Testing Power Supplies! Re: Norsk Data Nord-10/S restoration effort on the way!

From: Joe R. <rigdonj_at_cfl.rr.com>
Date: Mon Nov 15 14:53:19 2004

At 01:11 PM 11/15/04 -0500, you wrote:
>>>>>> "Joe" == Joe R <rigdonj_at_cfl.rr.com> writes:
>
> Joe> Testing power supplies before powering up a system is just plain
> Joe> paranoid! (And I'm trying to be polite here!) There are a
> Joe> thousand parts in modern computers that can cause just as much
> Joe> damage!!!!!!!!
>
>I disagree -- at least in the case of linear regulator supplies, which
>is what you'll normally find in a classic computer.
>
>A very simple and very nasty failure in a linear regulator is a short
>in the pass transistor. If that happens, you get a much higher output
>voltage, and there is no current limiting. It's a fair bet that this
>will fry a large fraction of the semiconductors in your system.
>
>If the supply has a "crowbar overvoltage protection" circuit in it,
>that's a different matter. But testing a linear supply is trivial.
>So why not do it?

  I disagree. Testing a power supply is far from trivial and merely reading the open circuit voltage tells you virtually nothing. Power supplies need to be tested under load (like batteries) and things like ripple and regulation need to be checked. In fact, you can damage many PSUs by not having a suffient load on them. And without a load others will go into overvoltage and trip the crowbar circuits and thus appear to be bad. That's why I said that you need the specs for the supplies so you can be sure of having an adaquete load to prevent damage (or know if they can be operated with no load) and what the permissible regulation and ripple specs are as well as the current capacity. If you don't know exactly what they're supposed to do, how do you think you can test them? AND as I pointed out, I've had tested powers supplies fail TWICE just in the last week after operating a short time so even if you do an adaquete test you can't be sure that they won't fail in use.

  AND we haven't even talked about how you're supposed to know such simple things as the rated voltage outputs and pinouts of odd-ball things like the Nord that was being discussed. Tell me, do YOU know what the outputs of the Nord PSU are supposed to be and where the test points are???

>
>If you let the magic smoke out of the computer, it's too late...

  And you can still let the smoke out even after days of testing as I pointed out in my message. And that's based on an ACTUAL example and not some hypothetical discussion.

   Joe
Received on Mon Nov 15 2004 - 14:53:19 GMT