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

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Nov 15 22:56:44 2004

>
>
> 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.
>
> 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". :)

CLose :-). The idea is to be non-destructive to the rest of the machine,
even if it ends up destroying parts of the defective PSU. You want to
short-circuit the PSU outputs (normally only the +5V output) if it goes
overvoltage. Typically the circuit is something like :


  -------------+-----------------------+--------------------o +5V
               | |
             ----- |
  5V1 zener ^ | |
             / | \ |
               | |
               | \ | /
               +--------------------\ V SCR
               | --------
               | |
               / |
               \ |
   100 Ohm / |
               \ |
               | |
 --------------+-----------------------+-------------------o oV

If the 5V rail goes overvoltage, the zener breaks down and turns on the
SCR which short-circuits the 5V rail (and carries on shorting it until
the PSU output current drops to close to zero).

-tony
Received on Mon Nov 15 2004 - 22:56:44 GMT