Triac and load switching

Diodes and transistors are very important and useful semiconductor devices, without which there would be no phones, computers and everything else, but they have one feature, since a diode passes electricity only in one direction, in principle, like a transistor, they cannot be used in circuits alternating current. In our everyday life, many appliances are consumers of alternating current, for example, the motor of a washing machine, which rotates at different speeds, while it operates from the same 220 volts alternating current.
Схеме использования симистора для регулировки оборотов вращения двигателя.jpg
Схеме использования симистора для регулировки оборотов вращения двигателя.jpg


To understand the operation of a triac, you need to first talk about a thyristor, essentially these are two transistors p-n-p and n-p-n in which the n and p junctions are connected as shown in the figure.
It turns out that when voltage is applied to the control electrode of such an assembly of transistors, we open an NPN transistor, which, with its emitter-collector current, opens a PNP transistor.
As is obvious from the drawing, the thyristor conducts electric current only in one direction, but has a control contact past the anode and cathode. It also follows from the connection diagram of two transistors that to open the thyristor, you need to apply current to the control contact, and as soon as the thyristor opens, you can remove the voltage at the control contact, while the thyristor will maintain itself in the open state while its current flows.

If you disconnect the thyristor from the current source, it will close and reopen only after voltage is applied to the control signal.
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The principle of operation of a triac

If we connect two thyristors back-to-back, we get a symmetrical triode thyristor or TRIAC (from the English TRIAC - triode for alternating current). The anode of each triac is connected to the cathode of its “colleague” and the control contacts are also connected to each other. With this connection, we allow one half-wave of alternating current to pass through one thyristor, and the second half-wave through the second triac.

If a consumer (for example, an incandescent lamp or an electric motor) receives both half-waves from a sine wave, then the total power is calculated. If we take two identical thyristors, then they will be open constantly, since one thyristor in half-wave will tear off the second thyristor with its reverse current, so we need to take two different thyristors.

Types of thyristor control

Anode controlled thyristor (requires negative voltage to open)
Thyristor with cathode control (requires positive voltage to open)

To control such a triac (from an anode and cathode thyristor), you need to have time to supply a control voltage in each half-wave, but for this you can use the voltage from our source and connect it to the button through a resistor. Then, when the upper half-wave passes, we will open the anode thyristor, and when the necessary half-wave passes, we will open the cathode thyristor. The use of such a switch is not entirely justified, since a regular switch can be used, and the use of this effect is better used for power control.

Using a triac, you can control the rotation speed of an AC commutator motor (washing machine), but you cannot control DC or asynchronous motors using a triac, since to control an asynchronous motor you need to change the frequency, for example, with an inverter, and to control a DC motor you need to change the voltage .
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Power adjustment

When we connect the load, by default the triac is closed, but through the potentiometer (variable resistor) and resistor R2 the capacitor begins to charge, when the voltage on the capacitor reaches 32 volts, it opens the dinistor (thyristor without a control output, which opens when a certain voltage is reached)

The dinistor supplies current to the triac, which opens and supplies current to the load, but due to the fact that the capacitor spent some time charging, the sine wave opens as if in a truncated form.

So if you turn the potentiometer to maximum resistance, the capacitor will not have time to charge at all in half a wave and the triac will not open. And if you twist the variable resistor to the minimum resistance, the capacitor will charge almost instantly and we will get 100% of each half-wave.

When the lower half-wave flows, the capacitor is charged with a voltage of reverse polarity, which is why for the power regulator to operate, a triac-type dinistor is needed, i.e. which breaks off when the voltage reaches a certain value, independent of polarity.
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