Absolutely any thyristor can be in two stable states - closed or open

In a closed state, it is in a state of low conductivity and almost no current flows; in an open state, on the contrary, the semiconductor will be in a state of high conductivity, current passes through it with virtually no resistance

We can say that a thyristor is an electrical power controlled switch. But in essence, the control signal can only open the semiconductor. To lock it back, it is necessary to fulfill conditions aimed at reducing the forward current to almost zero.

Structurally, the thyristor is a sequence of four layers p And n type forming the structure p-n-p-n and connected in series.

One of the extreme areas to which the positive pole of the power is connected is called anode, p – type
The other, to which the negative voltage pole is connected, is called cathode, – n type
Control electrode connected to the inner layers.

In order to understand the operation of a thyristor, let’s consider several cases, the first: no voltage is supplied to the control electrode, the thyristor is connected according to the dinistor circuit - positive voltage is supplied to the anode, and negative voltage to the cathode, see figure.

In this case, the collector p-n junction of the thyristor is in the closed state, and the emitter junction is open. Open junctions have very low resistance, so almost all the voltage coming from the power supply is applied to the collector junction, due to the high resistance of which the current flowing through the semiconductor device is very low.

On the current-voltage characteristic graph, this state is relevant for the section marked with the number 1 .

As the voltage level increases, up to a certain point the thyristor current almost does not increase. But reaching a conditional critical level - switch-on voltage U on, factors appear in the dinistor during which a sharp increase in free charge carriers begins in the collector junction, which almost immediately carries avalanche nature. As a result, a reversible electrical breakdown occurs (point 2 in the figure shown). IN p-in the region of the collector transition, an excess zone of accumulated positive charges appears, in n-region, on the contrary, accumulation of electrons occurs. An increase in the concentration of free charge carriers leads to a drop in the potential barrier at all three junctions, and injection of charge carriers begins through the emitter junctions. The avalanche-like nature increases even more, and leads to switching of the collector junction to the open state. At the same time, the current increases in all areas of the semiconductor, resulting in a voltage drop between the cathode and anode, shown in the graph above by the segment marked with the number three. At this point in time, the dinistor has a negative differential resistance. On resistance Rn The voltage increases and the semiconductor switches.

After opening the collector junction, the I-V characteristic of the dinistor becomes the same as on the direct branch - segment No. 4. After switching the semiconductor device, the voltage drops to one volt. In the future, an increase in the voltage level or a decrease in resistance will lead to an increase in the output current, one to one, as well as the operation of the diode when it is connected directly. If the supply voltage level is reduced, then the high resistance of the collector junction is restored almost instantly, the dinistor closes, the current drops sharply.

Turn-on voltage U on, can be tuned by introducing minor charge carriers into any of the intermediate layers, next to the collector junction.

For this purpose, a special control electrode, powered from an additional source from which the control voltage follows - U control. As can be clearly seen from the graph, as U control increases, the switching voltage decreases.

Basic characteristics of thyristors

U on turn-on voltage - at which the thyristor transitions to the open state
U o6p.max– pulsed, repeating reverse voltage causes electrical breakdown p-n junction. For many thyristors the expression will be true U o6p.max . = U on
Imax- maximum permissible current value
I Wed- average current value for a period U np- forward voltage drop when the thyristor is open
I o6p.max- reverse maximum current starting to flow when applied U o6p.max, due to the movement of minority charge carriers
I hold holding current - the value of the anode current at which the thyristor is turned off
Pmax- maximum power dissipation
t off- shutdown time required to turn off the thyristor

Lockable thyristors- has a classic four-layer p-n-p-n structure, but at the same time has a number of design features that provide such functionality as complete controllability. Thanks to this effect from the control electrode, turn-off thyristors can transition not only to the open state from closed, but also from open to closed. To do this, the control electrode is supplied with a voltage opposite to that which previously opens the thyristor. To lock the thyristor, a powerful but short-duration negative current pulse follows on the control electrode. When using turn-off thyristors, it should be remembered that their limit values ​​are 30% lower than those of conventional ones. In circuit design, lockable thyristors are actively used as electronic switches in converter and pulse technology.

Unlike their four-layer relatives - thyristors, they have a five-layer structure.

Thanks to this semiconductor structure, they are able to pass current in both directions - both from the cathode to the anode, and from the anode to the cathode, and the control electrode receives voltage of both polarities. Thanks to this property, the current-voltage characteristic of the triac has a symmetrical appearance in both coordinate axes. You can learn about the operation of a triac from the video tutorial at the link below.

The principle of operation of a triac

If a standard thyristor has an anode and a cathode, then the electrodes of the triac cannot be described in this way because each electrode is both an anode and a cathode at the same time. Therefore, the triac is capable of passing current in both directions. This is why it works great in AC circuits.

A very simple diagram explaining the principle of a triac is a triac power regulator.

After voltage is applied, alternating voltage is supplied to one of the triac outputs. A negative control voltage is supplied to the electrode, which is the control electrode, from the diode bridge. When the switching threshold is exceeded, the triac is unlocked and current flows into the connected load. At the moment when the voltage polarity changes at the input of the triac, it is locked. Then the algorithm is repeated.

The higher the control voltage level, the faster the triac operates and the pulse duration at the load increases. As the control voltage level decreases, the duration of the pulses on the load also decreases. At the exit triac regulator the voltage will be sawtooth with adjustable pulse duration. Thus, by adjusting the control voltage we can change the brightness of an incandescent light bulb or the temperature of a soldering iron tip connected as a load.

So the triac is controlled by both negative and positive voltage. Let's highlight its pros and cons.

Pros: low cost, long service life, no contacts and, as a result, no sparking or rattling.
Cons: quite sensitive to overheating and is usually mounted on a radiator. It does not work at high frequencies, since it does not have time to transition from an open state to a closed state. Reacts to external interference that causes false alarms.

It is also worth mentioning the features of installing triacs in modern electronic equipment.

At low loads or if short periods of flow occur in it. impulse currents, installation of triacs can be carried out without a heat sink. In all other cases, its presence is strictly required.
The thyristor can be fixed to the heat sink with a mounting clip or screw
To reduce the likelihood of false alarms due to noise, the length of the wires should be kept to a minimum. It is recommended to use a shielded cable or twisted pair for connection.

Or optothyristors are specialized semiconductors, design feature which is the presence of a photocell, which is the control electrode.

A modern and promising type of triac is the optosimistor. Instead of a control electrode, there is an LED in the housing and control occurs by changing the supply voltage to the LED. When the light flux hits the back power, the photocell switches the thyristor to the open position. The most basic function of an optosimistor is that there is complete galvanic isolation between the control circuit and the power circuit. This creates simply an excellent level of design reliability.

Power keys. One of the main points influencing the demand for such circuits is the low power that a thyristor can dissipate in switching circuits. In the locked state, virtually no power is consumed, since the current is close to zero. And in the open state, power dissipation is low due to low voltage values

Threshold devices– they implement the main property of thyristors - to open when the voltage reaches the desired level. This is used in phase power regulators and relaxation oscillators

For interruption and on-off blocking thyristors are used. True, in this case the schemes need some modification.

Experimental devices– they use the property of a thyristor to have negative resistance while in a transient mode

The principle of operation and properties of a dinistor, circuits based on dinistors

A dinistor is a type of semiconductor diodes belonging to the class of thyristors. The dinistor consists of four regions of different conductivity and has three p-n junctions. In electronics it has found rather limited use, although it can be found in designs energy saving lamps for base E14 and E27, where it is used in starting circuits. In addition, it is found in the ballasts of fluorescent lamps.

Thyristors are power electronic switches that are not fully controlled. Often in technical books you can see another name for this device - single-operation thyristor. In other words, under the influence of a control signal it is transferred to one state - conducting. To be more specific, it turns on the circuit. To turn it off, you need to create special conditions, which ensure that the forward current in the circuit drops to zero.

Features of thyristors

Thyristor switches conduct electricity only in the forward direction, and in the closed state it can withstand not only forward, but also reverse voltage. The thyristor structure is four-layer, there are three outputs:

1. Anode (denoted by the letter A).
2. Cathode (letter C or K).
3. Control electrode (U or G).

Thyristors have a whole family of current-voltage characteristics, from which one can judge the state of the element. Thyristors are very powerful electronic switches; they are capable of switching circuits in which the voltage can reach 5000 volts and the current can reach 5000 amperes (while the frequency does not exceed 1000 Hz).

Thyristor operation in DC circuits

A conventional thyristor is turned on by applying a current pulse to the control terminal. Moreover, it must be positive (relative to the cathode). The duration of the transient process depends on the nature of the load (inductive, active), the amplitude and rate of increase in the current pulse control circuit, the temperature of the semiconductor crystal, as well as the applied current and voltage to the thyristors present in the circuit. The characteristics of the circuit directly depend on the type of semiconductor element used.

In the circuit in which the thyristor is located, a high rate of voltage rise is unacceptable. Namely, the value at which the element spontaneously turns on (even if there is no signal in the control circuit). But at the same time, the control signal must have a very high slope.

Shutdown methods

Two types of thyristor switching can be distinguished:

1. Natural.
2. Forced.

And now in more detail about each type. Natural occurs when the thyristor operates in an alternating current circuit. Moreover, this switching occurs when the current drops to zero. But forced switching can be accomplished in a large number of different ways. Which thyristor control to choose is up to the circuit designer, but it’s worth talking about each type separately.

The most typical method of forced switching is to connect a capacitor that was previously charged using a button (key). The LC circuit is included in the thyristor control circuit. This chain contains a fully charged capacitor. During the transient process, current fluctuations occur in the load circuit.

Forced switching methods

There are several other types of forced switching. A circuit that uses a switching capacitor with reverse polarity is often used. For example, this capacitor can be connected to the circuit using some kind of auxiliary thyristor. In this case, a discharge will occur to the main (working) thyristor. This will lead to the fact that the capacitor has a current directed towards the forward current of the main thyristor, which will help reduce the current in the circuit down to zero. Consequently, the thyristor will turn off. This happens because the thyristor design has its own characteristics that are unique to it.

There are also circuits in which LC circuits are connected. They discharge (and with fluctuations). At the very beginning, the discharge current flows towards the worker, and after their values ​​are equalized, the thyristor is turned off. The current then flows from the oscillatory circuit through the thyristor into the semiconductor diode. In this case, as long as the current flows, some voltage is applied to the thyristor. It is equal in magnitude to the voltage drop across the diode.

Thyristor operation in alternating current circuits

If the thyristor is connected to an alternating current circuit, the following operations can be performed:

1. Turn on or off an electrical circuit with an active-resistive or active load.
2. Change the average and effective value of the current that passes through the load, thanks to the ability to regulate the moment at which the control signal is applied.

Thyristor switches have one feature - they conduct current only in one direction. Therefore, if it is necessary to use them in circuits, it is necessary to use back-to-back connections. The effective and average current values ​​may vary due to the fact that the moment at which the signal is applied to the thyristors is different. In this case, the power of the thyristor must meet the minimum requirements.

Phase control method

With the phase control method with forced-type switching, the load is adjusted by changing the angles between the phases. Artificial switching can be done using special circuits, or it is necessary to use fully controlled (lockable) thyristors. Based on them, as a rule, they are made which allows adjustment depending on the level of charge of the battery.

Pulse width control

It is also called PWM modulation. When the thyristors open, a control signal is sent. The junctions are open and there is some voltage across the load. During closing (during the entire transient process), no control signal is supplied, therefore, the thyristors do not conduct current. When implementing phase control, the current curve is not sinusoidal; the shape of the supply voltage signal changes. Consequently, there is also a disruption in the work of consumers who are sensitive to high-frequency interference (incompatibility appears). The thyristor-based regulator has a simple design, which will allow you to change the required value without any problems. And there is no need to use massive LATRs.

Lockable thyristors

Thyristors are very powerful electronic switches used for switching high voltages and currents. But they have one huge drawback - management is incomplete. More specifically, this is manifested by the fact that in order to turn off the thyristor, it is necessary to create conditions under which the forward current will decrease to zero.

It is this feature that imposes some restrictions on the use of thyristors, and also complicates circuits based on them. To get rid of this kind of shortcomings, special designs of thyristors have been developed that are locked by a signal along one control electrode. They are called two-operation, or lockable, thyristors.

Switch-off thyristor design

Four-layer p-p-p-p structure Thyristors have their own characteristics. They make them different from conventional thyristors. We are now talking about complete controllability of the element. Volt-ampere characteristics(static) in the forward direction is the same as for simple thyristors. But the thyristor can pass a much larger direct current. But the function of blocking large reverse voltages is not provided for turn-off thyristors. Therefore, it is necessary to connect it counter-parallel with

A characteristic feature of a turn-off thyristor is a significant drop in forward voltages. To switch off, a powerful current pulse (negative, in a ratio of 1:5 to the direct current value) must be supplied to the control terminal. But only the pulse duration should be as short as possible - 10... 100 μs. Turn-off thyristors have a lower maximum voltage and current value than conventional ones. The difference is approximately 25-30%.

Types of thyristors

Lockable ones were discussed above, but there are many more types of semiconductor thyristors that are also worth mentioning. In a wide variety of designs ( charging device, switches, power regulators) certain types of thyristors are used. Somewhere it is required that control be carried out by supplying a stream of light, which means an optothyristor is used. Its peculiarity is that the control circuit uses a semiconductor crystal that is sensitive to light. The parameters of thyristors are different, they all have their own characteristics that are characteristic only of them. Therefore, you need to at least have a general idea of ​​what types of these semiconductors exist and where they can be used. So, here is the entire list and the main features of each type:

1. Thyristor diode. The equivalent of this element is a thyristor, to which a semiconductor diode is connected back-to-back.
2. Dinistor (diode thyristor). It can go into full conduction if a certain voltage level is exceeded.
3. Triac (symmetrical thyristor). Its equivalent is two thyristors connected back-to-back.
4. The high-speed inverter thyristor has a high switching speed (5... 50 μs).
5. Controlled thyristors You can often find designs based on MOS transistors.
6. Optical thyristors that are controlled by light flows.

Implementing element protection

Thyristors are devices that are critical to the rate of rise of forward current and forward voltage. They, like semiconductor diodes, are characterized by the flow of reverse recovery currents, which very quickly and sharply drops to zero, thereby exacerbating the likelihood of overvoltage. This overvoltage is a consequence of the fact that the current abruptly stops in all elements of the circuit that have inductance (even ultra-low inductances characteristic of installation - wires, board tracks). To implement protection, it is necessary to use a variety of circuits that allow protection from high voltages and currents in dynamic operating modes.

As a rule, the voltage source that is included in the circuit of a working thyristor has such a value that it is more than enough to avoid including some additional inductance in the circuit in the future. For this reason, in practice, a switching trajectory formation chain is more often used, which significantly reduces the speed and level of overvoltage in the circuit when the thyristor is turned off. Capacitive-resistive circuits are most often used for these purposes. They are connected in parallel with the thyristor. There are quite a few types of circuit modifications of such circuits, as well as methods for their calculations, parameters for the operation of thyristors in various modes and conditions. But the circuit for forming the switching trajectory of a turn-off thyristor will be the same as that of transistors.

Thyristors are widely used in semiconductor devices and converters. Various power sources, frequency converters, regulators, exciters for synchronous motors and many other devices were built on thyristors, and recently they have been replaced by transistor-based converters. The main task for a thyristor is to turn on the load at the moment the control signal is supplied. In this article we will look at how to control thyristors and triacs.

Definition

A thyristor (thyristor) is a semiconductor semi-controlled switch. Semi-controlled means that you can only turn on the thyristor; it turns off only when the current in the circuit is interrupted or if reverse voltage is applied to it.

It, like a diode, conducts current in only one direction. That is, to be included in an alternating current circuit to control two half-waves, you need two thyristors, one for each, although not always. A thyristor consists of 4 semiconductor regions (p-n-p-n).

Another similar device is called a bidirectional thyristor. Its main difference is that it can conduct current in both directions. In fact, it consists of two thyristors connected in parallel towards each other.

Main characteristics

Like any other electronic components, thyristors have a number of characteristics:

Voltage drop at maximum anode current (VT or Uoc).

Direct voltage in closed state (VD(RM) or Uзс).

Reverse voltage (VR(PM) or Urev).

Direct current (IT or Ipr) is the maximum current in the open state.

The maximum forward current capacity (ITSM) is the maximum peak on-state current.

Reverse current (IR) is the current at a certain reverse voltage.

Direct current in the closed state at a certain forward voltage (ID or Isc).

Constant unlocking control voltage (VGT or UУ).

Control current (IGT).

Maximum control current of the IGM electrode.

Maximum permissible power dissipation at the control electrode (PG or PU)

Principle of operation

When voltage is applied to the thyristor, it does not conduct current. There are two ways to turn it on - apply a voltage between the anode and cathode sufficient to open it, then its operation will be no different from a dinistor.

Another way is to apply a short pulse to the control electrode. The opening current of the thyristor lies in the range of 70-160 mA, although in practice this value, as well as the voltage that needs to be applied to the thyristor, depends on the specific model and instance of the semiconductor device and even on the conditions in which it operates, such as the ambient temperature environment.

In addition to the control current, there is such a parameter as the holding current - this is the minimum anode current to keep the thyristor in the open state.

After opening the thyristor, the control signal can be turned off; the thyristor will be open as long as direct current flows through it and voltage is applied. That is, in an alternating circuit the thyristor will be open during that half-wave the voltage of which biases the thyristor in the forward direction. When the voltage goes to zero, the current will also decrease. When the current in the circuit drops below the holding current of the thyristor, it will close (turn off).

The polarity of the control voltage must match the polarity of the voltage between the anode and cathode, which you observe in the oscillograms above.

The control of a triac is similar, although it has some features. To control a triac in an alternating current circuit, two control voltage pulses are needed - for each half-wave of a sine wave, respectively.

After applying a control pulse in the first half-wave (conditionally positive) of the sinusoidal voltage, the current through the triac will flow until the beginning of the second half-wave, after which it will close, like a regular thyristor. After this, you need to apply another control pulse to open the triac on the negative half-wave. This is clearly illustrated in the following waveforms.

The polarity of the control voltage must match the polarity of the applied voltage between the anode and cathode. Because of this, problems arise when controlling triacs using digital logic circuits or from microcontroller outputs. But this can easily be solved by installing a triac driver, which we will talk about later.

Common thyristor or triac control circuits

The most common circuit is a triac or thyristor regulator.

Here the thyristor opens after there is enough value on the capacitor to open it. The opening moment is adjusted using a potentiometer or a variable resistor. The greater its resistance, the slower the capacitor charges. Resistor R2 limits the current through the control electrode.

This circuit regulates both half cycles, meaning you get full power control from almost 0% to almost 100%. This was achieved by installing a regulator, thus regulating one of the half-waves.

A simplified circuit is shown below, here only half of the period is regulated, the second half-wave passes without change through the diode VD1. The operating principle is similar.

A triac regulator without a diode bridge allows you to control two half-waves.

According to the principle of operation, it is almost similar to the previous ones, but it is built on a triac with its help, both half-waves are regulated. The differences are that here the control pulse is supplied using a bidirectional DB3 dinistor after the capacitor is charged to the desired voltage, usually 28-36 Volts. Charging speed is also adjustable variable resistor or potentiometer. This scheme is implemented in most.

Interesting:

Such voltage regulation circuits are called SIFU - pulsed phase control system.

The figure above shows an option for controlling a triac using a microcontroller, using the example. The triac driver consists of an optosimistor and an LED. Since an optosimistor is installed in the driver output circuit, a voltage of the required polarity is always supplied to the control electrode, but there are some nuances here.

The fact is that to regulate the voltage using a triac or thyristor, you need to apply a control signal at a certain point in time, so that the phase cut occurs to the desired value. If you shoot control pulses at random, the circuit will of course work, but adjustments will not be achieved, so you need to determine the moment when the half-wave crosses zero.

Since the polarity of the half-wave at the current moment in time does not matter to us, it is enough to simply track the moment of transition through zero. Such a node in the circuit is called a zero detector or null detector, and in English-language sources “zero crossing detector circuit” or ZCD. A version of such a circuit with a zero-crossing detector using a transistor optocoupler looks like this:

There are many optodrivers for controlling triacs, the typical ones are the MOC304x, MOC305x, MOC306X line, produced by Motorola and others. Moreover, these drivers provide galvanic isolation, which will protect your microcontroller in the event of a breakdown of the semiconductor key, which is quite possible and probable. This will also increase the safety of working with control circuits by completely dividing the circuit into “power” and “operational”.

Conclusion

We talked about basic information about thyristors and triacs, as well as their control in circuits with “changes”. It is worth noting that we did not touch upon the topic of turn-off thyristors; if you are interested in this issue, write comments and we will consider them in more detail. Also, the nuances of using and controlling thyristors in power inductive circuits were not considered. To control the "constant" it is better to use transistors, since in this case you decide when the key opens and when it closes, obeying the control signal...

To understand how the circuit works, you need to know the action and purpose of each of the elements. In this article we will look at the operating principle of a thyristor, different types and operating modes, characteristics and types. We will try to explain everything as clearly as possible, so that it is clear even for beginners.

A thyristor is a semiconductor element that has only two states: “open” (current flows) and “closed” (no current). Moreover, both states are stable, that is, the transition occurs only under certain conditions. The switching itself occurs very quickly, although not instantly.

In terms of its mode of action, it can be compared to a switch or a key. But the thyristor switches using voltage, and turns off when the current is lost or the load is removed. So the operating principle of a thyristor is not difficult to understand. You can think of it as an electrically controlled key. Well, not really.

A thyristor usually has three outputs. One control and two through which current flows. You can try to briefly describe the principle of operation. When voltage is applied to the control output, the circuit is switched through the anode-collector. That is, it is comparable to a transistor. The only difference is that in a transistor, the amount of current passed depends on the voltage applied to the control terminal. And the thyristor is either completely open or completely closed.

Appearance

The appearance of the thyristor depends on the date of its production. The elements from the times of the Soviet Union are metal, in the form of a “flying saucer” with three terminals. Two terminals - the cathode and the control electrode - are located on the “bottom” or “cover” (whichever side you look at). Moreover, the control electrode is smaller in size. The anode may be located on the opposite side of the cathode, or stick out sideways from under the washer that is on the body.

Two types of thyristors - modern and Soviet, designation on diagrams

Modern thyristors look different. This is a small plastic rectangle with metal plate on top and three pins on the bottom. In the modern version there is one inconvenience: you need to look in the description which of the terminals is the anode, where is the cathode and the control electrode. Typically, the first is the anode, then the cathode and the one on the far right is the electrode. But this is usually the case, that is, not always.

Principle of operation

According to the principle of operation, a thyristor can also be compared to a diode. It will pass current in one direction - from the anode to the cathode, but this will only happen in the “open” state. In the diagrams, a thyristor looks like a diode. There is also an anode and a cathode, but there is also an additional element - a control electrode. Of course, there are differences in the output voltage (when compared with a diode).

In alternating voltage circuits, the thyristor will pass only one half-wave - the upper one. When the lower half-wave arrives, it resets to the “closed” state.

The principle of operation of a thyristor in simple words

Let's consider the principle of operation of a thyristor. The starting state of the element is closed. The “signal” to transition to the “open” state is the appearance of voltage between the anode and the control terminal. There are two ways to return the thyristor to the “closed” state:

• remove the load;
• reduce the current below the holding current (one of the technical characteristics).

In circuits with variable voltage, as a rule, the thyristor is reset according to the second option. Alternating current V household network has a sinusoidal shape when its value approaches zero and resets. In circuits powered by DC sources, it is necessary to either forcibly remove the power or remove the load.

That is, the thyristor works differently in circuits with constant and alternating voltage. In a constant voltage circuit, after a short-term voltage appears between the anode and the control terminal, the element goes into the “open” state. Then there can be two scenarios:

• The “open” state is maintained even after the anode-control output voltage has disappeared. This is possible if the voltage applied to the anode control terminal is higher than the non-unlocking voltage (this data is in the technical specifications). The flow of current through the thyristor is stopped, in fact only by breaking the circuit or turning off the power source. Moreover, the shutdown/break of the circuit can be very short-lived. After the circuit is restored, no current flows until voltage is applied to the anode control terminal again.
• After removing the voltage (it is less than the unlocking voltage), the thyristor immediately goes into the “closed” state.

So in DC circuits there are two options for using a thyristor - with and without holding the open state. But more often they use the first type - when it remains open.

The operating principle of a thyristor in alternating voltage circuits is different. There, the return to the locked state occurs “automatically” - when the current drops below the holding threshold. If the voltage is constantly applied to the anode-cathode, at the output of the thyristor we obtain current pulses that occur at a certain frequency. This is exactly how they are built impulse blocks nutrition. Using a thyristor, they convert the sine wave into pulses.

Functionality check

You can check the thyristor either using a multimeter or by creating a simple test circuit. If you have before your eyes when calling specifications, you can also check the resistance of the transitions.

Testing with a multimeter

First, let's analyze the continuity test with a multimeter. We switch the device to dialing mode.

Please note that the resistance value varies from series to series - you should not pay special attention to this. If you want to check the resistance of the transitions, look at the technical specifications.

The figure shows the test diagrams. The figure on the far right is an improved version with a button that is installed between the cathode and the control terminal. In order for the multimeter to record the current flowing through the circuit, briefly press the button.

Using a light bulb and a DC source (a battery will also work)

If you don’t have a multimeter, you can test the thyristor using a light bulb and a power source. Even a regular battery or any other constant voltage source will do. But the voltage must be sufficient to light the light bulb. You will also need resistance or a regular piece of wire. A simple circuit is assembled from these elements:

• We supply plus from the power source to the anode.
• We connect a light bulb to the cathode, and connect its second terminal to the negative of the power source. The light does not light because the thermistor is locked.
• Briefly (using a piece of wire or resistance) connect the anode and the control terminal.
• The light comes on and continues to light even though the jumper is removed. The thermistor remains open.
• If you unscrew the light bulb or turn off the power source, the light bulb will naturally go out.
• If the circuit/power is restored, it will not light up.

Along with the test, this circuit allows you to understand the principle of operation of the thyristor. After all, the picture turns out to be very clear and understandable.

Types of thyristors and their special properties

Semiconductor technologies are still being developed and improved. Over several decades, new types of thyristors have appeared, which have some differences.

• Dinistors or diode thyristors. They differ in that they have only two outputs. Opened by feeding to the anode and cathode high voltage in the form of an impulse. They are also called “uncontrolled thyristors”.
• SCRs or triode thyristors. They have a control electrode, but the control pulse can be supplied:
  • To the control output and cathode. Name - with cathode control.
  • To the control electrode and anode. Accordingly, control of the anode.

There are also different types of thyristors according to the locking method. In one case, it is sufficient to reduce the anode current below the holding current level. In another case, a blocking voltage is applied to the control electrode.

By conductivity

We said that thyristors conduct current only in one direction. There is no reverse conduction. Such elements are called reverse-non-conducting, but there are not only such elements. There are other options:

• They have a low reverse voltage and are called reverse-conducting.
• With non-standardized reverse conductivity. They are installed in circuits where reverse voltage cannot occur.
• Triacs. Symmetrical thyristors. Conduct current in both directions.

Thyristors can operate in switch mode. That is, when a control pulse arrives, supply current to the load. The load, in this case, is calculated based on the voltage in open form. The maximum power dissipation must also be taken into account. In this case, it is better to choose metal models in the form of a “flying saucer”. It is convenient to attach a radiator to them for faster cooling.

Classification by special operating modes

The following subtypes of thyristors can also be distinguished:

• Lockable and non-lockable. The operating principle of an unlockable thyristor is slightly different. It is in the open state when the plus is applied to the anode, the minus is on the cathode. Goes into the closed state when the polarity changes.
• Fast-acting. They have a short transition time from one state to another.
• Pulse. It transitions very quickly from one state to another, and is used in circuits with pulsed operating modes.

The main purpose is to turn on and off a powerful load using low-power control signals

The main area of ​​use of thyristors is as an electronic switch, used to close and open an electrical circuit. In general, many common devices are built on thyristors. For example, a garland with running lights, straighteners, pulsed sources current, rectifiers and many others.

Characteristics and their meaning

Some thyristors can switch very high currents, in which case they are called power thyristors. They are made in a metal case for better heat dissipation. Small models with a plastic body are usually low-power options that are used in low-current circuits. But, there are always exceptions. So for each specific purpose, the required option is selected. They select, of course, according to parameters. Here are the main ones:

There is also a dynamic parameter - the time of transition from a closed to an open state. In some schemes this is important. The type of speed may also be indicated: by unlocking time or by locking time.

Content:

The discovery of the properties of semiconductor transitions can rightly be called one of the most important in the twentieth century. As a result, the first semiconductor devices appeared - diodes and transistors. As well as the schemes in which they are used. One such circuit is the connection of two bipolar transistors of opposite types - p-n-p c n-p-n. This circuit is shown below in image (b). It illustrates what a thyristor is and the principle of its operation. It contains positive feedback. As a result, each transistor increases the amplification properties of the other transistor.

Transistor equivalent

In this case, any change in the conductivity of transistors in any direction increases like an avalanche and ends in one of the boundary states. They are either locked or unlocked. This effect is called triggering. And as microelectronics developed, both transistors were combined in 1958 on the same substrate, generalizing the transitions of the same name. The result was a new semiconductor device called a thyristor. The principle of operation of a thyristor is based on the interaction of two transistors. As a result of combining transitions, it has the same number of pins as transistor (a).

In the diagram, the control electrode is the base of the transistor structure n-p-n. It is the base current of the transistor that changes the conductivity between its collector and emitter. But control can also be performed on the basis p-n-p transistor. This is the device of a thyristor. The choice of control electrode is determined by its features, including the tasks performed. For example, some of them do not use any control signals at all. Therefore, why use control electrodes...

Dinistor

These are tasks where two-electrode varieties of thyristors - dinistors - are used. They contain resistors connected to the emitter and base of each transistor. Further on the diagram these are R1 and R3. For each electronic device there are restrictions on the amount of applied voltage. Therefore, up to a certain value, the mentioned resistors keep each of the transistors in a locked state. But with a further increase in voltage, leakage currents appear through the collector-emitter junctions.

They are picked up by positive feedback, and both transistors, that is, the dinistor, are unlocked. For those who want to experiment, an image with a diagram and component values ​​is shown below. You can assemble it and check its working properties. Let's pay attention to resistor R2, which differs in the selection of the desired value. It complements the leakage effect and therefore the trigger voltage. Consequently, a dinistor is a thyristor, the operating principle of which is determined by the magnitude of the supply voltage. If it is relatively large, it will turn on. Naturally, it is also interesting to know how to turn it off.

Difficulty turning off

Turning off the thyristors was, as they say, difficult. For this reason, for quite a long time, the types of thyristors were limited to only the two structures mentioned above. Until the mid-nineties of the twentieth century, only these two types of thyristors were used. The fact is that turning off the thyristor can only happen when one of the transistors is turned off. And for a certain time. It is determined by the rate of disappearance of charges corresponding to the gated transition. The most reliable way to “nail” these charges is to completely turn off the current flowing through the thyristor.

Most of them work this way. Not on DC, but on a rectified one, corresponding to the voltage without filtering. It changes from zero to the amplitude value, and then decreases to zero again. And so on, according to the frequency of the alternating voltage that is rectified. At a given moment between zero voltage values, a signal is sent to the control electrode and the thyristor is unlocked. And when the voltage passes through zero, it locks again.

To turn it off constant voltage and a current at which there is no zero value, a shunt is needed that operates for a certain time. In its simplest form, it is either a button connected to the anode and cathode, or connected in series. If the device is unlocked, there is residual voltage on it. By pressing the button it is reset to zero and the current through it stops. But if the button does not contain a special device and its contacts open, the thyristor will certainly turn on again.

This device should be a capacitor connected in parallel with the thyristor. It limits the rate of voltage rise across the device. This parameter is the most regrettable when using these semiconductor devices, since the operating frequency with which the thyristor is able to switch the load is reduced, and, accordingly, the switched power. This phenomenon occurs due to the internal capacitances characteristic of each of the models of these semiconductor devices.

The design of any semiconductor device inevitably forms a group of capacitors. The faster the voltage increases, the greater the currents that charge them. Moreover, they occur in all electrodes. If such a current in the control electrode exceeds a certain threshold value, the thyristor will turn on. Therefore, the parameter dU/dt is given for all models.

• Turning off the thyristor, as a result of the supply voltage passing through zero, is called natural. The remaining shutdown options are called forced or artificial.

Variety of models

These switching options add complexity to thyristor switches and reduce their reliability. But the development of the thyristor variety turned out to be very fruitful.

Nowadays, industrial production of a large number of varieties of thyristors has been mastered. Their scope of application is not only powerful power circuits (in which lockable and diode-thyristor, triac), but also control circuits (dinistor, optothyristor). The thyristor in the diagram is depicted as shown below.

Among them there are models whose operating voltages and currents are the highest among all semiconductor devices. Since industrial power supply is unthinkable without transformers, the role of thyristors in its further development is fundamental. Lockable high-frequency models in inverters ensure the generation of alternating voltage. Moreover, its value can reach 10 kV with a frequency of 10 kilohertz at a current strength of 10 kA. The dimensions of the transformers are reduced several times.

The switchable thyristor is turned on and off solely by influencing the control electrode with special signals. The polarity corresponds to the specific structure of this electronic device. This is one of the simplest varieties, referred to as GTO. In addition to it, more complex turn-off thyristors with built-in control structures are used. These models are called GCT and also IGCT. Use in these structures field effect transistors classifies turn-off thyristors as devices of the MCT family.

We tried to make our review informative not only for well-read visitors to our site, but also for dummies. Now that we are familiar with how a thyristor works, we can put this knowledge to practical use. For example, in simple repairs of household electrical appliances. The main thing is that while you get carried away with your work, do not forget about safety precautions!