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Subject: Re: Unusual latchup problem
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Wed, 06 Nov 2002 08:56:34 GMT
NNTP-Posting-Date: Wed, 06 Nov 2002 09:56:34 MET
latch-up is something else, this is a normal supply problem.
did you change the power supply to something reliable, like a laboratory
power supply? Will the problem still exist then?
If you use a computer power supply, without proper current load on the +5V
or +3.3V the other voltages might not be in the right range. Also the -12V
is usually very weak. Put two 12V/50W Halogen in parallel on the +5V to gnd,
which suck 4-5A as a basic load, (also on the 3.3V if there is). And also
load the +12V with another 12V/10W bulb. Usually the -12V is done from the
12V with an extra transformer requiring the current in the +12V double than
sequencing is usually not needed with opamps.
> 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
> 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
> 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
> from a steep rise on the P/S might cause the latchup. However the rise
> 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
> to troubleshoot the problem or otherwise find the faulty device, I would
> greatly appreciate.
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