>
> Appologies in advance for a very basic EE question...
>
> What issues should I be aware of if I want to tap power directly from
> the my PC's power supply to drive my breadboard(s)?  The PC provides a
> very convenient source of regulated +5v and +12v, and is easily
> obtained through one of the 4-pin drive connectors.  A cable could be
> very easily made from sacrificed fan taps and 120v line power cable
> (16-18 ga.)
>
> I imagine the distance (approx 5 feet) from the regulator might have
> some issues, but are they serious, and can they be overcome with a
> large decoupling capacitor for the entire breadboard?  If so what
> values would be apropriate?
>
> -p.

Many PC switching supplies require a minimum load on +5V, +12V or sometimes 
both for the switcher to start "switching".  The information is sometimes 
found on a sticker on the supply or if you are lucky and it is a brand-name 
supply you can find a datasheet with Google.

The simple way to make the minimum load is with large power resistors across 
the appropriate rails.  Just remember P=I*I*R or P=(V*V)/R and size the 
resistors appropriately.  Also, don't touch them either because they will be 
hot.

Also, most PC supplies aren't that well regulated.  +/-5% to +/-10% and 
possibly worse.  You will also get a lot of switching noise and its 
amplitude and possibly its frequency will depend on how you load the supply.

I would recommend that you keep your eyes open for a good deal on a nice 
linear bench supply (+5V @ 1 to 5A, +15V @ 1A adjustable and -15V @ 1A 
adjustable is nice, plus some kind of current limiting control is wonderful) 
but in the short term you can use a PC supply.  Just remember it won't be 
the greatest and you may be able to toast a bagel on your dummy load 
resistors...

Rob Young
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