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Scr-replacing-latching-switch

2015-10-16 23:42  
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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.
Scr-replacing-latching-switch

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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