Re: Info on Jumping a PC power supply?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Mike Pogue <mpogue@...> wrote:

>
> Sam...,
>
> I believe that this is only the case, if you don't meet the

minimum

> loading requirements on the various output lines, which includes
> avoiding "crossloading". Crossloading happens when you pull a lot

of

> current from the +5 side, and not a "balanced" amount from the +12

side

> (or vise versa).
>
> If all the minimum loading requirements are met, and there's no
> cross-loading, I'm not sure why using an ATX supply wouldn't work.
> (It may be hard to achieve this goal, however).
>
> The appeal, of course, is that ATX supplies can be so darn cheap

(<

> $20 US), and produce so much current (more than would be needed for
> small motors). I think it would be hard to find a brand-new,

assembled

> linear supply for that price. (If you are aware of one, please let

me

> know!)
>
> Mike
>
> sam3216549872003 wrote:
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffspinner"

<j.spinner@>

> > wrote:
> >
> >>I'm trying to jump out a pc power supply to run some stepper
> >
> > motors.
> >
> >>Does anyone know where I should install jumpers to power up a
> >
> > supply
> >
> >>that is used on a desktop PC?
> >>
> >
> >
> > Unfortunately a computer power supply is not a good choice for

any

> > thing other than a computer.
> >
> > Although I have heard of several people using computer power

supplies

> > for things other than a computer. This is never a good idea.
> >
> >
> > The problem is that a computer power supply regulates the voltage

on

> > only one output using a PMW controller, either the 5V or 12V

output.

> >
> > This means that if you are using the 12V, and it is regulated on

the

> > 5V output your available current will drop very quickly.
> >
> > If it is regulated on the 12V side, the 5V side will tend to

drift

> > higher and higher.
> >
> > The more current that is pulled from the 12V side the higher the
> > voltage on the 5V side.
> >
> > This can damage the capacitors on the 5V side.
> >
> > On ATX power supplies the 3.3V output is normally derived from

the 5V

> > output.
> >
> > The -12V and -5V outputs are normally linear regulators.
> >
> > You are better off building a transformer power supply.
> >
> > It will give you a much better available current.
> >
> > Sam
> >
> >
>

I understand how a PC power supply works, the more current you pull
from the regulated side, the more available current you will have on
the other side. I.e. if the voltage is regulated on the 5 v, side
and you need 10 amps at 12 V, you will need to pull an appropriate
current from the 5 v side.

It also means if your stepping motors are not energize for some
reason your 12 vote side might go up as high as 20 v or more. That
is if you have removed the 15 vote Zener Diode that limits the
voltage output, otherwise the power supply may shut down. You will
then have to shut the power supply down and restart it.

Also the electronic efficiency of these power supplies when used in
this manner will drop considerably. As much as 50% of the wattage
will be lost to heat.

Alternatively you could disassemble your power supply, remove
unnecessary circuits, and re-bias the regulator to the proper side of
the circuit. This information is available on the web.

Note high voltage exist within these power supplies, 160 V DC or
more. It is not recommended that you disassemble your power supply
unless you're very familiar with the safety procedures required.