Electronics >> Thyristors

Thyristors are semiconductor devices that use internal feedback to generate switching action.

Main types of Thyristors are: Silicon Controlled Rectifier (SRC), Triac, and Diac.

Thyristors can be used to switch large currents; therefore they can be used for overvoltage protection, motor controls, heaters, fans, lighting systems, and other high current loads.

Silicon Controlled Rectifier (SRC):

Silicon Controlled Rectifier (SRC) is a semiconductor device that has two p-type and two n-type doped regions, as well a triggering input lead; called (Gate).

Q1 is a (PNP) transistor, and Q2 is a (NPN) transistor.

The working of a SCR could be demonstrated by the (PNP) and (NPN) transistor circuit. (The base of each transistor is connected to the Collector of the other.)

Any change in the base current of (Q2) is amplified and fed back through (Q1). This will amplify the original change. The feed back continues changing the base current of (Q2) until both transistors go into either saturation or cut-off mode.

Similar to a transistor; it requires a minimum of 0.7 V to trigger the gate of a SCR. Manufacturer's datasheet displays the triggering voltage as (V_GT), as well the datasheet would indicate the minimum current needed to turn on the SCR. This current value is called: Gate Trigger Current (I_GT).

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Triac:

Triacs operate the same as if two SCRs were placed in reverse position to each other, and were connected in parallel. This characteristic allows the Triac to control the flow of current in both directions.

Triac's internal structure causes the device to have a higher Gate Trigger Voltage and current in comparison to SCRs.

(i.e. Gate Trigger Voltages of 2 V or 2.5 V)

Triacs can handle large currents; however SCRs generally have a much higher current rating.

This semiconductor device is mainly used when conduction on both half cycles in a circuit is necessary (alternating current), and are often used in industrial applications.

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Diac:

A Diac will pass current in both direction, and its internal make-up is equivalent to two four layer (P, N, P, N) diodes connected in parallel.

The diac's conduction starts when the charge across it exceeds the breakover voltage. (When a large enough voltage is supplied to the device in order for it to starts the conduction.)

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