Desktop PC power supply

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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

Re: Desktop PC power supply
says...
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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.

Re: Desktop PC power supply
On 3/2/2017 10:13 AM, MJC wrote:

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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.

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


Re: Desktop PC power supply
says...
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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 ?



Re: Desktop PC power supply
On 3/2/2017 10:50 AM, Ralph Mowery wrote:

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



Re: Desktop PC power supply
says...
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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.


Re: Desktop PC power supply
On Thu, 02 Mar 2017 09:58:26 -0500, Bob Engelhardt

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I'll assume it's an ATX style power supply.
<http://www.corsair.com/en-us/blog/2012/august/80-plus-platinum-what-does-it-mean-and-what-is-the-benefit-to-me

  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:
<http://802.11junk.com/jeffl/pics/drivel/slides/dummy-load-inductance.html
--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Desktop PC power supply
wrote:

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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.  

Re: Desktop PC power supply
On 3/2/2017 11:45 AM, Pat wrote:
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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.

Re: Desktop PC power supply
In sci.electronics.repair, on Thu, 02 Mar 2017 08:13:54 -0800, Jeff

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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:  
https://www.amazon.com/AideTek-multimeter-capacitance-Inductance-self-discharge/dp/B0187LWDBM/ref=sr_1_1?ie=UTF8&qid14%88498176&sr=8-1&keywords=dm4070+lcr+meter
  and
https://www.amazon.com/AideTek-multimeter-capacitance-Inductance-self-discharge/dp/B01N2KDB18/ref=sr_1_2?ie=UTF8&qid14%88498176&sr=8-2&keywords=dm4070+lcr+meter

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.  

Re: Desktop PC power supply
wrote:

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I think I bought it from this vendor probably 3 years ago:
<http://www.ebay.com/itm/322363697016
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).

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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:
<http://www.ebay.com/itm/252290685197
<http://www.banggood.com/M328-LCD-12864-Transistor-Tester-DIY-Kit-Diode-Triode-Capacitance-LCR-ESR-Meter-p-1041588.html
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":
<https://www.google.com/search?q=m328&tbm=isch

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Desktop PC power supply
snipped-for-privacy@cruzio.com says...
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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.




Re: Desktop PC power supply
On 3/2/2017 11:13 AM, Jeff Liebermann wrote:

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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

Re: Desktop PC power supply
On Thu, 02 Mar 2017 19:12:38 -0500, Bob Engelhardt

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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:
<http://h20564.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c03412787
<http://h20564.www2.hp.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-c02781555
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.  

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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.  

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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.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Desktop PC power supply
On 3/3/2017 12:48 AM, Jeff Liebermann wrote:
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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.

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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.

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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.


Re: Desktop PC power supply
On Fri, 03 Mar 2017 01:46:08 -0500, Bob Engelhardt

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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.  
<http://802.11junk.com/jeffl/pics/drivel/slides/kill-a-watt.html
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.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Desktop PC power supply
On 3/3/2017 11:31 AM, Jeff Liebermann wrote:
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I checked my Kill-a-watt against my Fluke "True RMS" clamp-on.  They  
read the same current, so that gives me a lot more confidence in the  
Kill-a-watt.

Now, for the "I'm feeling really stupid" part: in the maze of cables  
under my desk, I had put the Kill-a-watt on the wrong one!  I was  
measuring the power on everything: PC, printer, modem, etc.

When I used the right cable & measured just the PC, the power was  
155W+-.  Versus the 200W previously.  So my PSU efficiency is 33%+-  
(54/155).  Better than the previous/wrong 25%, but not dramatically so.

Thanks to all the commenters.

Bob


Re: Desktop PC power supply
Bob Engelhardt wrote:


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I think it HAS to be that the Kill-a-Watt is not measuring a non-linear load  
properly.  If the PS was burning 150 W internally, a small fan would NOT be  
able to cool the hot components effectively.  Saying that 2 75 W bulbs in  
the housing would not burn up is not germane.  There are several components  
in the PS that will be generating all the heat.  Switching transistors,  
diodes and maybe transformers are what will get hot.  Real Kill-a-Watt  
meters are SUPPOSED to properly measure non-linear loads up to some  
reasonable crest factor, like 10.  Probably most non-PFC corrected power  
supplies won't be too much worse than a crest factor of 10 (I hope).

So, you might check this with a real electromagnetic wattmeter or a Kill-a-
Watt that is known to handle non-linear loads accurately.

Or, if you want to go nuts, get a digital scope or other scope, digitize the  
current waveform and numerically integrate the true power draw.

Jon

Re: Desktop PC power supply
On Thursday, March 2, 2017 at 6:58:36 AM UTC-8, Bob Engelhardt wrote:
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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.

Re: Desktop PC power supply
On 3/2/2017 7:34 PM, whit3rd wrote:
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Very modest machine.  And I'm much more confident about the output  
measurements.

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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.




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