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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Unusual latchup problem
Date: Wed, 06 Nov 2002 04:47:27 GMT
NNTP-Posting-Date: Tue, 05 Nov 2002 23:47:27 EST
Jean-Rene David wrote:
> This is a circuit I didn't design but which I'm troubleshooting.
> It seems I have a latchup problem on a circuit with no connection to the
> exernal world. There are four supplies (+-12, +-5) but no part is connected
> to both (I removed them all) and no +-12V part is connected to any +-5V
> part. All the +-12V parts are opamps or instrumentation amplifiers.
> Now the symptoms are strange in the following way: when I fire up the power
> supply with the circuit plugged in, the -12V latches up 90% of the time.
> When I turn the supply on and THEN connect it to the circuit, the -12V
> latches-up about 10% of the time.
> Since the circuit is not connected to the external world, I thought ringing
> from a steep rise on the P/S might cause the latchup. However the rise time
> when turning the P/S on with the circuit plugged in (the case that latches
> up) has SLOWER rise time (about 3ms) than the other case (a few
> Also, when the -12V *doesn't* latchup, it actually goes up to 0.7V for
> 50-400ms after the +12V rise before coming back down again to -12V. The
> delay seems random.
> There are more than a dozen opamps on the thing and though I will do it if
> I have to, I don't really feel like adding protection diodes or current
> limiting resistors on each one.
> If anyone has any idea on 1) the possible cause of this problem or 2) a way
> to troubleshoot the problem or otherwise find the faulty device, I would
> greatly appreciate.
If the problem is really latch up of a load connected to the -12
supply, something should pop or get very hot, at the very least. And
latch up seldom drags a supply below 1.2 volts, usually not lower than
1.5 volts. Are you sure that the problem is not just insufficient
load on the 5 volts supply to keep the -12 running? Some supplies
have very weird inter relations between the various outputs. Add a
big power resistor (or a 12 volt lamp) to the +5 supply and see if
the symptoms change.
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