The crowbar

226

04:16:55

The two transistors are set up as a scr as the voltage goes above the zeners threshold the gate will trigger and short the circuit blowing the fuse and saving the components 

published 11 years ago

Voltransistor

11 years ago

Nice circuit. It's no really a SCR. An SCR has the collector of the PNP transistor also connected to the base of the NPN transistor, instead of negative source. It's like a darlington pair, but I forgot what it's called.

thebugger

11 years ago

It's not called anything it does nothing but waste power in heat.

Dufniall89

11 years ago

The reason for this circuit is to voltage protect normally it would just blow the fuses

Voltransistor

11 years ago

@thebugger-it only shorts out for a millisecond just to burn the fuse out. The purpose is the fuses have very big voltage drop and LEDs have very small voltage drop and the LED needs to be protected(the fuse, zener, and the transistors takes the bullet for the LED). Once the fuse blows, there's no more short circuit and it only wastes 1 millisecond, maybe less(depends on the fuse speed).

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