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Bench power supply build from ATX power supply acceptable voltage Answered

Hi,
I built a bench  power supply from an ATX power supply and I am not sure if the voltage is correct.
I am getting 5.74 volts off of the 5 volt supply and 12.7 volts from the 12 volt supply. Is that too high to use or should I try to bring it down a little? When I built the supply I didn't have a 10 ohm power resistor so I used a 6 ohm 10 watt resistor.
Could this be the reason for the higher voltage?

Thanks for any help you can give me.

Dan

9 Replies

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Gelfling6 (author)2012-02-06

that 6-Ohm resistor must be getting pretty hot, and I'm no expert, but it might be why you're getting the higher voltages.. (though, <1V is not critical.).. though, your volt meter might be registering higher, too.. (most store bought meters aren't calibrated at the factory.) I have a Fluke meter that ranges the other direction, with a stock 300Watt supply that had a soldered-in (on-board) 33-Ohm, 5W carbon-film resistor across the +5V. Output on the +5 is roughly 4.87, +12 is roughly 11.90...

check the +3.3V.. if it runs higher, check it with a different meter. there, is where you have some critical because the 3.3v chips would run a greater risk of burn-out..
(I.E. If it's running 4V, I'd say it's not a good thing to try running a MP3 player that runs on only 2 AA batteries, because the 1V over, would cause problems.)
Hope this helps a little..

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Gelfling6 (author)Gelfling62012-04-28

And this comes from a geocacher, who accidentally fried a Magellan Explorist-500LE, by plugging a 9V wall-wart supply (which was supposed to be for a cheapy Net-Book) into the power cord to recharge the Explorist (which the regular wall-wart supply, was supposed to be 5V.) (Oh-yeah, I now have an ancient, $100+ brick now..)

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steveastrouk (author)Gelfling62012-04-28

There's a vague chance you only blew the input fuse on the PCB - worth a shot.

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snips56 (author)steveastrouk2012-10-02

@steveastrouk assuming that that is what it is, what would you estimate is the cost to correct?

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steveastrouk (author)snips562012-10-02

pennies The works in taking it apart and looking.

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Gelfling6 (author)steveastrouk2012-07-16

I'll have to check.. No ideas where to look inside the explorist.. Hope someone posted something on the net, and it's 'Google'-able.

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snips56 (author)Gelfling62012-10-02

Did you find out anything further about that? I did the same thing as the power supplies got mixed up when we were moving and I ended up plugging in a 9v to the charger cord and the unit won't start up now. Also, @steveastrouk assuming that that is what it is, what woudl you estimate is the cost to correct?

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FrankBlourtango (author)2012-07-17

Your voltages will be high with no load. Don't worry about that at all. And with your resistor, just make sure you have a hi watt one. When I did my power supply out of a PC power supply I got some gnarly ceramic ones from Radio Shack. Your power supply is fine, have at it!

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mpilchfamily (author)2012-02-08

The power supply itself may be keeping the rails a bit high while there is no load. Try plugging in a fan and then see what the power supply reads. Some cheap old and/or old PSUs may have trouble regulating the power so it could be higher or lower then what it should be. The average life of a PC power supply is considered to be about 5 years. The older the PSU gets the less effective the capacitors, that help smooth out the voltage, will be. Not good for a PC but fine for a bench top PSU. But the PSU should keep the voltages at around +/-5% of there rated voltages. if your reading more the +/- 10% then i wouldn't use it. For now your readings are within reason.

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