Thread: PC Power supply
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Rod Speed Rod Speed is offline
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Default PC Power supply

"Paul" wrote in message
pinnerite wrote:
The power supply in my home build low profile HTPC has become very noisy.

It is a Zalman ZM500-GT 500W and very good until recently. I want to
replace it with a low noise job but do not want to spend more than I need
to. Very few of those advertised on the Internet quote how many dbs to

Does anyone have any knowledge of what might be a good choice?


Somebody makes a fanless 700W supply, but I somehow
doubt you need all that power for an HTPC. I'm
showing you the first one, to show how the science
has advanced. At one time, the highest for this
sort of design, was around 500W. But they're past
that now, by a tiny bit.

+3.3V@20A, +5V@20A, +12V@58A, , +5VSB@3A

+3.3V@20A, +5V@20A, +12V@37A, , +5VSB@3A

Using a Kill-O-Watt meter, you can check your mains
consumption on the HTPC, while it is performing a
representative workload, and get some idea
exactly what class of load this is. Maybe the thing
is drawing 200W, and either supply would work.

It's a bit more tricky than that. With my PVR which has
a digital TV capture card which can do 4 multichannel
streams, the PC would shut down when 4 were recording
at once, but not with 3. Fixed with a beefier power supply.

For example, my newest PC idles at 100W, and when
both the GPU and the 156W CPU are engaged, it can
go up to 400W. But, if I were engineering for a
light-dedicated application (like running TV tuners
but not transcoding), then 200W would cover it.
When my video card transcodes, it only draws 60W of
the 180W max.

The PSUs above, like with normal PSUs, have a combined
rating on the two low rails.

+3.3V@20A, +5V@20A
\--- 100W max ---/

That means, if the +5V rail goes to max, the
3.3V rail can't supply any current :-) In real
life, it is expected neither rail goes to max,
and the combined is around 50W or so. Some other
PSUs, the combined rating is 130W or so, and that's
the kind I buy. But then, I do want the fanless,
so I have to make sure that my low-rails don't
draw too much. Seasonic uses that 100W limit
on some of their fan-equipped PCs, so this is
"just a Seasonic thing". Because the designs
are double-forward conversion, the 3.3V/5V supply
is a separate card that runs off 12VDC from the
main converter.

Mains -+-- 12VDC ---------
| |
| 3.3V/5V ------- Double forward
| ------- conversion
+--- 5VSB --------- Standby converter

The highest low-rail load I've had, was 5V @ 25A
on an Athlon system plus an AGP video card. Back
in the day, they made ATX PSUs with up to 5V @ 40A
for that reason. But today, 3.3V and 5V consumption
is not even close to that. A clamp-on DC ammeter
allows hobbyist engineering of low-power PCs (allows
exact measurement of loads per rail). The Kill-O-Watt
method, is a "lazy guy approximate method", to establish a
load class and give a very rough ballpark of how
challenging the load is.

At 230V mains, the efficiency at 50% load on those
is fantastic. Ballpark 95-96% (assumes most load is
on +12V rail, less loading on 3.3V/5V). With double
forward conversion, the PSU "looks better" if you
only pull +12V from it :-)