Thread: PSU load tester
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Old June 30th 06, 01:21 AM posted to alt.comp.hardware
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Default PSU load tester

There are quite a few ways to create the load but if you
want to use load resistors, so be it. How did you plan on
mounting and cooling them? Merely hooking up this load for
a few minutes isn't going to tell you much rather than
running long term and this means you have significant heat
(750W) to be rid of.


Honestly, I was thinking just mount the resistors to a big slab of aliminium
or a big heatsink out of something.
No PCB or anything, just a series of resistors wired together, with a few
molex connectors attached.

There are resistors inside extruded
aluminum casings to help 'sink them but that's a massive
heatsink if you were to try to passively remove 750W, I'd
suggest something more like a large aluminum tube with a fan
blowing through the middle, or oil cooling or (your
inventiveness, budget and skill in fabricating something
will determine what your (subjective) best approach is.


I saw those resistors you speak of.. 50W, 30ohm on ebay for $2/piece.. I
was thinking bolt them to a big heatsink of some variety.. whatever I could
get my hands on from something dead (amp/hifi, etc). Budget is as cheap as
possible.. $10 - $15 would great.. ideally no more than about $25..

Skill in fabricating is zero.. I can drill holes in a chunk of metal and
put a few bolts in.. if it involves cutting or shaping anything or making a
PCB, it's outta the question..

Understood, but remember that a resistive load tester will
only qualify the PSU for running a constant, resistive load.
A computer is a highly variable capacitive load too. A
constant resistive load is *easier* for a PSU if it has the
capacity to do it at all.


A large purely resistive load seems to be the norm for most places when it
comes to testing a PSU.. I did see a variation that used 2 car headlamps
and about 8 transistors.. I don't understand what the purpose of the
transistors is though...

The schematic is he http://www.kbt-dc-supplies.com/tester.php

If I could make it an RC load that would be great.. but it would all still
have to be simple and mount on a big heatsink.. delicate PCBs are not an
option.. (I have no means to make them, and I don't have a small delicate
iron either).

I will try your suggestions re parallell resitors and ohms law.. I did try
applying ohms law.. but when it worked out to 0.67ohms to draw 18amps @ 12v
I thought I royally screwed up my calculations.. (I was expecting a couple
of hundred ohms).

The main things I want to test are thermal/overload protection, fan
speed/noise at different loads, and voltage readings at different loads..
(e.g. do I still get 12V with it loaded to 700W or does it look more like
10V).

We can expect a typical 750W PSU to have well over 10A on at
least 2, probably 3 or more rails. Make sure your (DMM
with current feature) is rated for this current. Most
aren't, on consumer grade DMM 10A is a common limit.


The other option would be to meaure the voltage drop across
one of the resistors and calculate it out (Ohms law again).


Upon looking, my current DMM only handles 10A.. so I'll have to find a
better one or just calculate amperage like you said..

I suppose analogue meter movements still only handle 10A too ? Obviously I
can't afford a $1000 fluke bench meter.. (as much as I would like one)

Speaking of equipment.. I read it's possible to use a software O'Scope which
works off the sound card.. (one probe connects to line out, the other to
line in).. is this suitable for checking AC ripple in the psu output ? (I
don't need precision, just a good estimate) I don't have access to a CRO to
use/borrow and it's another tool that is way beyond my budget.. (if it's
more than $50 it's past my budget)

Thanks for the help..

Chris