6. Thyristors, triacs, diacs

There are several thyristors displayed on 6.1. Triacs look the same, while diacs look like small power rectifying diodes. Their symbols, and pin-out is found in figure 6.2.

Fig. 6.1: Several thyristors and triacs

A thyristor is an improved diode. Besides anode (A) and cathode (k) it has another lead which is commonly described as a gate (G), as found on picture 6.2a. The same way a diode does, a thyristor conducts current when the anode is positive compared to the cathode, but only if the voltage on the gate is positive and sufficient current is flowing into the gate to turn on the device. When a thyristor starts conducting current into the gate is of no importance and thyristor can only be switched off by removing the current between anode and cathode. For example, see figure 6.3. If S1 is closed, the thyristor will not conduct, and the globe will not light. If S2 is closed for a very short time, the globe will illuminate. To turn off the globe, S1 must be opened. Thyristors are marked in some circuits as SCR, which is an acronym for Silicon Controlled Rectifier.
A triac is very similar to a thyristor, with the difference that it can conduct in both directions. It has three electrodes, called anode 1 (A1), anode 2 (A2), and gate (G). It is used for regulation of alternating current circuits. Devices such as hand drills or globes can be controlled with a triac.

Thyristors and triacs are marked alphanumerically, KT430, for example.
Low power thyristors and triacs are packed in same housings as transistors, but high power devices have a completely different housing. These are shown in figure 6.1. Pin-outs of some common thyristors and triacs are shown in 6.2 a and b.
Diacs (6.2c), or two-way diodes as they are often referred to, are used together with thyristors and triacs. Their main property is that their resistance is very large until voltage on their ends exceeds some predefined value. When the voltage is under this value, a diac responds as a large value resistor, and when voltage rises it acts as a low value resistor.

Fig. 6.2: Symbols and pin placements for: a - thyristor, b - triac, c - diac


Fig. 6.3: Thyristor principle of work

6.1 Practical examples

Picture 6.5 detects when light is present in a room.  With no light, the photo-transistor does not conduct. When light is present, the photo-transistor conducts and the bell is activated. Turning off the light will not stop the alarm. The alarm is turned off via S1.

Fig. 6.5: Alarm device using a thyristor and a photo-transistor

A circuit to flash a globe is shown in figure 6.6  This circuit flashes a 40w globe several times per second. Mains voltage is regulated using the 1N4004 diode. The 220u capacitor charges and its voltage rises. When this voltage reaches the design-voltage of the the diac (20v), the capacitor discharges through the diac and into the triac. This switches the triac on and lights the bulb for a very short period of time, after a period of time (set by the 100k pot), the capacitor is charged again, and the whole cycle repeats. The 1k trim pot sets the current level which is needed to trigger the triac.

Fig. 6.6: Flasher

A circuit to control the brightness of a globe or the speed of a motor is shown in figure 6.7

Fig. 6.7: Light bulb intensity or motor speed controller

If the main use for this circuit is to control the brightness of a light bulb, RS and CS are not necessary.