bill@flash.UUCP (bill) (07/14/90)
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. :-) --