement for such generators is high frequency stability of self-oscillations. Meanwhile, as a result of changes in supply voltages, replacement and aging of elements, the influence of other factors (temperature, humidity, interference, etc.), the stability of the frequency of self-oscillations of the generator is usually low.

In synchronization or frequency division mode, the frequency of repetition of the generated pulses is determined by the frequency of the external clock voltage (sinusoidal or pulsed) supplied to the generator circuit. The pulse repetition rate is equal to or a multiple of the clock voltage frequency.

A generator of periodically repeating rectangular pulses of a relaxation type is called a multivibrator.

The multivibrator circuit can be implemented both on discrete elements and in an integrated design.

Multivibrator on discrete elements. In such a multivibrator, two amplifying stages covered by feedback are used. One feedback branch is formed by a capacitor and a resistor , and the other And (Fig. 6.16).

states and ensures the generation of periodically repeating pulses, the shape of which is close to rectangular.

In a multivibrator, both transistors can be in active mode for a very short time, since as a result of positive feedback, the circuit jumps into a state where one transistor is open and the other is closed.

Let us assume for definiteness that at the moment of time transistor VT1 open and saturated, and the transistor VT2 closed (Fig. 6.17). Capacitor due to the current flowing in the circuit at previous moments of time, it is charged to a certain voltage. The polarity of this voltage is such that to the base of the transistor VT2 a negative voltage is applied relative to the emitter and VT2 closed. Since one transistor is closed, and the other is open and saturated, the self-excitation condition is not satisfied in the circuit, since the gains of the cascades
.

In this state, two processes take place in the circuit. One process is associated with the flow of the capacitor recharge current from the power supply through the resistor circuit - open transistor VT1 .The second process is due to the charge of the capacitor through a resistor
and the base circuit of the transistor VT1 , as a result, the voltage at the collector of the transistor VT2 increases (Fig. 6.17). Since the resistor included in the base circuit of the transistor has a greater resistance than the collector resistor (
), capacitor charging time less capacitor recharge time .

open, because its base is connected to the positive pole of the power source through a resistor .

Basic
and collector
transistor voltage VT1 while not changing. This state of the circuit is called quasi-stable.

At the point in time as the capacitor recharges, the voltage at the base of the transistor VT2 reaches the opening voltage and the transistor VT2 switches to the active mode of operation, for which
. When opening VT2 collector current increases and decreases accordingly.
. Decrease
causes a decrease in the base current of the transistor VT1 , which in turn leads to a decrease in the collector current . Current reduction accompanied by an increase in the base current of the transistor VT2 because the current flowing through the resistor
, branches off to the base of the transistor VT2 And
.

After the transistor VT1 leaves the saturation mode, the self-excitation condition is fulfilled in the circuit:
. In this case, the process of switching the circuit proceeds like an avalanche and ends when the transistor VT2 goes into saturation mode, and the transistor VT1 - in cut-off mode.

In the future, a practically discharged capacitor (
) is charged from a power source through a resistor circuit
- the base circuit of an open transistor VT2 exponentially with time constant
. As a result, over time
there is an increase in the voltage across the capacitor before
and the front of the collector voltage is formed
transistor VT1 .

Closed state of the transistor VT1 ensured by the fact that initially charged to voltage capacitor through an open transistor VT2 connected to the base-emitter gap of the transistor VT1 , which maintains a negative voltage at its base. Over time, the blocking voltage at the base changes as the capacitor recharged through the resistor circuit - open transistor VT2 . At the point in time transistor base voltage VT1 reaches the value
and it opens.

In the circuit, the self-excitation condition is again satisfied and a regenerative process develops, as a result of which the transistor VT1 goes into saturation mode VT2 closes. Capacitor is charged to a voltage
, and the capacitor almost empty (
). This corresponds to the time , from which the consideration of processes in the scheme began. On this, the full cycle of the multivibrator operation ends, since in the future the processes in the circuit are repeated.

As follows from the timing diagram (Fig. 6.17), in a multivibrator, periodically repeating rectangular pulses can be removed from the collectors of both transistors. In the case when the load is connected to the collector of the transistor VT2 , pulse duration determined by the process of recharging the capacitor , and the duration of the pause - the process of recharging the capacitor .

Capacitor recharge circuit contains one reactive element, so , where
;
;.

Thus, .

Recharge process ends at time , When
. Therefore, the duration of the positive pulse of the collector voltage of the transistor VT2 is determined by the formula:

.

In the case when the multivibrator is made on germanium transistors, the formula is simplified, since
.

Capacitor recharge process , which determines the length of the pause between transistor collector voltage pulses VT2 , proceeds in the same equivalent circuit and under the same conditions as the process of recharging the capacitor , only with a different time constant:
. Therefore, the formula for calculating similar to the formula for calculating :

.

Usually, in a multivibrator, the pulse duration and pause duration are adjusted by changing the resistance of the resistors And .

The duration of the fronts depends on the opening time of the transistors and is determined by the charge time of the capacitor through the collector resistor of the same shoulder
. When calculating a multivibrator, it is necessary to fulfill the condition of saturation of an open transistor
. For transistor VT2 without current
capacitor recharge current
. Therefore, for a transistor VT1 saturation condition
, and for the transistor VT2 -
.

Frequency of generated pulses
. The main obstacle to increasing the pulse generation frequency is the long duration of the pulse front. Reducing the duration of the front of the pulse by reducing the resistance of the collector resistors can lead to non-fulfillment of the saturation condition.

With a high degree of saturation in the considered multivibrator circuit, there may be cases when, after switching on, both transistors are saturated and there are no oscillations. This corresponds to the hard mode of self-excitation. To prevent this, you should choose the operating mode of an open transistor near the saturation limit in order to maintain sufficient gain in the feedback circuit, and also use special multivibrator circuits.

If the pulse duration equal to the duration , which is usually achieved at , then such a multivibrator is called symmetrical.

The duration of the front of the pulses generated by the multivibrator can be significantly reduced if diodes are additionally introduced into the circuit (Fig. 6.18).

When, for example, the transistor closes VT2 and the collector voltage begins to increase, then to the diode VD2 a reverse voltage is applied, it closes and thereby disconnects the charging capacitor from the collector of the transistor VT2 . As a result, the capacitor charge current no longer flows through the resistor , and through a resistor . Therefore, the duration of the front of the collector voltage pulse
now determined only by the process of closing the transistor VT2 . Diode works the same way. VD1 when the capacitor is charged .

Although in such a circuit the rise time is significantly reduced, the charge time of the capacitors, which limits the duty cycle of the pulses, remains practically unchanged. Time constants
And
cannot be reduced by lowering . Resistor in the open state of the transistor through an open diode is connected in parallel with the resistor .As a result, when
the power consumed by the circuit increases.

Multivibrator on integrated circuits(Fig. 6.19). The simplest circuit contains two inverting logic elements LE1 And LE2, two timing chains
And
and diodes VD1 , VD2 .

At the point in time
and at the exit LE2
. As a result, the input LE1 through a capacitor , which is charged to a voltage
, voltage is applied and LE1 goes to zero
. Since the output voltage LE1 decreased, then the capacitor starts to disintegrate. As a result, the resistor a negative polarity voltage will appear, the diode will open VD2 and capacitor quickly discharge to voltage
. After the end of this process, the input voltage LE2
.

At the same time, the process of charging the capacitor takes place in the circuit and over time, the input voltage LE1 decreases. When at a moment in time voltage
,
,
. Processes begin to repeat themselves. The capacitor is charging again. , and the capacitor discharged through an open diode VD1 . Since the resistance of an open diode is much less than the resistance of resistors , And , capacitor discharge And going faster than their charge.

Input voltage LE1 in the time interval
determined by the process of charging the capacitor :, Where
;
is the output resistance of the logic element in the unity state;
;
, where
. When
, the formation of a pulse at the output of the element ends LE2, hence the pulse duration

.

The duration of the pause between pulses (time interval from before ) is determined by the process of charging the capacitor , That's why

.

The duration of the front of the generated pulses is determined by the switching time of the logic elements.

On the timing diagram (Fig. 6.20), the amplitude of the output pulses does not change:
, since the output impedance of the logic element was not taken into account in its construction. Given the finiteness of this output resistance, the amplitude of the pulses will change.

The disadvantage of the considered simplest multivibrator circuit on logical elements is the hard mode of self-excitation and the associated possible absence of an oscillatory mode of operation. This disadvantage of the circuit can be eliminated if an additional logic element AND is introduced (Fig. 6.21).

When the multivibrator generates pulses, then the output LE3
, because the
. However, due to the hard mode of self-excitation, such a case is possible when, when the power supply voltage is turned on, due to the low rate of voltage rise, the capacitor charge current And turns out to be small. In this case, the voltage drop across the resistors And may be less than the threshold
and both elements LE1 And LE2) will be in a state where the voltages at their outputs
. With this combination of input signals at the output of the element LE3 there will be tension
, which through a resistor applied to the input of the element LE2. Because
, That LE2 is transferred to the zero state and the circuit begins to generate pulses.

To build rectangular pulse generators, along with discrete elements and integrated circuits, operational amplifiers are used.

so the voltage at the inverting input
depends not only on the voltage at the output of the amplifier, but is also a function of time, since
.

A positive voltage is applied to the non-inverting input
. Voltage
remains constant, and the voltage at the inverting input
increases over time, tending to the level
, since the process of recharging the capacitor takes place in the circuit .

However, for now
, the state of the amplifier determines the voltage at the non-inverting input and the output remains at the level
.

At the point in time the voltages at the inputs of the operational amplifier become equal:
. Further slight increase
leads to the fact that the differential (difference) voltage at the inverting input of the amplifier
turns out to be positive, so the output voltage decreases sharply and becomes negative
. Since the voltage at the output of the operational amplifier has changed polarity, the capacitor subsequently recharges and the voltage on it, as well as the voltage at the inverting input, tend to
.

At the point in time again
and then the differential (difference) voltage at the input of the amplifier
becomes negative. Since it acts on the inverting input, the voltage at the output of the amplifier abruptly again takes on the value
. The voltage at the non-inverting input also jumps
. Capacitor , which by the time charged to a negative voltage, recharges again and the voltage at the inverting input increases, tending to
. Since at the same time
, then the voltage at the output of the amplifier remains constant. As follows from the timing diagram (Fig. 6.23), at the time the full cycle of the circuit operation ends and in the future the processes in it are repeated. Thus, at the output of the circuit, periodically repeating rectangular pulses are generated, the amplitude of which at
is equal to
. Pulse duration (time interval
) is determined by the capacitor recharge time according to the exponential law from
before
with time constant
, Where
is the output impedance of the operational amplifier. Because during the pause (interval
) the capacitor is recharged under exactly the same conditions as during the formation of pulses, then
. Therefore, the circuit operates as a symmetrical multivibrator.

happens with time constant
. With a negative output voltage (
) open diode VD2 and the capacitor recharge time constant , which determines the duration of the pause,
.

The standby multivibrator or single vibrator has one stable state and provides the generation of rectangular pulses when short trigger pulses are applied to the input of the circuit.

Single vibrator on discrete elements consists of two amplifying stages covered by positive feedback (Fig. 6.25).

transistor VT1 depends on the collector current of the transistor VT2 . Such a circuit is called an emitter-coupled single vibrator. The circuit parameters are calculated in such a way that in the initial state, in the absence of input pulses, the transistor VT2 was open and saturated, and VT1 was in cutoff mode. Such a state of the circuit, which is stable, is ensured when the following conditions are met:
.

Let us assume that the one-shot is in a stable state. Then the currents and voltages in the circuit will be constant. transistor base VT2 through a resistor connected to the positive pole of the power supply, which in principle ensures the open state of the transistor. To calculate the collector
and basic currents, we have a system of equations

.

Determining from here the currents
And , we write the saturation condition in the form:

.

Considering that
And
, then the resulting expression is significantly simplified:
.

On a resistor due to the flow of currents ,
voltage drop is generated
. As a result, the potential difference between the base and emitter of the transistor VT1 is defined by the expression:

If the scheme satisfies the condition
, then the transistor VT1 closed. Capacitor while charged to voltage. The polarity of the voltage across the capacitor is shown in fig. 6.25.

When the transistor VT1 open, capacitor is connected to the base-emitter region of the transistor VT2 so that the base potential becomes negative and the transistor VT2 goes into cutoff mode. The switching process of the circuit is of an avalanche-like nature, since at this time the condition of self-excitation is fulfilled in the circuit. The switching time of the circuit is determined by the duration of the processes of switching on the transistor VT1 and turn off the transistor VT2 and is fractions of a microsecond.

When the transistor closes VT2 through a resistor collector and base currents stop flowing VT2 . As a result, the transistor VT1 remains open even after the end of the input pulse. At this time, the resistor voltage drops
.

The state of the circuit when the transistor VT1 open and VT2 closed, is quasi-stable. Capacitor through a resistor , open transistor VT1 and resistor is connected to the power source in such a way that the voltage on it has the opposite polarity. The capacitor recharge current flows in the circuit , and the voltage on it, and therefore on the base of the transistor VT2 tends to a positive level.

Voltage change
is exponential: where
. Initial voltage at the base of the transistor VT2 determined by the voltage to which the capacitor is initially charged and residual voltage on the open transistor:

The voltage limit to which the voltage at the base of the transistor tends VT2 , .

Here it is taken into account that through the resistor not only the current of recharging the capacitor flows , but also the current open transistor VT1 . Hence, .

At the point in time voltage
reaches trigger voltage
and transistor VT2 opens. Appeared collector current creates an additional voltage drop across the resistor , which leads to a decrease in voltage
. This causes a decrease in the base and collector currents and a corresponding increase in voltage
. Positive increment of transistor collector voltage VT1 through a capacitor transferred to the base circuit of the transistor VT2 and contributes to an even greater increase in its collector current . The circuit again develops a regenerative process, ending with the fact that the transistor VT1 closes, and the transistor VT2 goes into saturation mode. This completes the impulse generation process. The pulse duration is determined by putting
: .

After the end of the pulse, the process of charging the capacitor proceeds in the circuit through a circuit of resistors
,and emitter circuit of an open transistor VT2 . At the initial moment, the base current transistor VT2 equal to the sum of the capacitor charge currents : current , limited by the resistance of the resistor
, and the current flowing through the resistor . As the capacitor charges current decreases and, accordingly, the base current of the transistor decreases VT2 tending to a stationary value determined by the resistor . As a result, at the moment of opening the transistor VT2 voltage drop across the resistor turns out to be greater than the stationary value, which leads to an increase in the negative voltage at the base of the transistor VT1 . When the voltage across the capacitor reaches
the circuit returns to its original state. The duration of the process of recharging the capacitor , which is called the recovery stage, is determined by the relation .

Minimum repetition period of single vibrator pulses
, and the maximum frequency
. If the interval between input pulses is less than , then the capacitor will not have time to recharge and this will lead to a change in the duration of the generated pulses.

The amplitude of the generated pulses is determined by the voltage difference across the collector of the transistor VT2 in closed and open states.

A single vibrator can be implemented on the basis of a multivibrator if one feedback branch is made not capacitive, but resistor and a voltage source is introduced
(Fig. 6.27). Such a circuit is called a single vibrator with collector-base connections.

To the base of the transistor VT2 a negative voltage is applied and it is closed. Capacitor charged to voltage
. In the case of germanium transistors
.

Capacitor , acting as a boost capacitor, charged to a voltage
. This state of the circuit is stable.

When applied to the base of the transistor VT2 unlocking pulse (Fig. 6.28) in the circuit, the processes of opening the transistor begin to proceed VT2 and closing the transistor VT1 .

When switching the circuit, the front of the output pulse is formed, which is usually removed from the collector of the transistor VT1 . In the future, the process of recharging the capacitor takes place in the circuit .Voltage on it
, and hence the voltage at the base transistor VT1 changes exponentially
,Where
.

When at a moment in time the base voltage reaches the value
, transistor VT1 opens, the voltage on its collector
decreases and closes the transistor VT2 . In this case, a cutoff of the output pulse is formed. The pulse duration is obtained by putting
:

.

Because
, That . Cut duration
.

Subsequently, the capacitor charge current flows in the circuit through a resistor
and the base circuit of an open transistor VT1 . The duration of this process, which determines the recovery time of the circuit,
.

The amplitude of the output pulses in such a one-shot circuit is almost equal to the voltage of the power source.

Single vibrator on logical elements. To implement a one-shot on logical elements, NAND elements are usually used. The block diagram of such a single vibrator includes two elements ( LE1 And LE2) and a timing chain
(Fig. 6.29). Inputs LE2 combined and it works like an inverter. Exit LE2 connected to one of the inputs LE1, and a control signal is applied to its other input.

its input circuit. The circuit generates a square wave at a short-term decrease (time ) input voltage
. After a time interval equal to
(not shown in Figure 6.29), at the output LE1 the voltage will increase. This voltage jump through the capacitor passed to the input LE2. Element LE2 switches to state "0". Thus, at the input 1 LE1 after a time interval
tension starts
and this element will remain in the state of one, even if after the expiration of time
voltage
will again become equal to the logical "1". For normal operation of the circuit, it is necessary that the duration of the input pulse
.

As the capacitor charges output current LE1 decreases. Accordingly, the voltage drop across :
. At the same time, the voltage increases
aiming for tension
, which when switching LE1 to state "1" was less
due to the voltage drop across the output resistance LE1. This circuit state is temporarily stable.

At the point in time voltage
reaches the threshold
and element LE2 switches to state "1". To input 1 LE1 a signal is given
and it switches to the log state. "0". At the same time, the capacitor , which is in the time interval from before charged, begins to discharge through the output resistance LE1 and diode VD1 . After the time has passed , determined by the process of discharging the capacitor , the circuit returns to its original state.

Thus, at the output LE2 a rectangular pulse is generated. Its duration, depending on the time of decrease
before
, is determined by the relation
, Where
- output impedance LE1 in state "1". Circuit recovery time , where
- output impedance LE1 in state "0"; - internal resistance of the diode in the open state.

and the voltage at the inverting input is small:
, Where
voltage drop across the diode in the open state. At the non-inverting input, the voltage is also constant:
, and since
, then the output voltage is maintained constant
.

When applied at the time input pulse of positive polarity with amplitude
the voltage at the non-inverting input becomes greater than the voltage at the inverting input and the output voltage jumps to
. In this case, the voltage at the non-inverting input also increases abruptly to
. Simultaneously diode VD closed, condenser begins to charge and a positive voltage rises at the inverting input (Fig. 6.32). Bye
voltage is maintained at the output
. At the point in time at
there is a change in the polarity of the output voltage and the voltage at the non-inverting input takes its original value, and the voltage begins to decrease as the capacitor discharges .

Because
, That
.

The recovery time of the circuit is determined by the duration of the capacitor discharge process from
before
and taking into account the accepted assumptions
.

Oscillators on operational amplifiers provide the formation of pulses with an amplitude of up to tens of volts; the duration of the fronts depends on the bandwidth of the operational amplifier and can be fractions of a microsecond.

A blocking oscillator is a relaxation-type pulse generator in the form of a single-stage amplifier with positive feedback created using a transformer. The blocking generator can operate in standby and self-oscillating modes.

Standby operation blocking-generator. When operating in standby mode, the circuit has a single steady state and generates square wave pulses when trigger pulses are input. The steady state of the blocking generator on a germanium transistor is carried out by including a bias source in the base circuit. When using a silicon transistor, a bias source is not required, since the transistor is closed at zero voltage at the base (Fig. 6.33).

base voltage decreases. Such a connection is realized by appropriately connecting the beginning of the transformer windings (in Fig. 6.33, shown by dots).

In most cases, the transformer has a third (load) winding to which the load is connected. .

The voltages on the transformer windings and the currents flowing in them are interconnected as follows:
,
,
,
Where
,
– transformation coefficients;
- the number of turns of the primary, secondary and load windings, respectively.

The duration of the process of turning on the transistor is so short that during this time the magnetization current practically does not increase (
). Therefore, the equation of currents in the analysis of the transition process of turning on the transistor is simplified:
.

base current
and the actual current flowing in the base circuit of the transistor,
.

Thus, the initial change in the base current
as a result of the processes occurring in the circuit, leads to a further change in this current
, and if
, then the process of changing currents and voltages is avalanche-like. Therefore, the condition for self-excitation of the blocking generator:
.

In the absence of load (
) this condition is simplified:
. Because
, then the self-excitation condition in the blocking generator is satisfied quite easily.

The process of opening the transistor, accompanied by the formation of the front of the pulse, ends when it goes into saturation mode. In this case, the condition of self-excitation ceases to be satisfied and, subsequently, the top of the pulse is formed. Since the transistor is saturated:
, then a voltage is applied to the primary winding of the transformer
and reduced base current
, as well as the load current
, turn out to be constant. The magnetization current during the formation of the pulse top can be determined from the equation
, whence, under zero initial conditions, we obtain
.

Thus, the magnetization current in the blocking generator, when the transistor is saturated, increases in time according to a linear law. In accordance with the current equation, the collector current of the transistor also increases linearly
.

As time passes, the degree of saturation of the transistor decreases as the base current remains constant.
, and the collector current increases. At some point in time, the collector current increases so much that the transistor switches from saturation to active mode and the condition for self-excitation of the blocking generator begins to be satisfied again. Obviously, the duration of the pulse top is determined by the time during which the transistor is in saturation mode. The saturation mode boundary corresponds to the condition
. Hence,
.

From here we get the formula for calculating the duration of the top of the pulse:

.

Magnetizing current
during the formation of the top of the pulse increases and at the end of this process, i.e. at
, reaches the value
.

Since the voltage of the power source is applied to the primary winding of the pulse transformer during the formation of the peak of the pulse , then the amplitude of the pulse on the load
.

When the transistor switches to active mode, the collector current decreases
. A voltage is induced in the secondary winding, resulting in a decrease in the base voltage and current, which in turn causes a further decrease in the collector current. A regenerative process develops in the circuit, as a result of which the transistor switches to cutoff mode and a pulse cutoff is formed.

The avalanche-like process of closing the transistor has such a short duration that the magnetization current during this time practically does not change and remains equal
. Therefore, by the time the transistor closes in the inductance stored energy
. This energy is only dissipated in the load , since the collector and base circuits of a closed transistor are open. In this case, the magnetizing current decreases exponentially:
, Where
is the time constant. flowing through resistor current creates a reverse voltage surge on it, the amplitude of which
, which is also accompanied by a voltage surge at the base and collector of a closed transistor
. Using the previously found relation for
, we get:

,

.

The process of dissipation of energy stored in a pulse transformer, which determines the recovery time of the circuit , ends after a time interval
, after which the circuit returns to the initial state. Additional surge of collector voltage
may be significant. Therefore, in the blocking generator circuit, measures are taken to reduce the value
, for which a damping circuit consisting of a diode is included in parallel with the load or in the primary winding VD1 and resistor , whose resistance
(Fig. 6.33). When the pulse is formed, the diode is closed, since a voltage of reverse polarity is applied to it, and the damping circuit does not affect the processes in the circuit. When a voltage surge occurs in the primary winding when the transistor closes, a forward voltage is applied to the diode, it opens and the current flows through the resistor . Because
, then the surge of the collector voltage
and reverse voltage surge on are significantly reduced. However, this increases the recovery time:
.

Not always a resistor is connected in series with the diode , and then the burst amplitude is minimal, but its duration increases.

impulses. We will consider the processes occurring in the scheme, starting from the moment of time when the voltage across the capacitor reaches the value
and the transistor will open (Fig. 6.36).

Since the voltage on the secondary (base) winding remains constant during the formation of the top of the pulse
, then as the capacitor charges, the base current decreases exponentially
, Where
is the resistance of the base-emitter region of a saturated transistor;
is the time constant.

In accordance with the current equation, the collector current of the transistor is determined by the expression
.

It follows from the above relations that in the self-oscillatory blocking oscillator, during the formation of the pulse top, both the base and collector currents change. As you can see, the base current decreases over time. The collector current, in principle, can both increase and decrease. It all depends on the relationship between the first two terms of the last expression. But even if the collector current decreases, it is slower than the base current. Therefore, when the base current of the transistor decreases, the time comes , when the transistor leaves the saturation mode and the process of forming the top of the pulse ends. Thus, the duration of the pulse top is determined by the relation
. Then we can write the equation of currents for the moment when the formation of the pulse top is completed:

.

After some transformations we have
. The resulting transcendental equation can be simplified under the condition
. Using the series expansion of the exponential and restricting ourselves to the first two terms
, we obtain a formula for calculating the duration of the top of the pulse
, Where
.

During the formation of the top of the pulse due to the flow of the base current of the transistor, the voltage across the capacitor changes and by the time the transistor closes, it becomes equal to
. Substituting into this expression the value
and integrating, we get:

.

When the transistor switches to the active mode of operation, the self-excitation condition begins to be satisfied again and an avalanche-like process of its closing takes place in the circuit. As in the waiting blocking generator, after the transistor is closed, the energy stored in the transformer is dissipated, accompanied by the appearance of surges in the collector and base voltages. After the end of this process, the transistor continues to be in the closed state due to the fact that a negative voltage of the charged capacitor is applied to the base. . This voltage does not remain constant, because in the closed state of the transistor through the capacitor and resistor recharging current flows from the power supply . Therefore, as the capacitor recharges the voltage at the base of the transistor increases exponentially
, Where
.

When the base voltage reaches
, the transistor opens and the process of pulse formation begins again. Thus, the duration of the pause , determined by the time the transistor is in the off state, can be calculated if we put
. Then we get
.For a blocking oscillator based on a germanium transistor, the resulting formula is simplified, since
.

Blocking generators have a high efficiency, since practically no current is consumed from the power source in the pause between pulses. Compared to multivibrators and single vibrators, they allow you to get a larger duty cycle and a shorter pulse duration. An important advantage of blocking generators is the possibility of obtaining pulses whose amplitude is greater than the power supply voltage. To do this, it is enough that the transformation ratio of the third (load) winding
. In a blocking generator, in the presence of several load windings, it is possible to carry out galvanic isolation between the loads and receive pulses of different polarity.

The blocking generator circuit is not implemented in an integrated design due to the presence of a pulse transformer.

The task of the calculation is to determine the structure of the electrical circuit, select the element base, determine the parameters of the electrical circuit of pulse generators.

Initial data:

type of technological process and its characteristics;

constructive use of the discharge circuit;

characteristics of the supply voltage;

parameters of the electrical impulse, etc.

Calculation sequence:

The sequence of calculation depends on the structure of the electrical circuit of the generator, which consists in whole or in part of the following elements: a source of direct (alternating) voltage, an autogenerator, a rectifier, a discharge circuit, a high-voltage transformer, a load (Fig. 2.14).

calculation of the voltage converter (Fig. 2.15, a);

calculation of the actual pulse generator (Fig. 2.16).

2.14. Complete block diagram of the pulse generator: 1 - voltage source; 2 - self-oscillator; 3 - rectifier; 4 - smoothing filter; 5 - discharge circuit with a high-voltage transformer; 6 - load.

Converter calculation (Fig. 2.15 a). Supply voltage U n \u003d 12V DC. We select the output voltage of the converter U 0 \u003d 300V at a load current J 0 \u003d 0.001 A, output power P 0 \u003d 0.3 W, frequency f 0 \u003d 400 Hz.

The output voltage of the converter is selected from the conditions of increasing the stability of the frequency of the generator and to obtain good linearity of the output voltage pulses, i.e. U n >>U incl. dash, usually U n =2U incl. dash.

The frequency of the output voltage is set from the conditions of optimal performance of the master oscillator of the voltage converter.

The values ​​of P 0 and U 0 make it possible to use the KY102 series dynistor VS in the generator circuit.

As a transistor VT, we use MP26B, for which the limit modes are as follows: U kbm = 70V, I KM = 0.4A, I bm = 0.015A, U kbm = 1V.

We offer the transformer core made of electrical steel. We accept V M = 0.7 T, η = 0.75, 25 s.

We check the suitability of the performed transformer for operation in the converter circuit according to the conditions:

U kbm ≥2.5U n ; I km ≥1.2I kn; I bm ≥1.2I bn. (2.77)

transistor collector current

Collector current maximum:

According to the output collector characteristics of the MP26B transistor for a given collector current β st \u003d 30, therefore, the base saturation current

A.

Base current:

I bm \u003d 1.2 0.003 \u003d 0.0036A.

Therefore, the MP26B transistor, according to condition (2.78), is suitable for the designed circuit.

The resistance of the resistors in the voltage divider circuit:

Om,; (2.79)

Ohm.

We accept the nearest standard values ​​of the resistances of the resistors R 1 =13000 ohms, R 2 =110 ohms.

The resistor R in the base circuit of the transistor regulates the output power of the generator, its resistance is 0.5 ... 1 kOhm.

Cross-section of the core of the transformer TV1:

Figure 2.15. Schematic diagram of the pulse generator: a - converter;

b - pulse generator

We choose the core Ш8×8, for which S c =0.52·10 -4 m2.

The number of turns in the windings of the transformer TV1:

Vit.; (2.81)

vit.; (2.82)

vit. (2.83)

Filter capacitor capacitance VC1:

The diameter of the wires of the windings of the transformer TV1:

We select the standard wire diameters d 1 \u003d 0.2 mm, d 2 \u003d mm, d 3 \u003d 0.12 mm.

Taking into account the thickness of the enamel insulation d 1 \u003d 0.23 mm, d 2 \u003d 0.08 mm, d 3 \u003d 0.145 mm.

Rice. 2.16. Calculation scheme of the pulse generator

Calculation of pulse generators (Fig. 2.16)

We take the voltage at the input of the generator equal to the voltage at the output of the converter U 0 = 300 V. The pulse frequency f = 1 ... 2 Hz. The pulse voltage amplitude is not more than 10 kV. The amount of electricity in the pulse is not more than 0.003 C. Pulse duration up to 0.1 s.

We select a diode VD type D226B (U arr = 400 V, I pr \u003d 0.3 A, U pr \u003d 1 V) and a thyristor of the KN102I type (U on = 150 V, I pr t = 0.2 A, U pr \u003d 1 .5 V, I on = 0.005 A, I off = 0.015 A, τ on = 0.5 10 -6 s τ off = 40 10 -6 s).

The direct resistance to direct current of the diode R d.pr \u003d 3.3 Ohm and the thyristor R t.pr \u003d 7.5 Ohm.

Pulse repetition period for a given frequency range:

. (2.86)

The resistance of the charging circuit R 3 must be such that

Ohm. (2.88)

Then R 3 \u003d R 1 + R d.pr \u003d 20 10 3 + 3.3 \u003d 20003.3 Ohm.

Charge current:

A. (2.89)

Resistor R 2 limits the discharge current to a safe value. His resistance:

Ohm, (2.90)

where U p is the voltage on the charging capacitor VC2 at the beginning of the discharge, its value is equal to U off. In this case, the condition R 1 >>R 2 (20·10 3 >>750) must be observed.

Discharge circuit resistance:

R p \u003d R 2 R t. pr \u003d 750 + 7.5 \u003d 757.5 Ohm.

The conditions for stable inclusion (2.91, 2.92) are satisfied.

, , (2.91)

, . (2.92)

Capacitor capacitance VC2:

. (2.93)

Capacitance VC2 for frequency f=1 Hz:

F

And for a frequency of 2 Hz:

C 2 \u003d 36 10 -6 F.

The amplitude of the current in the charge circuit of the capacitor VC2

, (2.94)

The amplitude of the current in the charge circuit of the capacitor VC2:

, (2.95)

Pulse energy:

J. (2.96)

The maximum amount of electricity in a pulse:

q m \u003d I p τ p \u003d I p R p C 2 \u003d 0.064 757.5 72 10 -6 \u003d 0.003 C (2.97)

does not exceed the specified value.

Let's calculate the parameters of the output transformer TV2.

Estimated transformer power:

Tue, (2.98)

where η t \u003d 0.7 ... 0.8 - the efficiency of a low-power transformer.

Sectional area of ​​the transformer core:

The number of turns of each transformer winding per

vit/V. (2.100)

The number of turns in the windings of the transformer TV2:

W 4 \u003d 150 N \u003d 150 16.7 \u003d 2505 vit.; (2.101)

W 5 \u003d 10000 16.7 \u003d 167 10 3 vit.

Wire diameter in windings (2.85):

mm;

mm.

We choose the standard diameters of wires with enameled insulation d 4 \u003d 0.2 mm, d 5 \u003d 0.04 mm.

Example. Determine the voltage and currents in the circuit of fig. 2.16.

Given: U c \u003d 300 V AC 400 Hz, C \u003d 36 10 -6 F, R d.pr \u003d 10 Ohm, R t.pr \u003d 2.3 Ohm, L w \u003d 50 mH, R 1 \u003d 20 kOhm , R 2 \u003d 750 Ohm.

Capacitor voltage at the time of charging:

, (2.102)

where τ st \u003d 2 10 4 36 10 -6 \u003d 0.72 s.

The impedance of the charge circuit of the capacitance VC2:

The charge current is:

A.

In this article, we will talk about the pulse generator for the Mayer cell.

Studying the element base of electronic boards, on which all the devices included in the complex installation used by Meyer in the hydrogen generator installed by him on the car were assembled, I assembled the "main part" of the device - a pulse generator.

All electronic boards perform certain tasks in the Cell.

The electronic part of the Meyer mobile hydrogen generator installation consists of two full-fledged devices designed as two independent blocks. This is a control and monitoring unit for a cell that produces an oxygen-hydrogen mixture and a control and monitoring unit for supplying this mixture to the cylinders of an internal combustion engine. The first photo is shown below.

The control and monitoring unit for the operation of the cell consists of a secondary power supply device that provides all the boards of the module with energy and eleven modules - boards consisting of pulse generators, control and management circuits. In the same block, behind the pulse generator boards, there are pulse transformers. One of eleven kits: the pulse generator and pulse transformer board is used specifically for only one pair of cell tubes. And since there are eleven pairs of tubes, there are also eleven generators.

.

Judging by the photographs, the pulse generator is assembled on the simplest element base of digital logic elements. Schematic diagrams published on various sites dedicated to the Mayer Cell, according to the principle of operation, are not so far from its original, with the exception of one thing - they are simplified and work uncontrollably. In other words, the pulses are applied to the electrode tubes until a “pause” occurs, which, at its discretion, is promptly set by the circuit designer with the help of adjustment. Meyer's "pause" is formed only when the Cell itself, consisting of two tubes, reports that it is time to make this pause. There is an adjustment of the sensitivity of the control circuit, the level of which is set quickly using the adjustment. In addition, there is a quick adjustment of the duration of the "pause" - the time during which the cell does not receive pulses. The Mayer generator circuit provides for automatic adjustment of the "pause" depending on the need for the amount of gas produced. This adjustment is carried out by a signal from the control unit and monitoring the supply of the fuel mixture to the internal combustion engine cylinders. The faster the internal combustion engine rotates, the greater the consumption of the oxygen-hydrogen mixture and the shorter the “pause” for all eleven generators.

On the front panel of the Mayer generator, there are slots of trimming resistors that adjust the pulse frequency, the duration of the pause between bursts of pulses and manually set the sensitivity level of the control circuit.

For replication of an experienced pulse generator, there is no need for automatic control of gas demand and automatic regulation of the “pause”. This simplifies the electronic circuit of the pulse generator. In addition, the modern electronic base is more advanced than it was 30 years ago, so with more modern microcircuits, it makes no sense to use the simplest logic elements that Meyer used earlier.

This article publishes a circuit of a pulse generator, assembled by me, recreating the principle of operation of the Mayer cell generator. This is not my first design of a pulse generator, before it there were two more complex circuits capable of generating pulses of various shapes, with amplitude, frequency and time modulation, circuits for controlling the load current in the circuits of the transformer and the Cell itself, circuits for stabilizing the amplitudes of pulses and the shape of the output voltage on the cell. As a result of the exclusion of, in my opinion, “unnecessary” functions, the simplest circuit was obtained, very similar to the circuits published on various sites, but differing from them in the presence of the Cell current control circuit.

As in other published circuits, there are two generators in the cell. The first is a generator - a modulator that forms bursts of pulses, and the second is a pulse generator. A feature of the circuit is that the first oscillator - the modulator does not operate in the oscillator mode, like other developers of the Meyer Cell circuits, but in the standby oscillator mode. The modulator works according to the following principle: At the initial stage, it allows the generator to work, and when a certain current amplitude is reached directly on the Cell plates, generation is disabled.

In Mayer's mobile installation, a thin core is used as a pulse transformer, and the number of turns of all windings is huge. Neither the dimensions of the core nor the number of turns are specified in any of the patents. In a stationary setup, Mayer has a closed torroid with known dimensions and the number of turns. That is what we decided to use. But since wasting energy on magnetization in a single-cycle generator circuit is wasteful, it was decided to use a transformer with a gap, based on the ferrite core from the TVS-90 line transformer used in transistor black and white TVs. It is most suitable for the parameters specified in Mayer's patents for fixed installation.

The circuit diagram of the Mayer Cell in my performance is shown in the figure.

.

There is no complexity in the design of the pulse generator. It is assembled on banal microcircuits - LM555 timers. Due to the fact that the generator is experimental and it is not known what load currents we can expect, for reliability, IRF is used as the output transistor VT3.

When the Cell current reaches a certain threshold, at which water molecules break, it is necessary to pause in the supply of impulses to the Cell. For this, a silicon transistor VT1 - KT315B is used, which prohibits the operation of the generator. Resistor R13 "Generation stall current" is designed to set the sensitivity of the control circuit.

Switch S1 "Duration coarse" and resistor R2 "Duration fine" are operational adjustments of the duration of the pause between bursts of pulses.

In accordance with Mayer's patents, the transformer has two windings: the primary contains 100 turns (for 13 volts of supply) of PEV-2 wire with a diameter of 0.51 mm, the secondary contains 600 turns of PEV-2 wire with a diameter of 0.18 mm.

With the specified parameters of the transformer, the optimal pulse repetition rate is 10 kHz. The inductor L1 is wound on a cardboard mandrel with a diameter of 25 mm, and contains 100 turns of PEV-2 wire with a diameter of 0.51 mm.

Now that you have "swallowed" all this, let's debrief this scheme. With this scheme, I did not use additional schemes to increase gas output, because they are not observed in the mobile Mayer Cell, of course, not counting laser stimulation. Either I forgot to go with my Cell to the "grandmother - whisperer" so that she would whisper the high performance of the Cell, or I did not choose the right transformer, but the efficiency of the installation turned out to be very low, and the transformer itself was very hot. Considering that the resistance of water is small, the Cell itself is not capable of acting as a storage capacitor. The cell simply did not work according to the "scenario" that Meyer described. Therefore, I added an additional capacitor C11 to the circuit. Only in this case, a signal shape appeared on the output voltage oscillogram, with a pronounced accumulation process. Why did I put it not in parallel with the Cell, but through a choke? The cell's current control circuit has to track the sudden increase in this current, and the capacitor will prevent this with its charge. The coil reduces the influence of C11 on the control circuit.

I used plain tap water, and used fresh distilled water. As I just did not pervert, but the energy costs at a fixed performance were three to four times higher than directly from the battery through a limiting resistor. The resistance of water in the cell is so small that the increase in the pulse voltage by the transformer was easily extinguished at low resistance, causing the magnetic circuit of the transformer to become very hot. It is possible to assume that the whole reason is that I used a ferrite transformer, and in the mobile version of the Mayer Cell there are transformers in which the core is almost absent. It acts more like a framework. It is not difficult to understand that Mayer compensated for the small thickness of the core with a large number of turns, thereby increasing the inductance of the windings. But water resistance will not increase from this, therefore, the voltage, which Meyer writes about, will not rise to the value described in the patents.

In order to increase the efficiency, I decided to “throw out” the transformer from the circuit, on which energy loss occurs. Schematic diagram of the Meyer cell without a transformer is shown in the figure.

.

Since the inductance of the coil L1 is very small, I also excluded it from the circuit. And "lo and behold" the installation began to produce a relatively high efficiency. I conducted experiments and came to the conclusion that for a given volume of gas, the installation expends the same energy as during electrolysis with direct current, plus or minus the measurement error. That is, I finally assembled an installation in which there is no energy loss. But why is it needed if the energy costs are exactly the same directly from the battery?

Completion

Let's finish the topic of very little water resistance. The Cell itself is not able to work as a storage capacitor because water, which acts as a capacitor dielectric, cannot be one - it conducts current. In order for the process of electrolysis to take place over it - decomposition into oxygen and hydrogen, it must be conductive. It turns out an unresolvable contradiction, which can be resolved only in one way: Reject the “Cell-capacitor” version. Accumulation in the Cell, like a capacitor, cannot occur, this is a Myth! If we take into account the area of ​​the capacitor plates formed by the surfaces of the tubes, then even with an air dielectric, the capacitance is negligible, and here water with its low active resistance acts as a dielectric. Don't believe? Take a physics textbook and calculate the capacitance.

It can be assumed that the accumulation occurs on the coil L1, but this also cannot be for the reason that its inductance is also very small for a frequency of about 10 kHz. The inductance of the transformer is several orders of magnitude higher. You can even think about why it was “plugged” into the circuit with a small inductance.

Afterword

Someone will say that everything is a miracle in bifilar winding. In the form in which it is presented in Mayer's patents, there will be no sense from it. Bifilar winding is used in protective power filters, not of the same conductor, but opposite in phase and is designed to suppress high frequencies. It is even available in all, without exception, power supplies for computers and laptops. And for the same conductor, bifilar winding is done in a wire resistor, to suppress the inductive properties of the resistor itself. The bifilar winding can be used as a filter that protects the output transistor, preventing high-power microwave pulses from passing into the oscillator circuit, supplied from the source of these pulses directly to the Cell. By the way, the L1 coil is an excellent filter for microwaves. The first pulse generator circuit that uses a step-up transformer is correct, only something is missing between the VT3 transistor and the Cell itself. I will devote the next article to this.

The most common generators are rectangular and linearly changing (sawtooth) voltage pulses.

Pulse signal generators (pulse generators) can operate in one of three modes: self-oscillating, waiting and synchronization.

In the self-oscillatory mode, the generators continuously form pulse signals without external influence. In standby mode, the generators generate a pulse signal only upon arrival of an external (triggering) signal. In synchronization mode, the generators generate voltage pulses whose frequency is equal to or a multiple of the frequency of the synchronizing signal.

Rectangular pulse generators are divided into multivibrators and blocking generators. Both of them can work both in self-oscillating and standby modes.

Self-oscillating multivibrators can be built on discrete, logic elements or operational amplifiers. A self-oscillating multivibrator based on an op-amp is shown in fig. 11.12.

Rice. 11.12. Self-oscillating multivibrator based on op-amp

In this circuit, with the help of resistors R 1 and R 2, positive feedback is introduced, which is a necessary condition for the occurrence of electrical oscillations. Depending on the output voltage (which can be equal to either + E pit or -E pit, where E pit is the supply voltage of the op-amp), either voltage U +1 or voltage U +2 is set at the non-inverting input of the op-amp. Capacitance C, included in the negative feedback circuit, is recharged with a time constant τ= RC. The pulse repetition period T is determined by the expression

.

Thus, this multivibrator generates rectangular voltage pulses.

Blocking generators used to obtain powerful rectangular pulses of short duration (from fractions of a microsecond to fractions of a millisecond) and a duty cycle of up to several tens of thousands. The main element of such generators is a pulse transformer (Fig. 11.13).

Rice. 11.13. Self-oscillating blocking oscillator

The blocking oscillator can operate in self-oscillating, standby or synchronization modes. During a pause (there is no output voltage), the capacitor is recharged along the E–R–W 2 circuit with a time constant τ 1 =RC. At the moment when the voltage on the capacitor C (and, therefore, on the base of the transistor) becomes equal to zero, the transistor begins to open (leave the cutoff mode), the collector current begins to flow, which causes a positive feedback signal to appear (through the transformer winding W 2) , under the influence of which the transistor jumps into saturation mode. In this case, the capacitor C is recharged along the circuit W 2 -C - input resistance of the transistor r in with time constant τ 2 = r in ·WITH. With an increase in the voltage across the capacitor C, the base current begins to decrease and at the end of the charge, the transistor goes out of saturation and closes. After that, the energy stored in the inductance is discharged to the load. Because r in << R, then the time the transistor is in the open state t u, and consequently, the pulse duration at the load is much less than the pulse repetition period.

Ramp generator . Linearly changing voltage (LIN) is a voltage that changes linearly during a period of time called a working stroke, and then returns to its original level during a period of time called a reverse stroke (Fig. 11.14).

Rice. 11.14. Ramp voltage

On fig. 11.14 the following designations are accepted: U 0 - initial level, U m - LIN amplitude, T p - working stroke time, T 0 - reverse stroke time.

Devices designed to form LIN are called LIN generators (CLAY). LIN generators are often referred to as sawtooth voltage generators.

The principle of constructing LIN generators is based on the charge of the capacitance by direct current. The basis of CLAY (Fig. 11.15) is a capacitance through which a direct current flows from a direct current source IT, due to which, when the key device of the KU is open, the voltage on the capacitance is determined by the expression

, (at i With = I= const), i.e. changes linearly.

CLAYS can work either in a waiting room (Fig. 11.15, A), or in self-oscillatory mode (Fig. 11.15, b). CLAY in self-oscillatory mode forms LIN regularly, and to obtain LIN in CLAY in standby mode, an external voltage pulse U input is required.

Rice. 11.15. Linear voltage generators,

operating in standby (a) and self-oscillating (b) modes

All CLAYS can be divided into three types:

a) with an integrating RC chain (Fig. 11.16);

b) with a current-stabilizing two-terminal network (Fig. 11.17);

c) with compensating feedback (OS) (Fig. 11.18).

Rice. 11.16. CLAY based on transistor switch

(with integrating RC circuit)

Until the point in time t 1 the transistor switch is in saturation mode, i.e. voltage U ke, and hence the voltage U exit, are equal to zero. When applied at the time t 1 of the blocking voltage pulse, the transistor enters the cut-off mode, and the capacitance C is charged from the source E k through the resistor R k, and the voltage across the capacitance tends to the level E k. At the moment of time t 2 the transistor again enters saturation mode, and the capacitance through the low resistance of the collector-emitter gap of the transistor is discharged.

Consider the principle of constructing a clay line with a current-stabilizing two-terminal network, which ensures the flow of direct current through it, regardless of the applied voltage (Fig. 11.17). The simplest current-stabilizing element is a transistor. With a constant base current (for example, i bae), even with a significant decrease in voltage u eq between the emitter and the collector (for example, from U 2 to U 1), the collector current of the transistor decreases slightly.

Rice. 11.17. CLAY with current-stabilizing two-pole

The disadvantage of this circuit is that when connected to the output (i.e., to capacitance C) of the load resistance, the linearity of the output voltage is distorted.

Consider CLAY with a compensating OS (based on OS) (Fig. 11.18). At the point in time t 1 key TO opens and a forward stroke is carried out and carried out, and at the moment of time t 2 key closes, capacitance WITH is discharged and the output voltage is set to zero. Capacity WITH is charged by direct current, which means that the voltage on it (as well as the voltage U exit) changes linearly (Fig. 11.18, b). Compensating voltage U To repeats the voltage across the capacitance U c when the key opens and the capacitance is charged from the source U. Since the compensating voltage is turned on opposite to the voltage on the capacitance, the voltage applied to the resistor R, all the time constantly and equally U.

Rice. 11.18. CLAY with compensating feedback

flowing through resistor R current is given by

i R =(E- U in )/ R.

If the op amp is close to ideal, ( K→ ∞,U in → 0 ,i – → 0 ), That i R = E/ R= const. Then the output voltage is given by

.

Requirements for pulse generators (PG) include the need to achieve high efficiency. In addition, they are determined by the properties of the interelectrode gap (IEG) - a sharply nonlinear element of the electrical circuit.

The stability of current pulses - the constancy of their duration depends on the constancy of the properties of the gap and the steepness of the leading edge of the voltage pulse. The greater this steepness, the more stable the current pulses. This implies another requirement for pulse generators - a high degree of steepness of the leading edge of the voltage pulse.

The supply of energy pulses to the interelectrode gap during EEE can be carried out according to the block diagram shown in Fig. 1, a.

Fig.1 Structural diagrams of the power supply for the EDM installation and timing diagrams of voltage and current

During the time τ and the switch K is closed and the power source gives the load (MEM) the power P u, which is n times greater than the average power for the pulse repetition period T.

The power of the power supply must be equal to P and = I m * U m , where I m and U m are the amplitude values ​​of the voltage and current during the pulse. It is consumed only in the time interval τ and.

If we neglect the losses in the energy storage device, then the energy given off by the storage device to the MEM will be A=P and *τ and, and the power of the source P=A/T= P and *τ and /T=P and /n, i.e. when an energy storage device is introduced into the block diagram, the power of the source can be reduced by n times.

The scheme of the electroerosive installation, which provides work with energy storage devices, is shown in fig. 1b.

During the pause P and *τ and the switch K is in position 1, and through the current limiter, the storage device consumes power P/n from the power source. In this case, the accumulator stores energy A=P and *τ and, which, when switching the switch K for the pulse time τ and to position 2, gives power P and =A/ τ and.

Work according to this scheme makes it possible to transform the power of the source P=P and /n into the power that is consumed at the load.

Pulse generators are distinguished by the principle of operation, design and pulse parameters. GI is conditionally divided into dependent, limited-dependent and independent. In the first of them, the parameters of the generated pulses are determined by the physical state of the interelectrode gap. In independent generators, pulses are not associated with the state of the MEP.

The electrical energy in the storage device can be stored in the form of an electric field of a capacitor or an electromagnetic field of an inductive coil. Combined storage devices containing active resistances, capacitance and inductance are also used - relaxation generators (Fig. 2).

Fig.2 Schematic diagrams of relaxation generators for EEE installations

In the process of their discharge, the energy accumulated in the reactive elements of the circuit (capacitor or inductive coil) is consumed.

RC-pulse generator (Fig. 2, a) consists of a series-connected power source G, key TO, current-limiting resistance R 1 and storage capacitor From 1 connected in parallel to the MEP.

The capacitive storage is charged from the power supply through the limiting resistance R1 due to which the winding current is much less than the pulse current I And. The charging current of the capacitor will be determined from the ratio i 1 =(dUc/dτ)*С. Capacitor voltage where U co is the initial voltage on the capacitor at the moment τ=0. By the end of charging voltage U c will be equal to the power supply voltage. Discharging occurs during the time τ= T/n. In the case of a large duty cycle of the pulses, the average value of the discharge current during the passage of the pulse τ and in n times the charging current, so the capacitive storage is essentially a current transformer.

In an inductive storage, the rate of current rise in the inductance is determined by its value and the applied voltage. Required current I and can also be obtained for small values ​​of the voltage drop across the inductance U to<

In the processes of electrical discharge machining, generators with capacitive storage are more widely used, since the inductive storage is inferior to the capacitive one in terms of energy performance.

Pulse circuit LC generator is shown in fig. 2b. Charging current flows to the capacitor WITH from the power supply G through the vibrator winding L. First he pulls the anchor I electromagnetic vibrator and increases the interelectrode gap, raising the electrode-tool.

By the end of the charging of the capacitor, the current through the vibrator winding gradually decreases, the electromagnetic force holding the vibrator armature weakens, and the electrodes begin to approach each other, reducing the MEP. After the breakdown of the MEP and the passage of the current pulse, the cycle of the generator is repeated. The pulse frequency is determined by the relation L And C in the generator circuit.

Generators made according to this scheme have high efficiency and productivity.

Introduction to the charging circuit of the RC generator inductance (transition to the generator RLC) increases the efficiency of the generator, since in this case the current-limiting resistance decreases. RLC- generators (Fig. 2, c) operate at a lower voltage than RC generators, since in the presence of resonance between L And WITH the voltage on the storage capacitor is greater than the voltage of the power supply.

Charging circuit transient equation RLC-generator has the form

From this equation it follows that the charge of a capacitor can occur according to an exponential or oscillatory law.

The oscillatory process occurs at . In this mode of operation of the charging circuit, the voltage on the capacitor at the end of the charging period τ charge is almost double the EMF.

In fact, the maximum voltage to which the capacitor can be charged depends on the ratio R 1 /(2L 1).

EEA also applies SS- a pulse generator in which a capacitor C 1 is used as a current-limiting element. Such a generator has a higher efficiency compared to LC- generator with electromagnetic vibrator. Frequency properties SS generators are determined mainly by the frequency characteristics of the rectifier diodes IN.

The main disadvantage of relaxation generators is the relationship between the frequency of current pulses and the physical state of the MEP. It can be eliminated if a controlled switch is introduced into the discharge circuit, which at a given time would connect a storage capacitor to the MEP.

To power EEE devices, there are static pulse generators that regulate time and energy parameters in a wide range in the absence of storage elements. They easily form rectangular and unipolar pulses. According to the generation method, they are divided into generators with independent excitation, self-generators and inverters.

Structurally, they are made mainly on transistor or thyristor devices.

The block diagram of a wide-range pulse generator is shown in fig. 2.3.

Fig.3 Structural diagram of a wide-range transistor pulse generator

It includes a power source, power blocks, the number of which can be equal to six, with a separating diode VD, an ignition unit, a master oscillator, a power preamplifier, an operating gap (MEP), a short circuit protection unit. The composition of the power blocks and the ignition block includes power transistors operating in the key mode and switching synchronously from the master oscillator. When the transistors are turned on, a low-power pulse is supplied from the ignition unit. It contributes to the breakdown of the gap and the formation of a low-voltage discharge. Before the breakdown, the separating diode D is locked. After the breakdown, the voltage across the gap decreases to 40-25 V, diode D opens and a current pulse passes through the gap, the value of which is determined by the number of power units connected in parallel. Their synchronous shutdown interrupts the discharge. In the event of a short circuit in the electrode gap of the MEC, all transistors of the power units are switched off. The supply of pulses to the MEP is resumed after the elimination of the short circuit.

For EEE of metals with high-energy pulses with a frequency of 50-100 Hz, static pulse generators are used - industrial frequency transformers with a valve.

Energy pulses with a duration of up to milliseconds are obtained using pulse generators, which, according to the principle of operation, are divided into commutator and inductor generators.

The magnetic commutator generator (MKG) includes an alternating-pole magnetic system on the stator and an armature winding. The armature winding on its circumference is unevenly distributed on narrow parts under the poles, which are much larger in MCG than in conventional machines, due to which the frequency of the generator current increases. When the generator armature rotates in its winding, located in a narrow section opposite the poles of the inductor, at the moment of passing its variable-pole inductor, a symmetrical pulsed EMF is induced.

The pulses are unipolarized using a collector (commutator) located on the same shaft as the armature, which consists of two systems of segments with brushes superimposed on them. The presence of pauses between pulses facilitates switching, since the transition of brushes from one system of segments to another occurs at the moment of absence of voltage in the armature winding.

A machine inductor pulse generator (MIT) is an electric machine of a brushless type that generates an alternating voltage of increased frequency. Its main feature is the absence of a rotating pole system, which is replaced by a toothed inductor. The armature winding and excitation are located on the generator stator. A variable magnetic flux occurs due to a change in the resistance of the magnetic circuit of the generator, due to the gearing of the rotating inductor.

Due to the use of a toothed inductor, an asymmetric alternating voltage curve is obtained with different amplitudes of half-waves of positive and negative polarity. With a sufficiently small amplitude of the reverse voltage half-wave, the breakdown of the MEP occurs only with voltage pulses of direct polarity, as a result of which the current pulses will always be unipolar.

Industrial power supplies for EEE installations.

The thyristor pulse generator type TG-250-0.15M is designed to convert a three-phase alternating current of industrial frequency into a pulsed current with a frequency of 150 Hz with an adjustable duty cycle. It is used as a power source with technological current for electroerosive machines of models 4723, 4A724, 4D723, 4D26.

The maximum productivity of the machine when powered by a thyristor pulse generator is 4000 mm 3 /min in the case of processing steel 45 with copper tools and 3500 mm 3 /min when processing with graphite tools.

The pulse generator includes blocks of valves, ignition, control, heating and resistance regulators, as well as transformers and inductive ballast resistances. The valve block is assembled according to the scheme of a three-phase semi-controlled bridge on diodes and thyristors. The ignition block, synchronously with the power ones, generates high-voltage pulses with an amplitude of 400-500 V, which break through the erosion gap and form a low-voltage discharge. To automatically maintain the working distance of the erosion gap, a supply control unit with voltage feedback is provided. Structurally, the pulse generator is made in the form of a metal double-sided service cabinet. Forced air cooling.

Manufacturer - software "Transformer", Zaporozhye.