Entertaining experiments: a family of thyristors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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You can often hear, and even read the word "thyristor" in popular radio engineering magazines. This is a semiconductor device. But such a device, unfortunately, does not exist, since thyristors are a class of devices. It includes a dinistor (diode thyristor), a trinistor (triode thyristor) and a triac (symmetrical trinistor). We will get to know them in the course of entertaining experiments. Let's start with dinistor.

Each semiconductor device from the class of thyristors is a "pie" of several layers that form a semiconductor structure of alternating pn junctions. The dinistor has three such transitions (Fig. 1), but the conclusions are made only from the extreme regions (p and n). The surface of the "pie" crystal with n-type electrical conductivity is usually soldered to the bottom of the case - this is the cathode of the dinistor, and the output from the opposite surface of the crystal is made through a glass insulator - this is the anode.

Externally, the dinistor (the KN102 series with the letter indices AI and its analogue with the designation 2H102 is common) is no different from the rectifier diodes of the D226 series. As in the case of a diode, a plus supply voltage is applied to the anode of the dinistor, and a minus to the cathode. And be sure to include a load in the dinistor circuit: a resistor, a lamp, a transformer winding, etc.

If you smoothly increase the voltage, the current through the dinistor will initially increase slightly (Fig. 2). The dinistor is practically closed. This state will continue until the voltage across the dinistor becomes equal to the turn-on voltage Uon. At this moment, an avalanche-like process of current growth begins in the four-layer structure and the dinistor goes into the open state. The voltage drop across it decreases sharply (this can be seen on the characteristic), and the current through the dinistor will now be determined by the load resistance, but it should not exceed the maximum allowable Iopen max. For all dinistors of the KN102 series, this current is 200 mA.

The voltage at which the dinistor opens is called the turn-on voltage (Uon), and the current corresponding to this value is the turn-on current (Ion). For each dinistor, the turn-on voltage is different, for example, for KN102A - 20 V, and for KN102I - 150 V. The same switching on for all dinistors of the series is 5 mA.

The dinistor can be in the open state until the direct current through it exceeds the minimum allowable current Iud, called the holding current.

The reverse branch of the characteristic of a dinistor is similar to the same branch of a conventional diode. The reverse voltage supply to the dinistor is higher than the permissible Uobr.max. can disable it. For all dinistors and Uobr.max. is 10 V, while the current Iobr.max. does not exceed 0,5 mA.

Now that you have become familiar with some of the parameters of the dinistor, you can assemble two generators and experiment with them.

Light flash generator (Fig. 3). It allows you to get light flashes of an incandescent lamp. When the plug X1 of the generator is inserted into the mains socket, the capacitor C1 will begin to charge (only in positive half-cycles). The charging current is limited by resistor R1. As soon as the voltage on it reaches the turn-on voltage of the dinistor, the capacitor will discharge through it and the EL1 lamp. Although the voltage on the capacitor is much higher (8 times!) The operating voltage of the lamp (2,5 V), it will not burn out, because the duration of the discharge current pulse is too short.

After the capacitor is discharged, the dinistor will close and the capacitor will start charging again. Soon a new flash will appear, followed by the next one, etc. With the details indicated in the diagram, flashes will follow every 0,5 s.

Replace the resistor with another, say, lower resistance. Flash frequency will increase. And with a larger resistor, it will decrease. A similar result will be obtained by reducing the capacitance of the capacitor or increasing it.

Returning to the original generator circuit, install an additional capacitor C2 (it can be paper or oxide) with a capacity of several microfarads for a voltage of at least 400 V. The flashes will disappear. The answer is simple. When this capacitor was not present, the resistor received Fig. 3 half-cycles of the mains voltage, i.e., it changed from zero to the maximum amplitude value. Therefore, after discharging the capacitor C1, the current through the dinistor at some point (when the sinusoid passes through zero) dropped to zero and the dinistor turned off. With the connection of capacitor C2, the voltage on the left output of the resistor according to the circuit already becomes pulsating, since the capacitor begins to act as a filter of a half-wave rectifier and the voltage across it does not drop to zero. And therefore, after opening the dinistor and the first flash of the lamp, a small current continues to flow through it, exceeding the holding current. The dinistor does not turn off, the generator does not work.

True, the generator can be made to work (and you can verify this) by increasing the resistance of the resistor, but then the flashes will follow too rarely. To increase the flash frequency, try reducing the capacitance of capacitor C1. The following will happen: the energy stored by the capacitor will not be enough to maintain sufficient brightness of the flashes.

The dinistor in this device can be, in addition to that indicated in the diagram, KN102B. Capacitor C 1 - oxide of any type for a rated voltage of at least 50 V, a diode - for a current of at least 50 mA and a reverse voltage of at least 400 V, a resistor - with a power of at least 2 W, a lamp - for an operating voltage of 2,5 V and current 0,26 A.

Audio frequency generator (Fig. 4). Its circuit is similar to the previous one, but the incandescent lamp is replaced by a higher resistance load - TON-2 (BF1) headphones, the capsules of which are removed from the headband (you may not remove it) and connected in series. The capacitance of the charge-discharge capacitor (C2) is significantly reduced, due to which the frequency of the generated signal has increased (up to 1000 Hz). The resistance of the limiting resistor (R2) in the dinistor circuit has also increased.

The remaining elements are a half-wave rectifier, in which capacitor C1 filters the rectified voltage, and resistor R1 helps to reduce the reverse voltage across the VD1 diode. If an alternating voltage of 45 ... 60 V is used to power the generator, resistor R1 is not needed.

Capacitor C1 can be paper, for example MBM, C2 - any type for a voltage of at least 50 V, diode - any with a permissible reverse voltage of at least 400V.

As soon as the X1 plug is inserted into the mains socket, a sound of a certain tone will appear in the headphones. Replace capacitor C2 with another, smaller capacitance - and the tone of the sound will increase. If you install a larger capacitor, the phones will hear the sound of a lower tone. The same results will be obtained by changing the resistance of the resistor R2 - check this. It should be noted that at present, microcircuits are produced that have characteristics close to dinistor ones, and in some cases they can replace them (see "Radio", 1998, No. 5, pp. 59-61).

And in conclusion - a few words about safety. When conducting experiments with generators, do not touch the terminals of the parts with the X1 plug connected to the network, do not touch the headphones, let alone put them on your head, and for all soldering or connecting parts, de-energize the structure and discharge (with tweezers or a piece of mounting wire) capacitors.

The next semiconductor device from the thyristor class is the trinistor. Its main difference from the dinistor is the presence of an additional output, called the control electrode (GE), from one of the transitions (Fig. 5) of the four-layer structure. What gives this conclusion?

Assume that the control electrode is not connected anywhere. In this embodiment, the trinistor retains the functions of a dinistor and turns on when the anode voltage Uon is reached (Fig. 6).

But it is worth applying at least a small positive voltage to the control electrode relative to the cathode and thus passing a direct current through the control electrode - cathode circuit, as the turn-on voltage decreases. The higher the current, the lower the turn-on voltage.

The smallest turn-on voltage will correspond to a certain maximum current Iu.e, which is called the rectification current - the direct branch is rectified so much that it becomes similar to the same branch of the diode.

After turning on (i.e., opening) the SCR, the control electrode loses its properties and it will be possible to turn off the SCR either by reducing the direct current below the holding current Isp, or by briefly turning off the supply voltage (a short-term short circuit of the anode with the cathode is acceptable).

The trinistor can be opened both by direct current passing through the control electrode, and by pulsed current, and the permissible pulse duration is millionths of a second!

Each trinistor (most often you will have to meet trinistors of the KU101, KU201, KU202 series) has certain parameters that are given in the reference books and by which the trinistor is usually selected for the assembled structure. Firstly, this is the allowable direct forward voltage ( Upr) in the closed state, as well as constant reverse voltage ( Uobr) - it is not specified for all trinistors, and in the absence of such a figure, it is undesirable to apply reverse voltage to this trinistor.

The next parameter is direct current in the open state (Ipr) at a certain allowable case temperature. If the trinistor heats up to a higher temperature, it will have to be installed on a radiator - this is usually reported in the design description.

No less important is such a parameter as the holding current (Iud), characterizing the minimum anode current at which the SCR remains on after the control signal is removed. The limiting parameters for the control electrode circuit are also negotiated - the maximum opening current (Iу.ot) and the constant opening voltage (Uу.ot) at a current not exceeding Iу.ot.

When operating trinistors of the KU201, KU202 series, it is recommended to include a shunt resistor with a resistance of 51 Ohm between the control electrode and the cathode, although in practice, in most cases, reliable operation is observed even without a resistor. And one more important condition for these trinistors is that with a negative voltage at the anode, the supply of control current is not allowed.

And now we will conduct some experiments to better understand the operation of the trinistor and the features of its control. Stock up on a trinistor, say, KU201L, a miniature 24 V incandescent lamp, a 18 ... 24 V DC voltage source at a load current of 0,15 ... 0,17 A and a 12 ... transformer from an old receiver or tape recorder with two secondary windings of 14 V at a current of up to 6,3 A, connected in series).

How to open a trinistor (Fig. 7). Set the variable resistor R2 to the lower position according to the diagram, and then connect the cascade on the trinistor to a DC source. By pressing the SB1 button, smoothly move the variable resistor slider up the circuit until the HL1 lamp lights up. This will indicate that the trinistor has opened. You can release the button, the lamp will continue to glow.

To close the trinistor and bring it to its original state, it is enough to turn off the power source for a short time. The lamp will turn off. By pressing the button again, you open the trinistor and light the lamp. Now try to extinguish it in another way - with the button released, close for a moment, say, with tweezers, the anode and cathode leads, as shown in Fig. 7 dashed line.

To measure the opening current of the trinistor, turn on the milliammeter in the open circuit of the control electrode (at point A) and, smoothly moving the variable resistor slider from the lower position to the upper (with the button pressed), wait until the lamp is ignited. The arrow of the milliammeter will fix the desired current value.

Or maybe you want to know what is the holding current of the trinistor? Then turn on the milliammeter in the open circuit at point B, and in series with it a variable resistor (nominal 2,2 or 3,3 kOhm), the resistance of which must first be output. With the trinistor open, increase the resistance of the additional resistor until the milliammeter needle jumps back to zero. The milliammeter reading before this moment is the holding current.

The trinistor is controlled by an impulse (Fig. 8). Slightly change the trinistor stage by excluding the variable resistor from it and introducing a capacitor C1 with a capacity of 0,25 or 0,5 microfarads. Now, a constant voltage is not applied to the control electrode, although the trinistor did not become uncontrollable from this.

After applying the supply voltage to the cascade, press the button. Capacitor C1 will charge almost instantly, and its charging current in the form of a pulse will pass through resistor R2 and a control electrode connected in parallel. But even such a short-term impulse is enough for the trinistor to open. The lamp will light up and, as in the previous case, will remain in this state even after the button is released. The capacitor will discharge through resistors R1, R2 and will be ready for the next current pulse.

Now take an oxide capacitor C2 with a capacity of at least 100 microfarads and for a moment connect it in the appropriate polarity to the anode and cathode terminals of the trinistor. A charging current pulse will also pass through the capacitor. As a result, the trinistor will be shunted (the indicated conclusions are closed) and, of course, it will close.

Trinistor in the power regulator (Fig. 9). The ability of the SCR to open at different anode voltages depending on the current of the control electrode is widely used in power regulators that change the average current flowing through the load.

To get acquainted with this "profession" of the trinistor, assemble a layout from the parts shown in the diagram. In a full-wave rectifier, both individual diodes and a ready-made diode bridge, for example, the KTs402, KTs405 series, can work. As you can see, there is no filtering capacitor at the rectifier output - it is not needed here. For visual control of the processes occurring in the cascade, connect an oscilloscope parallel to the load (HL1 lamp) operating in automatic (or standby) mode with internal synchronization.

Set the slider of the variable resistor R2 to the upper position according to the diagram (resistance is output) and apply an alternating voltage to the diode bridge. Press the SB1 button. The lamp will immediately light up, and the image of half-cycles of a sinusoid (diagram a) will appear on the oscilloscope screen, which is characteristic of full-wave rectification without a smoothing capacitor.

Release the button and the lamp will turn off. Everything is correct, because the trinistor closes as soon as the sinusoidal voltage passes through zero. If a filtering oxide capacitor is installed at the rectifier output, it will not allow the rectified voltage to decrease to zero (the voltage shape for this option is shown in the diagram by a dashed line) and the lamp will not go out after the button is released.

Press the button again and smoothly move the variable resistor slider down the circuit (enter the resistance). The brightness of the lamp will begin to decrease, and the shape of the "half-sine wave" will be distorted (diagram b). Now the current through the control electrode decreases compared to the original value, and therefore, the trinistor opens at a higher supply voltage, i.e. part of the half-sine wave, the trinistor remains closed. Since this reduces the average current through the lamp, its brightness decreases.

With further movement of the resistor engine, which means a decrease in the control current, the trinistor can open only when the supply voltage practically reaches its maximum (diagram c). The subsequent decrease in current through the control electrode will lead to non-opening of the trinistor.

As you can see, by changing the control current, and hence the amplitude of the voltage on the control electrode, it is possible to control the power at the load within a fairly wide range. This is the essence of the amplitude method of controlling the trinistor.

If it is necessary to obtain large control limits, the phase method is used, in which the phase of the voltage on the control electrode is changed in comparison with the phase of the anode voltage.

It is not difficult to switch to this control method - it is enough to connect an oxide capacitor C1 with a capacity of 100 ... 200 microfarads between the control electrode and the trinistor cathode. Now the trinistor will be able to open at small amplitudes of the anode voltage, but already in the second "half" of each half-cycle (diagram d). As a result, the limits of change in the average current through the load, and hence the power released on it, will expand significantly.

Trinistor analog. It happens that it is not possible to purchase the desired trinistor. It can be successfully replaced by an analog assembled from two transistors of different structures. If a positive (with respect to the emitter) voltage is applied to the base of the transistor VT2, the transistor will open slightly and the current of the base of the transistor VT1 will flow through it. This transistor will also open slightly, which will increase the base current of the transistor VT2. Positive feedback between transistors will lead to their avalanche opening.

Analog transistors are selected depending on the maximum load current and supply voltage. The control transition of both the analog and the trinistor is supplied with a voltage (or pulse signal) of only positive polarity. If, under the operating conditions of the device being designed, a negative signal may appear, the control electrode should be protected, for example, by turning on a diode (cathode - to the control electrode, anode - to the trinistor cathode).

The last device from the thyristor family is a triac (Fig. 11), symmetrical thyristor. Like the trinistor, it is made in a similar package with the same anode, control electrode, and cathode terminals. The triac has a complex multilayer structure with electron-hole transitions. From one of the transitions, a control output (UE) is made.

Since both extreme regions of the structure have the same type of conductivity, then, in the presence of an appropriate voltage on the electrodes of the triac, current pulses can pass through it in both directions.

Common triacs that you will have to meet in amateur radio practice are the KU208 series.

Author: B.Ivanov

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