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PSU load tester



 
 
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  #11  
Old June 30th 06, 07:41 AM posted to alt.comp.hardware
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Default PSU load tester

On Fri, 30 Jun 2006 15:04:49 +1000, "Rod Speed"
wrote:

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.


Depends on what's being built. It will be a lower heat
density than some alternatives, possibly easier to 'sink
and/or cheaper to 'sink in air rather than liquid.

Having (more) smaller resistance increments also allows a
finer adjustment of load unless there is more to the design
than just the wire or bulbs mentioned below. They are just
an example though, you can get other ohmic values and
wattages from the electronics houses, for example,
http://www.mouser.com/catalog/626/490.pdf

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


It could be cheaper, yes. 750W is a whole lot of light
though, the bulbs and sockets start to add up in cost too.
I'd think the wire the most cost effective but most fiddly
and least professional looking. We don't know how much that
matters though, some people are hung on esthetics and others
aren't.




  #12  
Old June 30th 06, 07:46 AM posted to alt.comp.hardware
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Default PSU load tester

On Fri, 30 Jun 2006 01:22:37 -0400, kony
wrote:


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.



I don't know what I was thinking but the above is clearly
wrong. You would have them in parallel, not series, and yes
1.2 Amp is the applicable current, it would be quite easy
to get wire large enough to handle that. It would actually
be harder to work with wire so small it couldn't handle 1
amp, you'd be up around 30 gauge at that point. Even common
CAT5 (24 gauge) would suffice.
  #13  
Old June 30th 06, 10:20 AM posted to alt.comp.hardware
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Default PSU load tester

kony wrote
Rod Speed wrote
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.


Depends on what's being built. It will be a lower heat
density than some alternatives, possibly easier to 'sink
and/or cheaper to 'sink in air rather than liquid.


The fundamental problem with them is the resistance.

Having (more) smaller resistance increments also allows a
finer adjustment of load unless there is more to the design
than just the wire or bulbs mentioned below. They are just
an example though, you can get other ohmic values and
wattages from the electronics houses, for example,
http://www.mouser.com/catalog/626/490.pdf


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


It could be cheaper, yes. 750W is a whole lot of light though,


Sure. I've got some 500W halogens and you need the sunnys |-)

the bulbs and sockets start to add up in cost too.
I'd think the wire the most cost effective but most fiddly and
least professional looking. We don't know how much that matters
though, some people are hung on esthetics and others aren't.


Sure, but the other advantage with bulbs is that
they handle the watts fine without anything special.


  #14  
Old June 30th 06, 10:21 AM posted to alt.comp.hardware
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Default PSU load tester

kony wrote:
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.


Nope, they're fine on the other rails too.

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.



  #15  
Old June 30th 06, 07:11 PM posted to alt.comp.hardware
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Default PSU load tester

On Fri, 30 Jun 2006 19:21:42 +1000, "Rod Speed"
wrote:



Well headlamps are typically 110W or less, IIRC, and that only on the 12V rail.


Nope, they're fine on the other rails too.



Yes, but I meant using _one_ headlamp, was why I'd mentioned
the upper wattage I'd seen per one. It's going to be quite
the contraption to hook up enough headlamps to put a full
load on all rails of a 750W PSU, particularly the 3.3V and
5V rails. That doesn't make it undo-able but not so
convenient either unless one worked for a place that makes
or sells headlamps.



  #16  
Old June 30th 06, 07:22 PM posted to alt.comp.hardware
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Default PSU load tester

On Fri, 30 Jun 2006 19:20:06 +1000, "Rod Speed"
wrote:

kony wrote
Rod Speed wrote
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.


Depends on what's being built. It will be a lower heat
density than some alternatives, possibly easier to 'sink
and/or cheaper to 'sink in air rather than liquid.


The fundamental problem with them is the resistance.



Yes it would take more of them. That's not really a problem
though, and as I'd written it means lower heat density. If
we want to find problems, any method is less than perfect in
one way or another.




Having (more) smaller resistance increments also allows a
finer adjustment of load unless there is more to the design
than just the wire or bulbs mentioned below. They are just
an example though, you can get other ohmic values and
wattages from the electronics houses, for example,
http://www.mouser.com/catalog/626/490.pdf


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


It could be cheaper, yes. 750W is a whole lot of light though,


Sure. I've got some 500W halogens and you need the sunnys |-)

the bulbs and sockets start to add up in cost too.
I'd think the wire the most cost effective but most fiddly and
least professional looking. We don't know how much that matters
though, some people are hung on esthetics and others aren't.


Sure, but the other advantage with bulbs is that
they handle the watts fine without anything special.


Yes that's a benefit, though you also wont' have any
assurance of what the wattage will be. Supposing best
wattage per cost were 55W or so and you wanted 24A of12V,
are they actually 55W at 12.0V or closer to 13.x V? They're
not a linear resistance either, change as the element heats
which is good for the lower voltages, possibly needing fewer
bulbs, but still quite a few bulbs.

What does each bulb cost though? Probably about $10 each
unless using the older bulky types with a front lens but
that will make this a giant contraption. Then again it
might be possible to muck around in a junkyard to get some
bulbs cheap but it does start adding onto the amount of work
to complete the project.
  #17  
Old June 30th 06, 08:04 PM posted to alt.comp.hardware
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Default PSU load tester

kony wrote
Rod Speed wrote


Well headlamps are typically 110W or less,
IIRC, and that only on the 12V rail.


Nope, they're fine on the other rails too.


Yes, but I meant using _one_ headlamp, was why
I'd mentioned the upper wattage I'd seen per one.


Still cant see what you are getting at there. If anything bulbs
give you more flexibility since you can plug in a variety of
different bulbs to get the load you want on a particular rail.

It's going to be quite the contraption to hook up enough
headlamps to put a full load on all rails of a 750W PSU,
particularly the 3.3V and 5V rails.


The resistance of the bulb wont change that much, if
anything the ones running on the the 3.3V and 5V rails
will have a lower resistance than they will on the 12V rail.

That doesn't make it undo-able but not so convenient either
unless one worked for a place that makes or sells headlamps.


Dunno, in some ways bulbs are more flexible since you
can just plug in different wattage bulbs as required.


  #18  
Old June 30th 06, 09:23 PM posted to alt.comp.hardware
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Default PSU load tester

kony wrote:
On Fri, 30 Jun 2006 19:21:42 +1000, "Rod Speed"
wrote:


Well headlamps are typically 110W or less, IIRC, and that only on the 12V rail.

Nope, they're fine on the other rails too.



Yes, but I meant using _one_ headlamp, was why I'd mentioned
the upper wattage I'd seen per one. It's going to be quite
the contraption to hook up enough headlamps to put a full
load on all rails of a 750W PSU, particularly the 3.3V and
5V rails. That doesn't make it undo-able but not so
convenient either unless one worked for a place that makes
or sells headlamps.


You don't need a "headlamp", all you need are the bulbs! Old time
headlights,(and taillights), did use separate bulbs. Finding suitable
sockets might be a chore, but you can easily solder 12 gage wire to the
bulbs. You don't need that size for the current, but it's stiff enough
to hold the bulbs where you want. Left "floating" in the air, they will
easily dissipate their rated wattage. Old fashioned "knife" switches are
ideal for connecting as many lamps as required in parallel, and they
have terminals, so you don't even have to solder!

You can still find 6V lamps if you like to "see the light" on the 5V
rail. :-)

Google for "automobile lamps" to find amperage specs. The "inrush
current" of ~1A taillight is ideal for simulating the 12V start-up of a
hard drive motor. (Don't look for the 12V rail droop with a DVM, in
spite of what w_tom says!) ;-)

--
Virg Wall, PE
  #19  
Old June 30th 06, 10:02 PM posted to alt.comp.hardware
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Default PSU load tester

On Sat, 1 Jul 2006 05:04:58 +1000, "Rod Speed"
wrote:

kony wrote
Rod Speed wrote


Well headlamps are typically 110W or less,
IIRC, and that only on the 12V rail.


Nope, they're fine on the other rails too.


Yes, but I meant using _one_ headlamp, was why
I'd mentioned the upper wattage I'd seen per one.


Still cant see what you are getting at there.


For one, the # of headlamps isn't necessarily lower than
other types of resistors, could go either way, lower or
higher. The one thing which seems more certain is it could
end up either costing more, or being very very large.

Suppose a typical 55W headlamp, it's roughly 4.5A at the
slightly over 12V electical system of an automobile. You'll
need at least 6 for the 12V rail alone. Next we need a way
to re-rate the resistance for what it would be (slightly
higher) due to the element not getting as hot from the other
(lower voltage) PSU rails, but for simplicities' sake let's
consider it a constant, 2.7Ohm

750W PSU is bound to have a fairly high 5V current (unless
we were told otherwise, that wattage rating has to account
for it, else we'd need even more amps on a different rail
instead which is still more bulbs). So let's suppose a
fairly modest 30A on 5V rail and 20A on 3.3V rail. Do you
see that we're now talking about a LOT of headlamps and that
this entire setup may require a whole table or more? Sure
it doesn't need heatsinks anymore but it requires it's own
zip code instead.




If anything bulbs
give you more flexibility since you can plug in a variety of
different bulbs to get the load you want on a particular rail.


"Plug in" is easier written than rewired. Yes it can work,
and work fine at that. It does have it's own set of
trade-offs though, and that many headlamps are not going to
be so cheap unless there is a lower-than-normal cost source
for them. Even being conservative and thinking $5 each,
that's well over the budget.

That was only a 55W headlamp, but beggers can't be choosers,
a much higher wattage lamp may cost a lot more too. Again,
these were (@ 55W) about 2.7 Ohm each, so you may be paying
$5 (or more) per 2.7Ohm. Contrast that with 29 cents each
for the first 10 Ohm resistor I'd linked. So you need 4 of
them to reach close to 2.7Ohm, but it's still closer to $1
than =$5.



It's going to be quite the contraption to hook up enough
headlamps to put a full load on all rails of a 750W PSU,
particularly the 3.3V and 5V rails.


The resistance of the bulb wont change that much, if
anything the ones running on the the 3.3V and 5V rails
will have a lower resistance than they will on the 12V rail.


Agreed, but he'll still need to know how much it changes for
determination of load, for # of bulbs to buy, and it's still
a whole lotta bulbs and space to build this to test a 750W
PSU. Maybe that's within the acceptible project needs for
the OP, I don't know... but if he has a way to heatsink
something else, it will tend to be more expensive than other
alternatives.



That doesn't make it undo-able but not so convenient either
unless one worked for a place that makes or sells headlamps.


Dunno, in some ways bulbs are more flexible since you
can just plug in different wattage bulbs as required.


Does he have different wattage bulbs? I don't recall this
being introduced into the thread yet, and wouldn't expect
the average person to have several different wattages of 12V
bulbs lying about, at least not more than their own
automobiles/etc needed. Of course he can buy them but then
the costs keep going up.
  #20  
Old June 30th 06, 10:37 PM posted to alt.comp.hardware
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Default PSU load tester

On Fri, 30 Jun 2006 17:02:55 -0400, kony
wrote:


Suppose a typical 55W headlamp, it's roughly 4.5A at the
slightly over 12V electical system of an automobile. You'll
need at least 6 for the 12V rail alone. Next we need a way
to re-rate the resistance for what it would be (slightly
higher) due to the element not getting as hot ...



snip

I meant " .. would be (slightly LOWER) due to the element
not getting as hot.
 




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