In article , "Skeleton Man"
wrote:

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.

I used heavy copper house wiring. You want solid copper, so the
wires work as a "ladder" or mechanical framework for the loading of
one output. I also used an 80mm fan blowing over my collection of
resistors. You could buy a terminal block, and bolt the wire "ladders"
to terminal block screws. The terminal blocks can be screwed down
to a piece of wood (a refinement I didn't bother with).


snip

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

The purpose of that schematic, is the transistors amplify the effect
of the pot on the left. By varying the bias created by the pot, the
designer is hoping to vary the conductivity of the transistors.

Be aware that light bulbs have a ratio between "cold start amps" and
"running amps", that makes them unsuitable for a precision load.
You could end up in a situation where the light bulbs trip out
the overload protection on the supply, if you connect them all
cold. If you had a bunch of light bulbs running in parallel,
switching them on one at a time, would reduce the magnitude of
that effect. The above schematic is ideal for the purpose, in
the sense that turning the pot from zero to a finite value,
gives you the ability to adjust for the desired current - with
just light bulbs, the cold start amps are there instantly when
switched on. The resistance of the tungsten wire inside the bulb
changes with temperature, as you would expect, and the cold
resistance is lower than the hot resistance.


snip

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)

I use one of these - 380947 on the lower right. It is a clamp-on
AC and DC ammeter. You just clamp it around a wire and it measures
the current, with no voltage drop. It detects the magnetic field.
The internal detector is presumably a Hall Probe. I used this on
my car last winter, set to DC, turned on Peak Hold, and when the
starter in my car would not turn, it told me the car battery was
supplying 180 amps. I've worked on my central air conditioning,
which is 220VAC, without getting electrocuted. Simply clamp around
an AC wire feeding the compressor or fan motor. And on PCs, you
can take all the red wires on the ATX 20 pin cable, put them inside
the clamp jaws, and the meter adds the current in the wires together,
giving you total DC amps through the wires. Very handy, fast to
connect to the problem area, and non-contact. The only thing you
cannot do, is connect the meter around an AC power cord, as
the current flow in the two wires flows in opposite directions
and the magnetic fields cancel - to measure AC appliance power,
you need access to _one_ of the wires in the cord:

http://www.extechproducts.com/produc...41_942_947.pdf



Chris

Another parameters you can measure is rail noise. But that is a
hard measurement to make, and requires great care in how the
oscilloscope is connected. The problem is, the switching of the
power supply, gets coupled into the test leads, and makes the
noise look worse than it is.

You can also look at transient response (look at voltage when
current is 10 amps, then increase current flow to 20 amps
instantly, and see whether the voltage takes a temporary dip).
Again, that would be a hard thing to measure if you don't have
a real storage oscilloscope.

I think a DC constant load, is the cheapest thing you can
construct. Anything else will blow your budget. I use my
PSU load tester, as a quick test for a couple hours, that
the PSU is stable, before I connect it to a new computer.
I designed the resistors to simulate the loading of a
real computer, and 750W is way above what mine does. That
is a lot of heat to get rid of.

Paul