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.

If you want to use ready made Rs I'd suggest a metal container filled
with water. The water will sink 750w for a while. And if it gets hot,
just pour more cold in.

However I wouldnt buy those. I'd get a reel of resistance wire,
constantan, and make the right resistances with that. Its easier,
quicker and cheaper. Just string the length of wire you need between 2
bolts with wires attached, and bear in mind the wire can get red hot,
so must be mounted sensibly.

A quick look at a 1kw electric fire will give you a ruogh idea what
you'll be dealing with when you've got most of 750w coming out of it,
and what sort of construction you need to survive it.


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).

sounds like it'd be too hard for you to switch your loads fast, it
would mean some electronics. Then again there is another way... make
one o your load Rs non inductive, which you do by avoiding loops or
coils, and connect it to the psu via a bare wire end resting on a rough
surfaced copper strip (eg slating tingle from builders merchants).
Running the bare wire end gently and quickly along the rough strip as
you test will create rapidly changing load currents. Basic but should
do the job just fine. Quickest way to make rough copper is get some
very coarse sandpaper and a mallet, and whack the sand into the copper
a bit.


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).

0.67 ohm is correct


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

another way is to use several identical resistance in parallel, and put
the meter in series with just one of them. This may suit you well.


I suppose analogue meter movements still only handle 10A too ?

pocket multimeters usually yes, dedicated ammeters come in whatever
range you want. Look for 'ammeter' on ebay. Age doesnt matter. Avoid
anything described as a 'hot wire ammeter' but youre unlikely to find
one of those antiques.


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)

Sounds like a good idea. Theyre not perfect but will give you a lot
more info than no scope. The sound input would go to the +5 or +12 line
via a small capacitor, and the monitoring puter's ground goes to the
psu 0v line.

I like Konstantin Zeldovich's scope, osc251, for which google. Theres
also the BIP scope. Both are free and need no external hardware.


NT