Re: PC power supplies

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 10/02/04 

Date: Sat, 02 Oct 2004 14:30:53 GMT

"Vesku" <noveskuspam@geekmail.de.invalid> wrote in message
news:cjmco3$ut$1@phys-news1.kolumbus.fi...
> I bought couple of used PC power supplies to get 12 V supply. Table on
> supply says 8 A with +12 V. But when connect to mains, voltage only 10 V
> without load. When connecting to load that should take 2..5 A with 12-14
> V supply, voltage at +12V goes down to about 6 V and current probably
> only one amp or so (I did not measure)
>
> Manufacturer Zeck, model ZKS-620T, I didn't find spec fron web.
>
> Should there be some load on +5V as well to have +12 functioning
> properly and if yes how much ?
>
> Is it possible to connect two or three separate PC power supplies +5V
> outputt in series to get +10 or +15 V ?
>
> br Vesku
The standard PC supply, does not regulate on the 12v. Instead the 5v rail
controls the pulses fed to the transformer, and the load on the 12v rail
is assumed to be moderately 'proportional' to the 5v load, and the voltage
is therefore held within reasonable limits. With no load on the 5v rail,
the supply will be feeding the smallest pulses i can manage into the
transformer. Hence the behaviour you are seeing.
Most require a reasonable load (perhaps an amp or more) on the 5v rail
before the behaviour can be guaranteed. Often there is a tendency to
oscillate with less load.
On many supplies, it is possible to rewire them internally, so that the
feedback connection is taken from the 12v rail (with a suitable resistive
devider), instead of the 5v rail, which then gives a regulated 12v output.
Yes you can generally 'series connect' a couple of 5v outputs.

Best Wishes

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