Crowbar? (was: Re: Testing Power Supplies!)

From: Joe R. <>
Date: Mon Nov 15 20:30:01 2004

At 06:07 PM 11/15/04 -0800, you wrote:
>Tony Duell wrote of crowbar protection circuits thus:
>> I'd rather not trust a protection device if I don't have to. Sure I like
>> crowbar circuits, and they have saved my chips a few times, but I don't
>> like to depend on them.
>Ah, twice in one week I get to expose my wanton ignorance. :) What
>typically makes up a crowbar circuit? How are they triggered? If I'm
>understanding this correctly, a crowbar quickly kills the PSU and then
>drains power from the system rapidly and nondestructively... but how it
>accomplishes this, or knows it _should_, I don't yet understand.

   A crowbar is a very simple yet effective circuit to prevent PSUs from outputing excessive voltage. It's nothing more than a SCR (Silicon Controlled Rectifier) across the PSU output and a zener diode from the gate of the SCR to ground (and a small resistor to supply a small amount of current to the gate-zener junction). The zener sets the trip voltage of the SCR. The resistor-zener combination forms a small constant voltage source that is applied to the SCR gate. Anytime the PSU output exceeds the zener voltage (plus the gate voltage of the SCR) the SCR fires and becomes a dead short (minus a fraction of a volt due to the SCR PN junction) and the PSU output is shorted out. Once the SCR fires it says on regardless of the trigger voltage. The only way to remove the short is to turn off power to the PSU and let it discharge and then reapply power. As you might have guessed these circuits are very fast since they're so simple and they're located right on the PSU output. The FAST response and nearly dead shor
t account for the name "crowbar". It has the effect similar to that of throwing a metal crowbar right across the PSU output.

  And yes the PSU had better be able to handle the almost dead short or have a fuse or circuit breaker in it or something is going to give!


>O'course, one could accomplish this with an actual crowbar applied
>across the PSU output... but I suspect this would not be described as
>"nondestructive". :)
> -O.-
Received on Mon Nov 15 2004 - 20:30:01 GMT

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