21 Jul
2011
21 Jul
'11
9:16 p.m.
On 20 Jul 2011 at 22:31, Tony Duell wrote:
The HP9800 machines that I lkeep on going on about, and some of their
periphearlas are like that. They use 723s either as linear or switching
regulator controllers and have crowbars on the outputs. They do use the
current limiting circuitry of the 723, and if that doesn't help there's a
mains fuse.
Actually, in my experience, a crowbar circuit is
often used on a PSU
with electronic overcurrent protection, and said protectiuon should
operate if the crowbar fires due to an overvotlage. However, there's a
second line of overcurrent protection (e.g. a fuse) that will operate
if the electronic protection doesn;t (e.g. becasue the PSU control
circuitry is totally malfunctioning).
I've got a couple of linear supplies that use just that design--an
SCR crowbar driven by a comparator following a LM723 regulator stage--
the 723 has overcurrent protection, so it all hangs together.
(and there is a fuse on the primary of the transformer as well) 
22 Jul
22 Jul
7:34 p.m.
Thinking about it some more, I guess it makes sense to
crowbar the output
that is rising and going to cause damage rather than to crowbar something
nearer the input and maybe end up having to wait longer for reservoir
capacitors to discharge before the output voltage drops, even if there may
be more damage in the power supply in some cases.
It depends. I'v ecertainyl seen linear PSUs where the crowbar SCR is
conencted acros the smoothing capacitor on the input side of the
regualtors (of course it's trigged by an overvoltage on the output line).
And it's common in alrge SMPSUs for the crowabar SCR to be on one of the
'lesser' outputs. It'll shut the supply down just ans well as if it were
on the main output and it doens't have to handle as much current.
But yes, in gnral, crowbars operate on the principle that the PSU needs
repairing anyway, and a little more damage isn't too seruous, wherehas
having to replace large numbers of logic chips is.
-tony
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