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PSU load tester



 
 
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  #1  
Old June 29th 06, 08:40 PM posted to alt.comp.hardware
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Default PSU load tester

Hi all,

I'm soon to be reviewing a new 750W psu but I don't have anything for load..
I have seen heavy mention of wire wound power resistors of 'various values'
but no mention of what those values are. I can get various load resistors
of around 100ohm/10W from ebay.. but oviously 100ohm isn't gonna be
sufficient.. I'm not sure if 10W is heavy enough either..

Any suggestions on the values I should use to create a good load... I'm
talking like 600W+ at least.. and if I can setup multiple banks of
resistors that would be good too so I can have 100W, 200W, 350W, 500W, etc..

Just to be clear I want a _load tester_ not a $10 PSU tester that tells me
if it's dead or alive..

I have a good DMM and will probably buy a second so I can measure V and A
simultaneously.. I also plan on getting a Kill-A-Watt P3 meter.. so the load
tester is the only real concern I have..

Thanks in advance..


Chris


  #2  
Old June 29th 06, 09:19 PM posted to alt.comp.hardware
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Default PSU load tester

On Thu, 29 Jun 2006 15:40:33 -0400, "Skeleton Man"
wrote:

I'm soon to be reviewing a new 750W psu but I don't have anything for load..


Common and readily available load: car headlights.
Either new or from a scrapyard.

--
Kind regards,
Gerard Bok
  #3  
Old June 29th 06, 10:38 PM posted to alt.comp.hardware
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Default PSU load tester

On Thu, 29 Jun 2006 15:40:33 -0400, "Skeleton Man"
wrote:

Hi all,

I'm soon to be reviewing a new 750W psu but I don't have anything for load..
I have seen heavy mention of wire wound power resistors of 'various values'
but no mention of what those values are. I can get various load resistors
of around 100ohm/10W from ebay.. but oviously 100ohm isn't gonna be
sufficient.. I'm not sure if 10W is heavy enough either..



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. 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.

Ohm's law will get you started,

V = I * R (or)

I = V/R

So suppose your PSU is rated for 24A on the 12V rail. The
series resistance to load it to 24A is;

24 = 12 / R

R = 0.5 Ohm

So you'll have to find either a .5 Ohm resistor rated for 12
x 24A = 288W (which is uncommon), or use series and/or
parallel resistors to reach 0.5 Ohm, and based on how many
are in series you calculate out how many watts rating each
would need, except that it is generally good to over-spec
power resistors, you shouldn't really run a resistor rated
for 20W at 20W continuously if you want it to last and be
easier to cool.

Same applies to the other power rails, you just calculate
out what resistance you need to achieve the rated current on
each, and the wattage rating (plus a margin) of each member
in that reistor array.

You may find that eBay doesn't allow the selection of
resistor wattage or resistance you need (want) for best
results. You might also find that it's easier to device the
housing, board, cooling method, etc, and go from there
towards determining what resistors you'd use.



Any suggestions on the values I should use to create a good load... I'm
talking like 600W+ at least.. and if I can setup multiple banks of
resistors that would be good too so I can have 100W, 200W, 350W, 500W, etc..


I'd think about running the resistors at roughly half their
wattage rating... so for 12V rail, "if" you used 10W
resistors you might pick 22Ohm so you had 12(V)/22(Ohm)=
0.55A, 0.55A * 12V = 6.6W each. The number of resistors you
put in parallel then determines how many amps of load on the
12V rail. Suppose you wanted a closer fractional current
addition per resistor, let's say each reistor is additional
0.5A load instead of 0.55A.

12 / 0.5 = 24 (Ohm).

or perhaps you want 1A per resistor,

12 / 1 = 12 Ohm, but to keep some margin you'd want 20W
instead of 10W resistors.



Just to be clear I want a _load tester_ not a $10 PSU tester that tells me
if it's dead or alive..


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.




I have a good DMM and will probably buy a second so I can measure V and A
simultaneously.. I also plan on getting a Kill-A-Watt P3 meter.. so the load
tester is the only real concern I have..



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).

Suppose you had a 0.5 ohm resistor and 0.21V difference in
voltage at one end of the resistor to the other. It matters
not if there are other resistors in series with it (on the
same rail).

V = I * R

0.21 = I * 0.5

I = 0.42A

The best combination of resistors to use can be calulated as
described above but will also depend on what you are willing
to sacrifice in convenience of implementation to get cheaper
parts (going on eBay to find them, for example).

If you want better resistor selections then consider some of
the major online electronics houses like

http://www.alliedelec.com/
http://www.digikey.com/
http://www.jameco.com/
http://www.mouser.com/
http://www.newark.com/

You can sometimes get lower prices at some of the
electronics surplus 'sites but it is hit-or-miss what they
have available. Something like the following might be
convenient, especially if you have some large heatsinks
lying around (or reasonably thick aluminum sheeting, say
0.1" or thicker) and can put it all onto your own PCB
(though if screwed or bolted down I suppose you could
air-wire it instead but it might look a bit messy).

http://www.bgmicro.com/prodinfo.asp?prodid=RES1406

As for switching between different current levels, again
just do the math for how many resistors you'd group together
for the target current, and get some heavy duty switches
rated for this high a DC current. Regular light switches
available at a local hardware store might be as cost
effective as anything but are a bit larger than some
alternative due to the bracket on them, but the bracket does
allow easier mounting if you were to put this all on a board
or something like that instead of a proper chassis.
  #4  
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





  #5  
Old June 30th 06, 03:05 AM posted to alt.comp.hardware
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Posts: n/a
Default PSU load tester

Something like the following might be
convenient, especially if you have some large heatsinks
lying around (or reasonably thick aluminum sheeting, say
0.1" or thicker) and can put it all onto your own PCB
(though if screwed or bolted down I suppose you could
air-wire it instead but it might look a bit messy).
http://www.bgmicro.com/prodinfo.asp?prodid=RES1406


Those do look convenient, but as I will be air-wiring everything, I don't
think it would be possible. I mean there's no way that wire small enough to
attach to the terminals of that package, would handle 1amp, letalone 24A.

That may be my problem in general.. finding and working with wire that will
handle such a high current.. I would assume the 24A from the PSU would be
combined from all the molex connectors and I couldn't draw that from a
single connector ?

Chris



  #6  
Old June 30th 06, 03:18 AM posted to alt.comp.hardware
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Posts: n/a
Default PSU load tester

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

  #7  
Old June 30th 06, 03:28 AM posted to alt.comp.hardware
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Posts: n/a
Default PSU load tester

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
  #8  
Old June 30th 06, 06:04 AM posted to alt.comp.hardware
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Posts: n/a
Default PSU load tester

Skeleton Man wrote:
Something like the following might be
convenient, especially if you have some large heatsinks
lying around (or reasonably thick aluminum sheeting, say
0.1" or thicker) and can put it all onto your own PCB
(though if screwed or bolted down I suppose you could
air-wire it instead but it might look a bit messy).
http://www.bgmicro.com/prodinfo.asp?prodid=RES1406


Those do look convenient,


Not really, you need too many of them for a decent wattage.

You'd be a lot better off with resistance wire or bulbs.

but as I will be air-wiring everything, I don't think it would be possible.
I mean there's no way that wire small enough to attach to the terminals
of that package, would handle 1amp, letalone 24A.


That may be my problem in general.. finding and
working with wire that will handle such a high current..


That wire is clearly in use from the power supply.

I would assume the 24A from the PSU would
be combined from all the molex connectors and
I couldn't draw that from a single connector ?


Yep, thats why there is more than one say 12V wire.


  #9  
Old June 30th 06, 06:22 AM posted to alt.comp.hardware
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Posts: n/a
Default PSU load tester

On Thu, 29 Jun 2006 22:05:21 -0400, "Skeleton Man"
wrote:

Something like the following might be
convenient, especially if you have some large heatsinks
lying around (or reasonably thick aluminum sheeting, say
0.1" or thicker) and can put it all onto your own PCB
(though if screwed or bolted down I suppose you could
air-wire it instead but it might look a bit messy).
http://www.bgmicro.com/prodinfo.asp?prodid=RES1406


Those do look convenient, but as I will be air-wiring everything, I don't
think it would be possible. I mean there's no way that wire small enough to
attach to the terminals of that package, would handle 1amp, letalone 24A.

That may be my problem in general.. finding and working with wire that will
handle such a high current.. I would assume the 24A from the PSU would be
combined from all the molex connectors and I couldn't draw that from a
single connector ?



It is a TO220 part with 0.2" lead spacing. You should be
able to wrap, offhand I'd guess about 10 gauge wire around
it if you had a need to, but you don't, 1 amp is handled
easily by quite small wire. You wouldn't have 1 amp though,
that's only the loss, not what remains through the circuit.

With 10 Ohm 20W resistors you have enough wattage rating
that you would not need to parallel resistors so they'd all
be in series. You'd be wiring from PSU to one resistor, to
the next, etc and so on until PSU ground, with the wire
connecting them carrying the entire current you were testing
(like 24A or whatever). Common 12 ga. copper wire would
suffice, likely even higher but I hesitate to recommend much
higher without knowing what this 750W PSU is, it could have
quite a bit (more) 5V current than 12V.

Just keep in mind that while this small TO220 package can be
convenient, if your heatsink isn't very dense it doesn't
necessarily mean you should mount them all very closely to
each other rather than space out some. Even so, supposing
that 24A figure I pulled out of thin air previously , that's
still 20 of them in series which is already a non-small
sized heatsink. You'd mentioned an amplifier heatsink... I
don't know of any amp heatsinks meant to handle 750W, or at
least not that would be so invaluable to be parted out for a
'sink instead of repaired and used as an amp. You'd be
wanting closer to 10 amplifier heatsinks, or to submerge
fewer heatsinks in liquid or strong fan-forced airflow on
them.
  #10  
Old June 30th 06, 06:48 AM posted to alt.comp.hardware
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Posts: n/a
Default PSU load tester

On Thu, 29 Jun 2006 20:21:07 -0400, "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.


Emphasis on BIG. Normally you wouldn't be able to get just
the slab of aluminum for double your proposed budget, but
for all I know you might have some already.

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..


I think your budget might be unrealistic for building a 750W
load tester, unless you did it like meow2222 suggested and
used some resistive wire and a crude wrap-it-around-bolts or
placed it in a liquid cooling bath. You may need quite a
bit of liquid though, to get a reasonably long test you may
find 750W heats it up quite a bit, as that liquid still has
to disperse it's heat too. For a moment I had a thought of
a bathtub but I'm not going to mention that, it would be
worth a little more work and expense to not have a 750W PSU
or the tester (person) near a large tub full of water.




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..


Well.... you gotta learn sometime if you want to tackle
these kinds of projects.

It's not too hard to cut aluminum though, at least if it's
not very thick you can get by with common woodworking tools
so long as you don't let them clog up.

Mainly it's just about what you can safely decide on to
remove the heat. Pick a method and work on it from there.
It need not be pretty if you don't care what it looks like,
but ideally you'd have something that can be reused and
doesn't short out the PSU (even though any decent PSU should
be able to shut down in such an event, it's not like you
want to risk it if that's avoidable).


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..


Yes, unfortunately, but at least it's a start and the only
reasonable target for the budget.

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...


Well headlamps are typically 110W or less, IIRC, and that
only on the 12V rail. Transistors can have their gain
raised to change current, or produce heat that's sunk away,
be a load themselves.




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 don't know about delicate (circuit board), it would have
to mount a few dozen resistive devices (or whatever), a
giant sized heatsink (or have the sink and board mounted
separately to a chassis and connected at the 'sunk parts),
and be handling 750W. The resistive elements would be quite
hot and their leads would be too so for that reason it
should have traces even larger than what would be necessary
to carry the current. That is, if you wanted it to last
long term.

Even so, it's obviously not a necessity and beyond what
you're willing to tackle for the project.




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).


That right.





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)


Just calculate it out then, that's essentially what a meter
does anyway, it has a precision internal resistance which it
measures the voltage across (and a fuse of course, which
would blow... well, not all meters have the high current
fuse unfortunately, some of the cheapest have only the
mA-range fuse).




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 ?


No, the magnitude might be too high. The frequency range is
a bit low too. Typical scopes work in the several to
several hundred MHz ranges.
 




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