Thyristors Chapter 15

This latch is stable in either of two states. Q1 Q2 Latch On state Off state

The four-layer diode { Q1 { Q2 p Q1 n p n Q2

Four-layer diode breakover IH V VK VB

The silicon controlled rectifier (SCR) Anode Anode Anode p Q1 n p Gate Gate Gate n Q2 Cathode Cathode Cathode

SCRs The most widely used thyristors Use gate triggering for turn-on instead of breakover triggering Data sheets list the gate trigger voltage (VGT) and the gate trigger current (IGT) Turn-off requires reducing the current to less than the holding current (IH)

An SCR does not turn off at the end of the trigger pulse. +VAA Load A B C D +VAA A B C D

SCR crowbar to protect a load from overvoltage VZ Power supply Protected load VT VGT The overvoltage that triggers the crowbar: VT = VZ + VGT

Adding gain to a crowbar circuit Power supply Protected load R2 R4 R1 is the trigger adjust

Adding an IC amplifier to a crowbar circuit 10 kW 10 kW A Power supply Protected load IC VA 10 kW VZ The circuit triggers when VA exceeds VZ.

SCR phase control vline Load 120 V R1 qfire vload R2 C qconduction

A snubber network can be used to limit the rate of voltage rise across the SCR. Load R1 RC snubber R2 C

I Bidirectional thyristors can conduct in either direction. Four-layer diodes in reverse parallel Diac

I The triac is a popular bidirectional thyristor. Gate SCRs in Triac reverse parallel Triac

Triac phase control vline Load 120 V R1 vload R2 C

Triac crowbar Fuse R1 Triac Protected load 120 V R2 Diac R3 R2 is the trigger adjust

The photo-SCR is triggered by light. RL RL Open gate Sensitivity adjust Maximum sensitivity

The gate-controlled switch is turned off with a reverse-biased trigger. RL A B C D A B C D RG

The silicon-controlled switch is a low-current device. Anode Anode gate Cathode gate Cathode Either gate can be used to trigger or open this device.

Unijunction transistor (UJT) B2 B2 RE E E p V n vin B1 B1 When vin reaches the standoff voltage, the resistance between the emitter and B1 drops dramatically.