http://www.wikihow.com/Convert-a-Com...b-Power-Supply
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That's really cool, I've wondered if that was possible before.
some additional comments from Gale:
I made one also, works great! I used a 200W pc/at power supply & bought used for $5. With a load just on the 12V output, it will read low (10.7V in my case) until I put a load on the 5V side. I opened the case and mounted a few 1-ohm x 25W resistors between +5V and ground and that boosted the 12V output back up to 12V & puts out 8 Amps and is a slick power supply for the price. The resistors get hot so I simply positioned them in the airstream for the case fan.
All of the +12V and +5V wires originate from the same source inside the power supply, so twist a few together for your output and snip off all the rest.
I bought a cigarette lighter socket and mounted it flush with the power supply case. I also put a 12V cigarette lighter socket in my trunk and hard wired it to the battery with a self resetting 20A breaker, and run a Coleman thermal electric cooler (5A draw) on long road trips and use the pc power supply to power it overnight along the way.
The spare socket in the trunk is also a safety for power input should the battery ever go flat & the manual passenger unlock feature gets autistic.
gale
92 735i 5-spd, 176k