Level gauges based on thyristors. Thyristor control, principle of operation

Sometimes you need to turn on a powerful load, such as a lamp in a room, with a weak signal from the microcontroller. This problem is especially relevant for developers. smart home. The first thing that comes to mind is relay. But don't rush, there is a better way :)

In fact, the relay is a complete mess. Firstly, they are expensive, and secondly, to power the relay winding you need an amplifying transistor, since the weak leg of the microcontroller is not capable of such a feat. Well, thirdly, any relay is a very bulky design, especially if it is a power relay designed for high current.

If we are talking about alternating current, then it is better to use triacs or thyristors. What it is? And now I’ll tell you.

If on the fingers, then thyristor look like diode, even the designation is similar. It allows current to flow in one direction and does not allow it to flow in the other. But it has one feature that fundamentally distinguishes it from a diode - control input.
If the control input is not applied opening current, That thyristor will not pass current even in the forward direction. But as soon as you give even a brief impulse, it immediately opens and remains open as long as there is direct voltage. If remove the voltage or change the polarity, the thyristor will close. The polarity of the control voltage should preferably match the polarity of the anode voltage.

If connect back-to-back parallel two thyristors, then it will work out triac- a great thing for switching AC loads.

On the positive half-wave of the sinusoid one passes, on the negative half-wave the other. Moreover, they pass only if there is a control signal. If the control signal is removed, then in the next period both thyristors will shut down and the circuit will break. Beauty and nothing more. So it should be used to control household loads.

But there is one subtlety here - we are switching a high-voltage power circuit, 220 volts. And we have the controller low voltage, runs on five volts. Therefore, in order to avoid excesses, it is necessary to carry out potential outcome. That is, make sure that there is no direct electrical connection between the high-voltage and low-voltage parts. For example, do optical separation. There is a special assembly for this - a triac optodriver MOC3041. Wonderful thing!
Look at the connection diagram - just a few additional parts and you have the power and control parts separated from each other. The main thing is that the voltage for which the capacitor is designed is one and a half to two times higher than the voltage in the outlet. You don’t have to worry about power interference when you turn the triac on and off. In the optodriver itself, the signal is supplied by an LED, which means you can safely light it from the microcontroller pin without any additional tricks.

In general, it is possible without decoupling and it will also work, but it is considered good form always make a potential outcome between the power and control parts. This includes the reliability and safety of the entire system. Industrial solutions are simply filled with optocouplers or all sorts of isolating amplifiers.

V. Krylov

Currently, thyristors are widely used in various automatic monitoring, signaling and control devices. A thyristor is a controlled semiconductor diode, which is characterized by two stable states: open, when the direct resistance of the thyristor is very small and the current in its circuit depends mainly on the voltage of the power source and the load resistance, and closed, when its direct resistance is high and the current is a few milliamps .

In Fig. Figure 1 shows a typical current-voltage characteristic of a thyristor, where section O A corresponds to the closed state of the thyristor, and section BB corresponds to the open state.

At negative voltages, the thyristor behaves like a regular diode (OD section).

If you increase the forward voltage on a closed thyristor with the control electrode current equal to zero, then when the value Uon is reached, the thyristor will open. This switching of the thyrostor is called switching along the anode. The operation of a thyristor in this case is similar to the operation of an uncontrolled semiconductor four-layer diode - a dinistor.

The presence of a control electrode allows the thyristor to open at an anode voltage less than Uon. To do this, it is necessary to pass the control current Iу through the control electrode-cathode circuit. The current-voltage characteristic of the thyristor for this case is shown in Fig. 1 dotted line. The minimum control current required to open the thyristor is called the rectifying current Irev. The rectifying current is highly dependent on temperature. In reference books it is indicated at a certain anode voltage. If during the operation of the control current the anode current exceeds the value of the switch-off current Ioff, then the thyristor will remain open even after the end of the control current; if this does not happen, the thyristor will close again.

If the voltage at the anode of the thyristor is negative, applying voltage to its control electrode is not allowed. A negative voltage (relative to the cathode) at which the reverse current of the control electrode exceeds several milliamps is also unacceptable.

An open thyristor can be switched to a closed state only by reducing its anode current to a value less than Ioff. In devices direct current For this purpose, special damping chains are used, and in the circuit alternating current The thyristor closes on its own at the moment the anode current passes through zero.

This is the reason for the widest use of thyristors in alternating current circuits. All the circuits discussed below relate only to thyristors connected to the alternating current circuit.

To ensure reliable operation of the thyristor, the control voltage source must meet certain requirements. In Fig. 2 shows the equivalent circuit of the control voltage source, and Fig. 3 - a graph with which you can determine the requirements for its load line.

On the graph, lines A and B limit the spread zone of the input current-voltage characteristics of the thyristor, which represent the dependence of the voltage on the control electrode Uу on the current of this electrode Iу with the anode circuit open. Straight B determines minimum voltage Uу, ​​at which any thyristor of this type opens at a minimum temperature. Direct Г determines the minimum current Iу sufficient to open any thyristor of a given type at a minimum temperature. Each specific thyristor opens at a certain point in its input characteristic. The shaded area is the geometric location of such points for all thyristors of a given type that satisfy technical specifications. Direct lines D and E determine the maximum permissible values ​​of voltage Uу and current Iу, respectively, and curve K - the maximum permissible value power dissipated at the control electrode. The load line L of the control signal source is drawn through the points that determine the no-load voltage of the source Eu.xx and its short-circuit current Iу.кз = Eu.xx/Rinternal, where Rinternal internal resistance source. The intersection point S of the load straight line L with the input characteristic (curve M) of the selected thyristor should be located in the area lying between the shaded area and lines A, D, K, E and B.

This area is called the preferred opening area. The horizontal straight line H determines the highest voltage at the control transition, at which not a single thyristor of this type opens at the maximum permissible temperature. Thus, this value, tenths of a volt, determines the maximum permissible amplitude of the interference voltage in the thyristor control circuit.

After opening the thyristor, the control circuit does not affect its state, so the thyristor can be controlled by pulses of short duration (tens or hundreds of microseconds), which simplifies control circuits and reduces the power dissipated at the control electrode. The pulse duration, however, must be sufficient to increase the anode current to a value exceeding the turn-off current Ioff for different types of load and operating mode of the thyristor.

The comparative simplicity of control devices when operating thyristors in alternating current circuits has led to the widespread use of these devices as control elements in voltage stabilization and regulation devices. The average value of the load voltage is regulated by changing the moment of supply (that is, the phase) of the control signal relative to the beginning of the half-cycle of the supply voltage. The repetition rate of control pulses in such circuits must be synchronized with the network frequency.

There are several methods for controlling thyristors, of which amplitude, phase and phase-pulse should be noted.

The amplitude control method consists in applying a positive voltage that varies in value to the control electrode of the thyristor. The thyristor opens at the moment when this voltage becomes sufficient for the rectification current to flow through the control junction. By changing the voltage on the control electrode, you can change the opening moment of the thyristor. The simplest scheme A voltage regulator built on this principle is shown in Fig. 4.

Part of the anode voltage of the thyristor, that is, the voltage of the positive half-cycle of the network, is used here as the control voltage. Resistor R2 changes the opening moment of thyristor D1 and, consequently, the average voltage across the load. When resistor R2 is fully inserted, the voltage across the load is minimal. Diode D2 protects the control junction of the thyristor from reverse voltage. It should be noted that the control circuit is not connected directly to the network, but in parallel with the thyristor. This is done so that the open thyristor shunts the control circuit, preventing unnecessary power dissipation on its elements.

The main disadvantages of the device in question are the strong dependence of the load voltage on temperature and the need for individual selection of resistors for each thyristor instance. The first is explained by the temperature dependence of the thyristor rectification current, the second by the large spread of their input characteristics. In addition, the device is capable of adjusting the opening moment of the thyristor only during the first half of the positive half-cycle of the network voltage.

The control device, the diagram of which is shown in Fig. 5, allows you to expand the control range to 180°, and the inclusion of a thyristor in the diagonal of the rectifier bridge allows you to regulate the voltage on the load during both half-cycles of the network voltage.

Capacitor C1 is charged through resistors R1 and R2 to a voltage at which a current equal to the rectification current flows through the control junction of the thyristor. In this case, the thyristor opens, passing current through the load. Due to the presence of a capacitor, the load voltage is less dependent on temperature fluctuations, but nevertheless, this device also has the same disadvantages.

With the phase method of controlling thyristors using a phase-shift bridge, the phase of the control voltage is changed relative to the voltage at the anode of the thyristor. In Fig. Figure 6 shows a diagram of a half-wave voltage regulator, in which the change in voltage across the load is carried out by resistor R2, connected to one of the arms of the bridge, from the diagonal of which the voltage is supplied to the control junction of the thyristor.

The voltage on each half of control winding III should be approximately 10 V. The remaining parameters of the transformer are determined by the voltage and load power. The main disadvantage of the phase control method is the low slope of the control voltage, which is why the stability of the thyristor opening moment is low.

The phase-pulse method of controlling thyristors differs from the previous one in that, in order to increase the accuracy and stability of the opening moment of the thyristor, a voltage pulse with a steep edge is applied to its control electrode. This method is currently most widespread. Schemes implementing this method are very diverse.

In Fig. 7 shows a diagram of one of the most simple devices using the phase-pulse method of thyristor control.

With a positive voltage at the anode of thyristor D3, capacitor C1 is charged through diode D1 and variable resistor R1. When the voltage on the capacitor reaches the turn-on voltage of dinistor D2, it opens and the capacitor is discharged through the control junction of the thyristor. This pulse of discharge current opens thyristor D3 and current begins to flow through the load. By changing the capacitor charge current with resistor R1, you can change the opening moment of the thyristor within the half-cycle of the network voltage. Resistor R2 prevents self-opening of thyristor D3 due to leakage currents at elevated temperatures. According to technical conditions, when thyristors operate in standby mode, the installation of this resistor is mandatory. Shown in Fig. 7, the circuit has not found wide application due to the large spread in the dinistor turn-on voltage, reaching up to 200%, and the significant dependence of the turn-on voltage on temperature.

One of the varieties of the phase-pulse method of controlling thyristors is the so-called vertical control, which is currently most widespread. It consists in the fact that at the input of the pulse generator a comparison is made (Fig. 8) of a constant voltage (1) and a voltage varying in magnitude (2). At the moment of equality of these voltages, a thyristor control pulse (3) is generated. The variable voltage can have a sinusoidal, triangular or sawtooth (as shown in Fig. 8) shape.

As can be seen from the figure, changing the moment of occurrence of the control pulse, that is, shifting its phase, can be done in three different ways:

changing the slew rate AC voltage(2a),

changing its initial level (2b) and

changing the value of constant voltage (1a).

In Fig. Figure 9 shows a block diagram of a device that implements the vertical method of controlling thyristors.

Like any other phase-pulse control device, it consists of a phase-shifting device FSU and a pulse generator GI. The phase-shifting device, in turn, contains an input device VU that perceives the control voltage Uу, a generator of alternating (in magnitude) voltage GPG and a comparing device SU. A variety of devices can be used as these elements.

In Fig. 10 given circuit diagram control device for a thyristor (D5) connected in series with a bridge rectifier (D1 - D4).

The device consists of a sawtooth voltage generator with a transistor switch (T1), a Schmitt trigger (T2, T3) and an output switch amplifier (T4). Under the influence of voltage removed from the synchronizing winding III of transformer Tr1, transistor T1 is closed. In this case, capacitor C1 is charged through resistors R3 and R4. The voltage across the capacitor increases along an exponential curve, the initial section of which, with some approximation, can be considered linear (2, see Fig. 8).

In this case, transistor T2 is closed and T3 is open. The emitter current of transistor T3 creates a voltage drop across resistor R6, which determines the level of operation of the Schmitt trigger (1 in Fig. 8). The sum of the voltages across resistor R6 and open transistor T3 is less than the voltage across zener diode D10, so transistor T4 is closed. When the voltage across capacitor C1 reaches the Schmitt trigger level, transistor T2 opens and T3 closes. At the same time, transistor T4 opens and a voltage pulse appears on resistor R10, opening thyristor D5 (pulse 3 in Fig. 8). At the end of each half-cycle of the mains voltage, transistor T1 is opened by the current flowing through resistor R2. In this case, capacitor C1 is discharged almost to zero and the control device returns to its original state. The thyristor closes at the moment the amplitude of the anode current passes through zero. With the beginning of the next half-cycle, the device’s operating cycle repeats.

By changing the resistance of resistor R3, you can change the charge current of capacitor C1, that is, the rate of increase in voltage across it, and therefore the moment the pulse that opens the thyristor appears. By replacing resistor R3 with a transistor, you can automatically regulate the voltage across the load. Thus, this device uses the first of the above methods of shifting the phase of control pulses.

A slight change to the circuit shown in Fig. 11 allows you to obtain regulation using the second method. In this case, capacitor C1 is charged through a constant resistor R4 and the rate of rise of the sawtooth voltage is the same in all cases. But when transistor T1 opens, the capacitor is discharged not to zero, as in the previous device, but to the control voltage Uу.
Consequently, the charge of the capacitor in the next cycle will begin from this level. By changing the voltage Uу, the opening moment of the thyristor is adjusted. Diode D11 disconnects the control voltage source from the capacitor during its charging.

The output stage on transistor T4 provides the necessary current gain. Using a pulse transformer as a load, several thyristors can be controlled simultaneously.

In the control devices under consideration, voltage is applied to the control transition of the thyristor for a period of time from the moment of equality of the direct and sawtooth voltages until the end of the half-cycle of the network voltage, that is, until the moment of discharge of capacitor C1. The duration of the control pulse can be reduced by turning on a differentiating circuit at the input of the current amplifier, made on transistor T4 (see Fig. 10).

One of the variants of the vertical method of controlling thyristors is the pulse-number method. Its peculiarity is that not one pulse, but a pack of short pulses is applied to the control electrode of the thyristor. The duration of the burst is equal to the duration of the control pulse shown in Fig. 8.

The repetition rate of pulses in a burst is determined by the parameters of the pulse generator. The pulse-number control method ensures reliable opening of the thyristor for any type of load and makes it possible to reduce the power dissipated at the control transition of the thyristor. In addition, if a pulse transformer is included at the output of the device, it is possible to reduce its size and simplify the design.

In Fig. Figure 12 shows a diagram of a control device using the pulse-number method.

A balanced diode-regenerative comparator is used here as a comparison unit and pulse generator, consisting of a comparison circuit on diodes D10, D11 and the blocking generator itself, assembled on transistor T2. Diodes D10, D11 control the operation of the circuit feedback blocking generator.

As in previous cases, when transistor T1 is closed, capacitor C1 begins to charge through resistor R3. Diode D11 is open with voltage Uу, and diode D10 is closed. Thus, the positive feedback winding circuit IIa of the blocking generator is open, and the negative feedback winding circuit IIb is closed and transistor T2 is closed. When the voltage on capacitor C1 reaches voltage Uу, diode D11 will close and D10 will open. The positive feedback circuit will be closed, and the blocking generator will begin to generate pulses that will be sent from winding I of transformer Tr2 to the control transition of the thyristor. The generation of pulses will continue until the end of the half-cycle of the mains voltage, when transistor T1 opens and capacitor C1 is discharged. Diode D10 will close and D11 will open, the blocking process will stop, and the device will return to its original state. By changing the control voltage Uу, you can change the moment of the start of generation relative to the beginning of the half-cycle and, consequently, the moment of opening of the thyristor. Thus, in this case, the third method of shifting the phase of control pulses is used.

The use of a balanced circuit of the comparison unit ensures temperature stability of its operation. Silicon diodes D10 and D11 with small reverse current allow you to obtain a high input impedance of the comparing node (about 1 MΩ). Therefore, it has virtually no effect on the charging process of capacitor C1. The sensitivity of the unit is very high and amounts to several millivolts. Resistors R6, R8, R9 and capacitor C3 determine the temperature stability of the operating point of transistor T2. Resistor R7 serves to limit collector current this transistor and improving the pulse shape of the blocking oscillator. Diode D13 limits the voltage surge on the collector winding III of transformer Tr2, which occurs when the transistor is turned off. Pulse transformer Tr2 can be made on a 1000NN ferrite ring of standard size K15X6X4.5. Windings I and III each contain 75, and windings II a and II b each contain 50 turns of PEV-2 0.1 wire.

The disadvantage of this control device is that it is relatively low frequency pulse repetition rate (approximately 2 kHz with a pulse duration of 15 μsec). You can increase the frequency, for example, by reducing the resistance of resistor R4, through which capacitor C2 is discharged, but at the same time the temperature stability of the sensitivity of the comparing unit is somewhat deteriorated.

The pulse-number method of controlling thyristors can also be used in the devices discussed above (Fig. 10 and 11), since with a certain choice of element ratings (C1, R4-R10, see Fig. 10) the Schmitt trigger when the voltage on capacitor C1 exceeds the level When the trigger is triggered, it generates not a single pulse, but a sequence of pulses. Their duration and frequency are determined by the parameters and trigger mode. This device is called a “multivibrator with a discharge trigger.”

In conclusion, it should be noted that significant circuit simplification of thyristor control devices while maintaining high quality indicators can be achieved using unijunction transistors.

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♦ It is known that the electric current in a household and industrial network varies according to a sinusoidal law. Form of alternating electric current frequency 50 hertz, presented on Fig 1 a).

Over one period, cycle, the voltage changes its value: 0 → (+Umax) → 0 → (-Umax) → 0 .
If you imagine simple generator alternating current (Figure 1 b) with one pair of poles, where the receipt of a sinusoidal alternating current determines the rotation of the rotor frame per revolution, then each position of the rotor at a certain time in the period corresponds to a certain amount of output voltage.

Or, each value of the sinusoidal voltage per period corresponds to a certain angle α frame rotation. Phase angle α , this is the angle that determines the value of a periodically changing quantity at a given time.

At the moment of the phase angle:

• α = 0° voltage U = 0;
• α = 90° voltage U = +Umax;
• α=180° voltage U = 0;
• α = 270° voltage U = — Umax;
• α = 360° voltage U = 0.

♦ Voltage regulation using a thyristor in alternating current circuits uses these features of sinusoidal alternating current.
As mentioned earlier in the article “”: a thyristor is semiconductor device, operating according to the law of a controlled electric valve. It has two stable states. Under certain conditions may have a conducting state (open) and non-conducting state (closed).
♦ A thyristor has a cathode, an anode and a control electrode. Using the control electrode, you can change the electrical state of the thyristor, that is, change electrical parameters valve
A thyristor can only pass electric current in one direction - from anode to cathode (the triac passes current in both directions).
Therefore, for the thyristor to operate, the alternating current must be converted (rectified using a diode bridge) into a pulsating voltage of positive polarity with the voltage crossing zero, as in Fig 2.

♦ The method of controlling a thyristor is to ensure that at the moment of time t(during the half-cycle Uс) through the transition Ue – K, switching current has passed Ion thyristor.

From this moment, the main cathode-anode current flows through the thyristor, until the next half-cycle transition through zero, when the thyristor closes.
Turn-on current Ion A thyristor can be obtained in different ways.
1. Due to the current flowing through: +U – R1 – R2 – Ue – K – -U (on the diagram, Fig. 3) .
2. From a separate unit for generating control pulses and feeding them between the control electrode and the cathode.

♦ In the first case, the control electrode current flows through the junction Ue – K, gradually increases (increasing along with the voltage Uс), until it reaches the value Ion. The thyristor will open.

phase method.

♦ In the second case, a short pulse generated in a special device is applied to the transition at the right time Ue – K, from which the thyristor opens.

This method of controlling a thyristor is called pulse-phase method .
In both cases, the current controlling the thyristor turn-on must be synchronized with the start of the transition mains voltage Uс through zero.
The action of the control electrode is reduced to controlling the moment the thyristor is turned on.

Phase method of thyristor control.

♦ Let's try it on simple example thyristor lighting controller (diagram on Fig.3) analyze the features of the operation of a thyristor in an alternating current circuit.

After the rectifier bridge, the voltage is a pulsating voltage, changing in the form:
0→ (+Umax) → 0 → (+Umax) → 0, as in Fig. 2

♦ The beginning of thyristor control comes down to the following.
When the network voltage increases Uс, from the moment the voltage crosses zero, a control current appears in the control electrode circuit Iup along the chain:
+U – R1 – R2 – Ue – K – -U.
With increasing tension Uс The control current also increases Iup(control electrode - cathode).

When the control electrode current reaches the value Ion, the thyristor turns on (opens) and closes the points +U and –U on the diagram.

The voltage drop across an open thyristor (anode - cathode) is 1,5 – 2,0 Volt. The control electrode current will drop almost to zero, and the thyristor will remain in a conducting state until the voltage Uс the network will not drop to zero.
With the action of a new half-cycle of the network voltage, everything will repeat from the beginning.

♦ Only the load current flows in the circuit, that is, the current through the lamp L1 along the circuit:
Uс – fuse – diode bridge– anode – thyristor cathode – diode bridge – light bulb L1 – Uс.
There will be a light bulb light up with each half-cycle of the mains voltage and go out when the voltage passes through zero.

Let's do some calculations as an example. Fig.3. We use the element data as in the diagram.
According to the thyristor reference book KU202N switching current Ion = 100 mA. In reality, it is much less and amounts to 10 – 20 mA, depending on the instance.
Let's take for example Ion = 10 mA .
Controlling the moment of switching on (brightness adjustment) occurs by changing the value of the variable resistance of the resistor R1. For different resistor values R1, there will be different breakdown voltages of the thyristor. In this case, the moment of switching on the thyristor will vary within the limits:

1. R1 = 0, R2 = 2.0 Com. Uon = Ion x (R1 + R2) = 10 x (0 + 2 = 20 volts.
2. R1 = 14.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (13 + 2) = 150 volts.
3. R1 = 19.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (18 + 2) = 200 volts.
4. R1 = 29.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (28 + 2) = 300 volts.
5. R1 = 30.0 Kom, R2 = 2.0 Kom. Uon = Ion x (R1 + R2) = 10 x (308 + 2) = 310 volts.

Phase angle α varies from a = 10, up to a = 90 degrees.
An approximate result of these calculations is given in rice. 4.

♦ The shaded part of the sine wave corresponds to the power released at the load.
Power control by phase method, possible only in a narrow range of control angle from a = 10°, to a = 90°.
That is, within from 90% to 50% power allocated to the load.

Start of regulation from phase angle a = 10 degrees is explained by the fact that at the moment of time t=0 – t=1, the current in the control electrode circuit has not yet reached the value Ion(Uc has not reached 20 volts).

All these conditions are fulfilled if there is no capacitor in the circuit WITH.
If you install a capacitor WITH(in the diagram in Fig. 2), the voltage regulation range (phase angle) will shift to the right as in Fig.5.

This is explained by the fact that at first (t=0 – t=1), all the current goes to charge the capacitor WITH, the voltage between Ue and K of the thyristor is zero and it cannot turn on.

As soon as the capacitor is charged, the current flows through the control electrode - the cathode, and the thyristor turns on.

The adjustment angle depends on the capacitance of the capacitor and moves approximately from a = 30 to a = 120 degrees (with capacitor capacity 50 µF).
The load power will vary approximately from 80% to 30%.

Of course, all the calculations given are very approximate, but the general reasoning is correct.

All of the above voltage diagrams, at different time values, were clearly visible on the oscilloscope screen.

If you have an oscilloscope, you can see for yourself

- a device with the properties of a semiconductor, the design of which is based on a single-crystal semiconductor having three or more p-n junctions.

Its operation implies the presence of two stable phases:

• “closed” (conductivity level is low);
• “open” (conductivity level is high).

Thyristors are devices that perform the functions of power electronic keys. Another name for them is single-operation thyristors. This device allows you to regulate the impact of powerful loads through minor impulses.

According to the current-voltage characteristic of the thyristor, an increase in the current in it will provoke a decrease in voltage, that is, a negative differential resistance will appear.

In addition, these semiconductor devices can connect circuits with voltages up to 5000 Volts and currents up to 5000 Amps (at a frequency of no more than 1000 Hz).

Thyristors with two and three terminals are suitable for operation with both direct and alternating current. Most often, the principle of their operation is compared with the operation of a rectifying diode and it is believed that they are a full-fledged analogue of a rectifier, in some sense even more effective.

The types of thyristors differ from each other:

• Control method.
• Conductivity (unilateral or bilateral).

General management principles

The thyristor structure has 4 semiconductor layers in a series connection (p-n-p-n). The contact connected to the outer p-layer is the anode, and the contact connected to the outer n-layer is the cathode. As a result, with a standard assembly, a thyristor can have a maximum of two control electrodes, which are attached to the internal layers. According to the connected layer, the conductors are divided into cathode and anode based on the type of control. The first type is most often used.

The current in thyristors flows towards the cathode (from the anode), so the connection to the current source is made between the anode and the positive terminal, as well as between the cathode and the negative terminal.

Thyristors with a control electrode can be:

• Lockable;
• Unlockable.

An indicative property of non-locking devices is their lack of response to a signal from the control electrode. The only way to close them is to reduce the level of current flowing through them so that it is inferior to the holding current.

When controlling a thyristor, some points should be taken into account. A device of this type changes operating phases from “off” to “on” and back in jumps and only under the condition of external influence: using current (voltage manipulation) or photons (in cases with a photothyristor).

To understand this point, you need to remember that a thyristor mainly has 3 outputs (thyristor): anode, cathode and control electrode.

The UE (control electrode) is precisely responsible for turning the thyristor on and off. The opening of the thyristor occurs under the condition that the applied voltage between A (anode) and K (cathode) becomes equal to or exceeds the operating voltage of the thyristor. True, in the second case, exposure to a pulse of positive polarity between Ue and K will be required.

With a constant supply of supply voltage, the thyristor can be open indefinitely.

To switch it to a closed state, you can:

• Reduce the voltage level between A and K to zero;
• Reduce the A-current value so that the holding current strength is greater;
• If the operation of the circuit is based on the action of alternating current, the device will turn off without outside intervention when the current level itself drops to zero reading;
• Apply a blocking voltage to the UE (relevant only for lockable types of semiconductor devices).

The closed state also lasts indefinitely until a triggering impulse occurs.

Specific control methods

• Amplitude .

It represents the supply of a positive voltage of varying magnitude to the Ue. The opening of the thyristor occurs when the voltage value is sufficient to break through the control transition of the rectifying current (Irect). By changing the voltage on the UE, it becomes possible to change the opening time of the thyristor.

The main disadvantage of this method is the strong influence of the temperature factor. In addition, each type of thyristor will require a different type of resistor. This point does not add ease of use. In addition, the opening time of the thyristor can be adjusted only while the first 1/2 of the positive half-cycle of the network lasts.

• Phase.

It consists of changing the phase Ucontrol (in relation to the voltage at the anode). In this case, a phase shift bridge is used. The main disadvantage is the low slope of Ucontrol, so it is possible to stabilize the opening moment of the thyristor only for a short time.

• Pulse-phase .

Designed to overcome the shortcomings of the phase method. For this purpose, a voltage pulse with a steep edge is applied to Ue. This approach is currently the most common.

Thyristors and safety

Due to the impulse nature of their action and the presence of reverse recovery current, thyristors greatly increase the risk of overvoltage in the operation of the device. In addition, the danger of overvoltage in the semiconductor zone is high if there is no voltage at all in other parts of the circuit.

Therefore, in order to avoid negative consequences, it is customary to use CFTP schemes. They prevent the appearance and retention of critical voltage values.

Two-transistor thyristor model

From two transistors it is quite possible to assemble a dinistor (thyristor with two terminals) or a trinistor (thyristor with three terminals). To do this, one of them must have p-n-p conductivity, the other - n-p-n conductivity. Transistors can be made from either silicon or germanium.

The connection between them is carried out through two channels:

• Anode from the 2nd transistor + Control electrode from the 1st transistor;
• Cathode from the 1st transistor + Control electrode from the 2nd transistor.

If you do without the use of control electrodes, then the output will be a dinistor.

The compatibility of the selected transistors is determined by the same amount of power. In this case, the current and voltage readings must necessarily be greater than those required for the normal functioning of the device. Data on breakdown voltage and holding current depend on the specific qualities of the transistors used.

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