VT100 PSU smelling.

js at cimmeri.com js at cimmeri.com
Fri Jan 22 18:19:09 CST 2016

On 1/22/2016 6:56 PM, Mattis Lind wrote:
> 2016-01-22 21:48 GMT+01:00 js at cimmeri.com<js at 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 

> 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-

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