15 Nov
2004
15 Nov
'04
2:56 p.m.
Tony Duell wrote of crowbar protection circuits thus:
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
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". :)