Posted on Sep 5, 2012

This circuit provides the tum-on characteristics of an SCR, but turns off with ease. The switch is comprised of three transistors with descending current ratings: Q3 has a high-current rating and Q2 has a medium rating. The current, !1, to be switched is 15 A.

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Momentarily depressing S2 removes Ql"s base drive, turning Ql off and allowing Q2 to tum on. Q2 then drives the base-emitter junction of Q3, turning Q3 on. Q3"s collector-emitter voltage, which ·serves as Ql"s base drive, is essentially zero, keeping Ql off. To tum Q3 off, depress Sl; this action momentarily shunts Q2"s base current to ground, reversing the chain of events that turned Q3 on.

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