26 Sep
2014
26 Sep
'14
12:21 p.m.
On Sep 26, 2014, at 3:03 PM, Tothwolf <tothwolf at concentric.net> wrote:
On Thu, 25 Sep 2014, drlegendre . wrote:
Just wondering: what tool do you use to show that short? A typical ohmmeter is a low
current source plus an ammeter. If you connect such a small power supply to a large
discharged capacitor, it will look like a short for a while. For example, if you have a
supply of a couple of volts current limited to 1 mA, it will take quite a number of
seconds for a 6800 uF capacitor to charge substantially. So in a test like that, hook up
the instrument and leave it connected for a minute. Does it still read 0 ohms? If yes,
then I would indeed suspect a short.
paul
Now this varies based upon construction of the
resistors, but it's almost impossible for any typical 2W resistor to fail dead-short -
they can drift up, they can drift down - but a hard-short is just not what we see. And
wirewound parts tend to act like fuses - they simply go red-hot and open up if they are
forced to carry high fault currents.
If the PSU isn't built for rapid over-current shutdown, and the only other
alternative is the 6800uF cap, then I'd put the money on the cap.
Have you checked for build errors, like missing insulators between boards & standoffs
/ mounts? The kind where all is well (often for many years) until the board coating
wears-though and creates a short to the standoff / mount point?
Those resistors are large ceramic wirewounds mounted vertically. I highly doubt any of
those could fail short.
This type of PSU has some form of crowbar circuit, so my money would be on either
overvoltage causing the crowbar to do its job, or a failure in the crowbar circuit itself.
I highly doubt that 6800uF electrolytic capacitor would fail short, and although that
always possible, I'd think one of the semiconductors would be more likely to fail
short.
From a google search, this very problem is apparently common to this model of PSU, but I
couldn't find anyone who had troubleshooted it to the point of finding the actual
cause.