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From: Grumps on 1 Apr 2008 08:47
Hello

I'm trying to find a PSU that has a very low minimum load requirement for
the +12V rail. This isn't for a PC, but the equipment does have an ATX power
connector and uses 5V (10A), 3.3V (15A), but normally nothing on the +12V
(although equipment options can put this up to 4A).

In my experimenting, a couple of hundred mA on the +12V is enough to get the
PSU running ok, but all of the datasheets that I've managed to find suggest
1A min load.

Any suggestions?
Ta.


From: Angelo Campanella on 1 Apr 2008 10:12
Grumps wrote:
> In my experimenting, a couple of hundred mA on the +12V is enough to get the
> PSU running ok, but all of the datasheets that I've managed to find suggest
> 1A min load.

The few hundred mA shows what is needed to keep the 12v value under
control (it will go higher of the drain is zero). The true minimum
current value will rise, usually due to power supply heating under heavy
other loads, or a hot ambient environment. The 1A value in the specs is
put there just as CYA by the vendor so that the 12v regulation value
will never change for any load including zero external (1A + whatever
you load it).

If the 12v you require need not be teribly well regulated, then you
fractional amp load is good enough. If you demand excellent regulation,
then add a few 100 ohm 2 watt resistors across the 12v terminals. Do
supply a minimum load.

The only way to determine whether you are heading for a problem with a
tiny current load is to hook up a DC voltmeter on the 12v terminals with
no load at all. Raed the value exactly. Then let the power supply run
for a long period of time, preferably with a normal or a high load by
other components and in a warm environment if possible. Let all get
totally warm, then read the voltmeter exactly. If the voltage is
acceptable, then all is OK. Likewise for your few hundred mil load. If
the 12v drifts too high, ot possibly develops noise, then the external
load is reaquired.


Angelo Campanella

From: Grumps on 1 Apr 2008 10:39
Angelo Campanella wrote:
> Grumps wrote:
>> In my experimenting, a couple of hundred mA on the +12V is enough to
>> get the PSU running ok, but all of the datasheets that I've managed
>> to find suggest 1A min load.
>
> The few hundred mA shows what is needed to keep the 12v value under
> control (it will go higher of the drain is zero). The true minimum
> current value will rise, usually due to power supply heating under
> heavy other loads, or a hot ambient environment. The 1A value in the
> specs is put there just as CYA by the vendor so that the 12v
> regulation value will never change for any load including zero
> external (1A + whatever you load it).
>
> If the 12v you require need not be teribly well regulated, then you
> fractional amp load is good enough. If you demand excellent
> regulation, then add a few 100 ohm 2 watt resistors across the 12v
> terminals. Do supply a minimum load.
>
> The only way to determine whether you are heading for a problem with a
> tiny current load is to hook up a DC voltmeter on the 12v terminals
> with no load at all. Raed the value exactly. Then let the power
> supply run for a long period of time, preferably with a normal or a
> high load by other components and in a warm environment if possible.
> Let all get totally warm, then read the voltmeter exactly. If the
> voltage is acceptable, then all is OK. Likewise for your few hundred
> mil load. If the 12v drifts too high, ot possibly develops noise,
> then the external load is reaquired.

Thanks, but that's not the whole story.
I have a few PSUs here (good makes, and no-names). If the +12V has no load,
then the +3.3V can drop to 2.5V. Increasing the 12V load gradually brings
the 3.3V rail up to spec.

I have managed to find a PSU with no minimum load requirement. BluTek Power,
are they any good?


From: Angelo Campanella on 1 Apr 2008 21:54
Grumps wrote:
> I have managed to find a PSU with no minimum load requirement. BluTek Power,
> are they any good?

I have no idea. I am proceeding just on old design experience. The only
way to find out is to run some load tests on them.

Ang. C.

From: kony on 2 Apr 2008 03:34
On Tue, 1 Apr 2008 13:47:10 +0100, "Grumps"
<grummps(a)hotmail.com> wrote:

>Hello
>
>I'm trying to find a PSU that has a very low minimum load requirement for
>the +12V rail. This isn't for a PC, but the equipment does have an ATX power
>connector and uses 5V (10A), 3.3V (15A), but normally nothing on the +12V
>(although equipment options can put this up to 4A).
>
>In my experimenting, a couple of hundred mA on the +12V is enough to get the
>PSU running ok, but all of the datasheets that I've managed to find suggest
>1A min load.
>
>Any suggestions?
>Ta.
>

You're wanting a PSU that regulates based on mostly, if not
solely, 5V voltage level. That was more common a few years
ago when PSU were designed to accomodate systems using more
5V current than 12V current. There are ways to hack a PSU's
feedback to make it do what you want but they are a bit
complex to describe in a usenet post, particularly not
having the unit here to reverse engineer.

Mainly, if your PSU mostly regulates from 5V rail feedback,
the 12V voltage level can float high and it wouldn't be a
problem, until that voltage gets to a level excessive enough
that it trips the overvoltage protection circuit. You'd
need to add a load to the PSU you want to use if it can't
run with that much of a 5V load before raising the 12V
current too much. Similarly, a PSU with a very low 12V
current rating (relatively, compared to some of today's PSU
that have 16 or 18A, even multiple times this much) compared
to it's 5V current rating would require less if any load on
12V to keep it under the protection shutoff threshold.

1A minimum load is for ATX regulation spec, you would be
able to make due with a lesser load, I suggest trying
something closer to a 47 Ohm, 10W power resistor you bolt to
the inside of the PSU case, preferribly near an airflow
path. That'll give you a quarter amp load, which as you
mentioned is close to that couple hundred mA you found
results in acceptible operation.
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