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LED on-off with SCR

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daviddoria

New Member
i don't think i see what's going on...

the 2 thinks we want to accomplish are:

1) LED turn on when circuit physically broken (open door)
2) LED stay on when door closed

i am right in that those are NO and NC momentary switches, correct?
 

k_noe

New Member
Both switches will be normally closed switches. The SCR will be triggered on by applying a voltage to the gate, ie. opening the lower switch which would be rigged to the door. After the SCR is trigger it will remain on whether or not a voltage is applied to the gate so the SCR will remain on whether the lower switch is open or close. To turn off the SCR the voltage will have to be removed from its anode which in this case will be done by someone manually opening the upper switch.
 

daviddoria

New Member
does an SCR look like a diode physically?

i found this:
silicon-controlled rectifier
(SCR) Three terminal active device that acts as a gated diode. The gate terminal is used to turn the device on allowing current to pass from cathode to anode.

so it's just like a transistor? apply current to the base and current flows from C to E? in this case, apply current to the gate and current flows from + to -?

how are these rated? do i need just like a 9v or 3v or whatever i run the circuit on SCR?

so if my understanding is correct of SCRs, then for this application, why does opening the lower switch apply voltage to the gate?
 

Sebi

Active Member
For thyristor firing need a positive impulse to gate. If the normally closed ON switch pushed, on gate rising positive voltage via resistor. The fired SCR remain in this state. Without anode-current no more conduct.
Unfortunately with NC switch we need continous current. With transistor possible reducing the quiescent current.(enhanced battery life)
 

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