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#11
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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