Bringing up a old Powersupply

From: Chuck McManis <cmcmanis_at_mcmanis.com>
Date: Fri Apr 23 00:35:37 1999

At 10:09 PM 4/22/99 -0800, Zane H. Healy wrote:
> Is this a good idea? [slow power up]

I'm on the fence about it. On the one hand you will find out if you've got
a short fairly harmlessly, on the other hand its going to have to withstand
power eventually.

> If so, how slowly is a good idea?

So my uneducated suggestion is to apply a lower voltage to it and see if it
sucks current (ie its shorted) and then if it isn't, give it full AC and
start checking the outputs.

>Also, in talking to a friend tonite he was suggesting I hook an O-Scope up
>to the PS to check for "ripple". Is this necessary?

YES. Especially under load. The thing that often fails in a PSU is the
filter capacitors and weak ones will show up as ripple and ripple will
render your machine inoperable.

> I've got a scope, but
> honestly do not have any idea how to use it, and haven't had time to learn.

This measuremnet is trivial. Set the probe for "AC coupling" and connect
ground to ground and then set your probe tip to the power rail. The display
should jump and then settle down to a straight line at "0" (you can adjust
this using the position adust knob when the coupling is set to "GND" or
with the probe connected to ground.

Now look at the trace and click the volts/div knob down until you either
reach the end of its travel or you see a sinusoid on the display. Using the
value given by the volts/div knob see how "tall" in divisions the signal is
on the scope. Multiply that number of divisions by the knob setting and
that will tell you how "big" the ripple is. It should be less than .1 volt.

After you've made the measurement use a power resistor to load the supply a
bit (ideally drawing half its rated power) and measure the ripple again.
Again it should be less than .1volt. If it isn't then you've got some
filter capacitors to replace.

-_Chuck
Received on Fri Apr 23 1999 - 00:35:37 BST