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Thyristors: The Basics and Functions Explained

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Understand the operation of thyristors and their various states such as on and off, gate triggering, overvoltage protection, phase control, and more in this comprehensive guide.

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Thyristors: The Basics and Functions Explained

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  1. Thyristors Chapter 15

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

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

  4. Four-layer diode breakover I IH V VK VB

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

  6. 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)

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

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

  9. Power supply Protected load Adding gain to a crowbar circuit R3 R1 R2 R4 R1 is the trigger adjust

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

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

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

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

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

  15. Load Triac phase control vline 120 V R1 vload R2 C

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

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

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

  19. 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.

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

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