Info on testing vintage power supplies

Brent Hilpert bhilpert at
Mon Sep 2 14:17:53 CDT 2019

On 2019-Sep-02, at 9:35 AM, Alan Perry via cctech wrote:
> Can anyone here provide a pointer to info on testing vintage power supplies? Search results on the web may eventually lead to the kind of info that I am looking for, but I have to get through too many pages of modern PC power supplies first.
> Specifically, I will be testing the power supplies in my Sun 3/260, which has 24V, 12V and 5V. I am wondering things like what is suitable loads and do I need to put a load on all three or can I test them one at a time and what I haven't thought of with regards to testing them.

Regarding incandescent bulbs, 12V bulbs can be used on the +5.
They tend to be cheaper and more readily available than 6V bulbs.

Dual-filament tail-light-with-brake-light bulbs such as the 1034 and 1157 are common and inexpensive.
The dual filaments allow for some load variation, the tail-light filament is lower current, the brake filament higher.
Parallel a couple such bulbs (and the filaments) for more current.
You don't have to buy sockets for them, you can solder wires to the base and contacts.

Note the current draw is not a straight linear relationship with reduced voltage.
At lower voltage they'll draw something more than the linearly-reduced current level.
E.g. for the 1034:

	@ 5V	@ 12V
	-----	-----
tail	0.35A	0.6A
brake	1.2A	2.4A

I don't know that you'll find any sort of generalised reference for your request.
While some general rules may cover many models, the only way to be comprehensive
over such a topic is to understand the principles and then look at the specific
design of the supply one is dealing with.

For switching supplies, usually only one of the outputs is actually regulated and the others
will 'follow' in regulation (designed for a low enough source-impedance-to-load-impedance ratio that
they'll be in range if the controlled output is in range).

Sometimes lesser-current outputs actually have a linear regulator between the switching transformer and the output. 

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