On 1/22/2016 6:56 PM, Mattis Lind wrote:
2016-01-22 21:48 GMT+01:00 j...@cimmeri.com<j...@cimmeri.com>:
On 1/22/2016 2:54 PM, Mattis Lind wrote:
Short: R27 in my VT100 PSU is hot and smelling. Why?
Long: I think it has been 20 years since I powered up this VT100 so I did
it carefully. Used a Variac and a bench supply. It switched just fine and
delivered the steady 5V out when the input was at approx 50V (115V input).
All the other voltages looked fine at full AC input. But there was this
little smell from R27.
You don't use Variacs with switch mode power supplies... not unless you
want to burn them up. I'll sometimes use one, but only to provide a very
fast soft start. Even that's risky.
Well. It depends if you read the schematics before you do so.
Assuming they're available for the particular PSU one is attempting to
test.
I powered the startup voltage from a 12 V bench supply. Normally the
startup voltage comes from a small mains transformer and a 7812. Then I
supplied the primary side voltage for the main switch transistor using a
variac.
I wasn't aware of this interesting method; I'd also misread your
original sentence and thought you were just using a variac alone to
bring up the entire SMPS.
Works perfectly well if you know what you are doing. Have done this
practice with many SMPS supplies.
Just curious -- what is *your* reason for bringing up an SMPS with the
help of a variac?
Thank you-
-J.
