On 3/2/2017 11:13 AM, Jeff Liebermann wrote:

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
"Intel’s ATX specification only requires that a 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:
http://802.11junk.com/jeffl/pics/drivel/slides/dummy-load-inductance.html


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