VT100 PSU smelling.

Mattis Lind mattislind at gmail.com
Sat Jan 23 04:38:14 CST 2016

>> 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.
Yes of course. If I have a PSU where I have doubts about the function and
there is no schematic I spend a few hours trace out the relevant portions.
Normally this is not a big chore. Then there are of course complex PSU and
simple ones. I would not like the idea of tracing out the VAX-11/750 PSU,
but a VT100 PSU is very small circuit on the primary side.

> 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.

OK. No I am not using onIy a variac. That wouldn't work very well. I
usually try to identify the control circuit and power it independently of
the switch circuit. That is usually no problem. Some PSU has a simple
voltage divider from mains input other like the VT100 has a normal small
iron core transformer to supply the control circuit supply.

> 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?

Many of the PSUs I work with have been not been powered up for decades. I
think it is a good practice to take it slowly in the beginning. Observing
it so that everything behaves well in the circuit using a very light load.
Verify that the switch control circuit and regulation circuit behaves well
and that the switching transistor waveforms looks good. Nothing is getting
abnormally hot or so. Also be a little bit kind to all the electrolytics in
the circuit etc.

That is why I do this and I think that this method have served me well.

Then of course I also use a protection transformer and very often some kind
of input current limitation, not just a variac and a bench supply.

> Thank you-
> -J.

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