At 07:57 PM 4/25/03 -0500, Picdude wrote:

>AFAIK, the only 12V in the system is the audio, serial port circuitry and
>fans, and I'm not using audio or serial.  The mobo allows for 12V tolerance
>of +/-10%, btw (ie: 10.8V to 13.2V).  I've added a couple fans beyond what's
>on the mobo, and calculated (but haven't measured) around 800mA, which with
>heatsinking (the 7812), seemed to hold out okay.  (But I'm thinking now that
>I can run the external fans right off the automotive supply and let the mobo
>still power the CPU fan).  I may use the audio later, which is why I would
>like to ensure it's designed right, by raising the voltage to 14.5V.

This sounds like one case where you should worry about drop-out because of
the audio stuff.  No matter how good the PSRR on those chips is, if you
have hash on the PSU rail, some of it gets into the audio.

I would do just as you have suggested: move the fans directly to the raw
(but filtered) auto 12V supply; raise the boost convertor output to 15V,
then use a 7812 or 78L12 to get clean 12V for your audio (and serial)
stuff.  You should not need more than a couple hundred mA total - probably
less.

BTW - I'd move the CPU fan (if 12V) to the auto 12V rail as well - I find
that any of those fans cause significant noise on the rail and that really
can make a difference to the audio noise floor.

dwayne


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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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