Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!wuarchive!mit-eddie!uw-beaver!sumax!quick!amc-gw!sigma!flash!bill
From: bill@flash.UUCP (bill)
Newsgroups: sci.electronics
Subject: regulators
Message-ID: <940@flash.UUCP>
Date: 13 Jul 90 21:29:53 GMT
References: <1990Jul12.024412.122@mintaka.lcs.mit.edu> <3172@rsiatl.UUCP>
Reply-To: bill@flash.UUCP (bill)
Distribution: na
Lines: 26

In article <3172@rsiatl.UUCP> jgd@rsiatl.UUCP (John G. DeArmond) writes:
}Your symptoms sure sound a lot like one or more regulators are oscillating.
}The pair should be able to handle 1.26 amps with room to spare if
}the load is partitioned well and the heat sinks are adequate.  Have
}you hung a wideband scope on the output and looked for oscillation?
}I've seen these babies oscillate well up into VHF.

In the mid-70s National Semiconductor came out with the LM125 series,
which would begin to oscillate at audio frequencies (two modes: one
about 40Hz, another quite a bit higher as I recall) after being run for a
while near, but below (I double-checked that) their limit.  Naturally,
bypassing didn't help. :-)

It cost me a little time and a few regulators, but I figured out the
conditions of failure and could reliably cause the regulator to start
oscillation after about an hour's operation. Once a device exhibited this
oscillation, it would ever after begin to oscillate under much less
stringent loads.

I remember talking to the designer(?) of this device, but was never 
able to convince him that the devices would fail in the way I described.

I ended up using some other device, and swore never to use a regulator
that close to the limit again. :-)

--