Desktop PC power supply

I have an old PC that recently lost its PS. An identical one was hard to find, but I didn't know the PC's power requirements, so I couldn't use a generic PS. I should say that I thought I couldn't use a generic one.

I measured the PC's power use so next time I'll be ready. It was quite surprising - the PS is "400W", but the PC only uses 54W. And there's

40A of 5V available, but only 5A are used. Etc.

The biggest surprise was the PS's efficiency, or lack thereof. A Kill-a-watt on the input showed 200w being used, and 54w output. 25% more or less. I thought that a SMPS would be way more efficient than that.

Bob

Reply to
Bob Engelhardt
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The energy has to go somewhere. If your measurement is to be trusted (real RMS?) then the PSU fan must be working like a hairdryer!

And what do you mean "lost its PS"? It vanished in a puff of smoke, leaving not even a specification plate behind?

Mike.

Reply to
MJC

I don't have a Kill-O-Watt but do have a meter similar to it. That one reads out Volts, Amps,and Watts. I have noticed that if I hookit to a resistive load like a heat gun the volts and amps are very close to the watts. If I hook it to a reactive load there is a big difference in the Watts vers the volts times amps. That I understand.

I have not paid any attention to a switching supply that way. Is it possiable the the watts and the V times I are not the same for your meter ?

Reply to
Ralph Mowery

I'll assume it's an ATX style power supply.

is 60% efficient at 50% load. Most decent quality power supplies made in the last decade are around 70% efficient at 50% load."

If you're getting 25%, either something is wrong, but I can't tell if it's your measurements, a defective design, a failed power supply, or how you're measuring the efficiency. I've made similar measurements of an ATX power supply using a dummy load on the +5v and +12V lines, and a Kill-a-watt meter. Typically, I obtained 75% efficiency at about 50% of full load on a cheap ATX PS.

As MJC mentioned, the heat has to go somewhere. If your efficiency at at 50% of maximum load (200 watts) is only 25%, then you should have

150 watts of hot air and flames coming out the back end of the PS with the temp controlled fan spinning furiously. Something is wrong.

Photo of one of my loads:

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

I was going to respond to the OP, but Jeff beat me to it. Switching supplies are notorious for having very poor power factors under light load. I looked up the specs on the kill-a-watt and it is supposed to read power factor correctly when seeing a pure sine wave. Of course, there is no guaranty the current waveform is a pure sine wave. It is more likely to have spikes all over the place. (That's one of the reasons switching supplies can cause RF noise.) My guess (and that's all it is) is your supply is using under well under 100 watts, but your kill-a-watt is confused by the strange current waveform and says it is using 200 watts.

Reply to
Pat

Wow. I want one of those.

The meter I mean.

It's a shame I have no use for it. In all these years I've never wanted to measure inductance and I've never needed to measure capacitance.

But what is the diff between these two things:

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and
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Same brand, same model, but one is 2 1/2 times the price!

Interestingly, the more expensive one takes much longer to ship, has no ratings, and only one picture, compared to 7. Different vendor. How do they hope to sell it. (I didnt' mean these differences. I thought maybe they're not actually the same.)

They both save Vici in the upper right but afaict, yours doesn't.

Reply to
micky

It would take a very long time to dry your hair with it. 150w vs 1500w in hair dryer. Yeah, the PSU exhaust is warm, but 150w is pretty easy to blow away.

It died. The problem was matching ALL the name plate specs. There are a lot of combinations.

Reply to
Bob Engelhardt

I assumed that the Kill-a-watt does true rms, but possibly not.

Reply to
Bob Engelhardt

I measured the current (inline meter) on all the MB power lines and the hard drives power lines. As I posted, the input was from a Kill-a-watt. One divided by the other for efficiency.

Although the PSU is using 200w, it's rated for 400w output, so my 54W is only 13% of rated power. (The name plate input is 10A at 115v). Maybe efficiency goes to hell for really small loads.

My intuition is that 2 75W light bulbs in a box could be cooled by a PSU fan. Without flames.

It's kinda wonky, but not crazily so.

Bob

Reply to
Bob Engelhardt

So, "maybe, maybe not" on the Kill-a-watt number. My scope is packed away waiting for its once-a-year use or I'd look at the PSU input current.

Reply to
Bob Engelhardt

Unless your machine is a loud-fans-and-multiple-XEONs type, 54W is a likely power draw, and 200W is unlikely.

It is possible (on old Apple computers, it's common) to repair a dead power supply. They're cramped and sometimes have unlabeled tiny chips, but a check for burst capacitors and shorted rectifiers, and of course fuses, will often bring 'em back to life.

If you find shorted power FETs, be aware that there's a gate drive which might have failed, too; those repairs get complicated.

Reply to
whit3rd

Very modest machine. And I'm much more confident about the output measurements.

I did actually try to repair the old one. I don't remember many specifics except that when I got a big spark and a burned trace I gave up.

Reply to
Bob Engelhardt

** A Kill-A-Watt reads "watts", power factor and "VA".

What does yours give for PF & VA ?

..... Phil

Reply to
Phil Allison

I did hook up the meter I have and the volts times amps was showing 120 watts but the power in watts was showing about 75 watts when hooked to a switching power supply. Did not measure the output to the load.

Reply to
Ralph Mowery

I didn't have time for a proper test today. I just stuffed the kill-a-watt meter into the bench power strip and used it to power an HP 8200 Elite SFF desktop. Something like this with an i3 CPU: I think it has power factor correction for: 320 W active PFC 87%/90%/87% efficient at 20%/50%/100% load. The input power consumption varied from 30 watts to 70 watts depending on what the machine was doing. Eyeball average was about 50 watts. Playing a video burned 70 watts. The power factor varied from 0.90 to

0.96. However, I was unable to obtain VA and watt reading pairs to determine if the PF was for real.

However, I had no way to measure the current at the motherboard and drives. I also couldn't find my ATX dummy load. I'll throw something together this weekend if I have time.

I don't use light bulbs as loads (except for UPS testing). Mostly, I use power resistors and head sinks. My load is intended to simulate a game machine, which typically burns about 350 watts with a high end video card installed. At that power level, I need either a fan or a bucket of water to cool the heat sink.

Well, like I suggested... something is wrong. It's unlikely that your PS has PF correction or is highly efficient. However, it shouldn't be as low as 13%. Something is wrong, but I can't tell where from here.

If you have the time and inclination, you might try testing a known good ATX power supply and see if the numbers magically improve. If the efficiency is fairly high, then the original power supply is broken somewhere. However, if it too shows 13% efficiency, then there's something wrong with either your instruments, or your methodology.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

I think I bought it from this vendor probably 3 years ago: It's a tolerable LCR meter. I bought it for a specific project (inductive vehicle detector) and then bought 2 more (for my office and vehicle).

You must live a sheltered life. Haven't you ever made your own inductors or capacitors and needed to measure the inductance or capacitance? In RF, that's common practice. Even simple things like estimating the length of coax cable or CAT5 by measuring the capacitance and dividing by capacitance per foot, is useful. Then there are loading coils, filters, drifting caps, etc. Also, if you trust the values printed on the parts, you might be in for a rude surprise when you test them.

Incidentally, I also use an earlier version of one of these M328 meters: I prefer a curve tracer for testing and matching active devices, but this is good enough for a quick test. It also does LCR. There are many variations. Google for "M328":

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

It's consistent: VA = 310+-, PF=.69, W=210+-. So, it knows that there's difference & it knows how to divide, but it still might not know how to do true RMS power.

Reply to
Bob Engelhardt

It's highly improbable that your machine is weaker than mine, so your

50W in versus my 200W means that mine is way too high. Unless my Kill-a-watt is wonky.

I just used the light bulbs as a common example of power. I have an intuitive feel for the power of a light bulb & if it could be kept in limit with a fan. Not so much a ceramic power resistor.

My 13% is my 54W draw versus the 400W rating of the PSU.

The PSU name plate ratings: 1200W in (10A), 400W out. I'm using 200W in, 54W out (25% efficient). 54 / 400 = 13% rated load. But even at

12% rated load, the efficiency shouldn't be 25%.

BTW - I don't really care, I'm just curious.

Reply to
Bob Engelhardt

There are many versions of that device and similar ones. I have one that was already built for about the same price I bouth several years ago. Too bad they did not have things like that around 20 or 30 years ago when I was working on equipment all the time. Retired now and just do electronics for a hobby, but it is nice to be able to get some of the equipment to play with. Especially in the $ 10 to $ 50 range.

Reply to
Ralph Mowery

That's a possibility. I have several kill-a-watt meters. When I upgraded to the latest greatest, I took them apart to determine the difference. Much the same except for the clock crystal, 5 instead of 4 buttons, different uP, and some beefier components.

However, I was having problems with erratic readings with the old model (top photo) which I traced to marginal soldering on the AC power connector pins and current sense "resistance wire". I resoldered everything that looked suspicious and the problems disappeared. You might want to disassemble yours and check for similar soldering problems.

Incidentally, the marginal soldering in the later unit (bottom photo) is stock. I later went through it and resoldered all the rough conections. That's when I found that the ground pin on the AC power plug had a cold solder connection. That should have any effect on the readings, but it does suggest that the soldering problem might be common.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

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