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SCR crowbar over-voltage protection

Transistors and thyristors · Intermediate · about 15 minutes · part of Electronics Technician

If the 5 V rail ever climbs past about 5.6 V, the Zener conducts, the SCR fires and short-circuits the rail like a crowbar thrown across it. That blows the fuse before the expensive logic downstream sees the over-voltage.

SCR crowbar over-voltage protection: the circuit on the simulator's bench

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How it works

Trip point ≈ V_Z (at about 1 mA) + V_GK + I·R_G ≈ 4.6 + 0.6 + 0.4 = 5.6 V.

Once fired, the SCR stays on (latched) as long as more than its holding current flows, which the shorted rail guarantees until the fuse opens.

Try it on the bench

  1. Raise V_supply to 6 V and watch it trip; then set it back to 5 V, replace the fuse (Replace burnt) and see it re-arm.

Parts on the bench

In the course

Electronics Technician, module “Power supplies”.

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