Desktop PC power supply

[Jon Elson]

Bob Engelhardt wrote:

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 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

 

[Bob Engelhardt]

On 3/3/2017 11:31 AM, Jeff Liebermann wrote:

On Fri, 03 Mar 2017 01:46:08 -0500, Bob Engelhardt wrote:

Unless my Kill-a-watt is wonky.

That's a possibility. I have several kill-a-watt meters. ...

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

 

On Thu, 02 Mar 2017 09:58:26 -0500, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

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

I had an older Pentium computer that came with a 100W PS. It worked fien
until I added more RAM and an extra Harddrive. Then it would randomly
crash or just do a reboot for no reason. Shortly afterwards, the PS just
died. I bought a 350W PS and never had another problem with that
computer. It's called RESERVE POWER. Under normal use, it only uses 54W,
but add a few drives and run it real hard, and the power use demand
rises.

This is about the same as an audio amplifier. You may have the output
tubes or transistors that will pump out 100W, but if your power supply
cant furnish the reserve power on high demand heavy bass loads, you will
hear a lot of distortion, or lose power, or worse....

 

[Michael A. Terrell]

oldschool@tubes.com wrote:

I had an older Pentium computer that came with a 100W PS. It worked
fine until I added more RAM and an extra Hard drive. Then it would
randomly crash or just do a reboot for no reason. Shortly afterwards,
the PS just died. I bought a 350W PS and never had another problem
with that computer. It's called RESERVE POWER. Under normal use, it
only uses 54W, but add a few drives and run it real hard, and the
power use demand rises.

It's called 'Not using under rated parts'. That 100W was marginal,
at best when the computer was built. That 350W rating may, or may not be
the real power it can deliver.


--
Never piss off an Engineer!

They don't get mad.

They don't get even.

They go for over unity! ;-)