Scr-replacing-latching-switch

  
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.

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 ├é┬Ěserves 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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