Converting computer power supplies

[dmcole]

Have hit something of a wall and maybe someone can help.

I need a 12-volt DC power source for a mega-LED project (figuring ~10a). Naturally thought of using an old computer power supply. Was given two but when I began looking them over found they weren't ATX power supplies and had weird wiring in the 20-pin connector (lots of brown wires). Opened one up and fiddled around; using the standard web pages as guides (10 ohm, 10 watt sandbar resistor on the 5v line for load -- I particularly liked this one: http://www.instructables.com/id/ATX--%3E-Lab-Bench-Power-Supply-Conversion/), was able to only get it to produce 7 volts on the 12 volt leads.

Frustrated, turned to CL and bought a new ATX PS for $10 yesterday. All the wire colors matched the colors the web pages talked about and wired it up accordingly. Nothing. Ran across an oblique reference to the 3-volt rail needing a load and added that (used a 12-volt auto lamp, because that's what I had laying around). The result was that the fan spins for about a second or two and then stops. Tried moving the sandbar to the 3-volt line and the auto lamp to the 5-volt line and nothing happened (fan didn't move at all).

I'd appreciate any insight into either of my problems -- how to get the non-ATX to deliver a solid 12 volts and/or how to get the ATX to run at all.

Thanks.

\dmc

[Wayne J]

I think maybe one of these sites will help....

http://www.directron.com/2powersupplies.html

http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

I used the second link to build mine.

[budude]

You may need a load on the 5V supply in addition to/instead of the 3.3V supply. Some supplies have sense leads that you can simply tie to the main output but you'll need to know the pinout (for the non-ATX supplies). As for the ATX supplies - I've used two so far and neither required anything more than connecting the PS_ON (green lead) to ground.

[dmcole]

Back in the day when I did end-user support, we techies had a phrase: "NPI syndrome." "NPI" stood for -- stands for -- "not plugged in." First question end-user support has to ask: "Is the cord for the device plugged into the wall and into the device itself?"

Fiddling with the true ATX PS just now, I noticed that it would work when I pushed the external power cord into the male socket on the PS, but then it would then stop. I thought the problem was with the power-sense lead. But then I tried a little experiment -- I wiggled the cord in the socket and it worked. When I stopped wiggling, it stopped working.

NPI.

Turns out this device doesn't need a load on either the 3v or the 5v rails, nor does a sensing wire need to be shorted anywhere. Just ground the green wire and it takes off.

Still would appreciate insights into why the 12v on the non-ATX PS is only producing 7v.

Thanks.

\dmc

[Wayne J]

Still would appreciate insights into why the 12v on the non-ATX PS is only producing 7v.

Thanks.

\dmc


Are you sure you have +12 and 0 together? If you have +12 and +5 together, it produces +7

[budude]

Can I ask a silly question? Are you sure you are measuring the 12v from ground? Just wondering if you are measuring from the 5v supply by any chance? Kind of a quinky dink that 12-5=7..... It doesn't really make sense that it sits ON at 7v - especially that you have two doing the same thing... Do you have a part number for the supply - a deep Googling might find it.

D'OH - Wayne beat me to it...

[Matt_Edwards]

i don't think you will get 10Amp out of the 12V lines on a ATX PS without further mods or wasting a lot of power. 12v @ 10A equates to 120W.

The main regulation on PC PS is done with 3.3 or 5 volts rails. That is why you are only seeing 7v on the 12v rail. increasing your regulated load will increase the 12V rail. Assuming your PS is spec'd at more than 10A for 12V, you nay need to add a couple of R47 100W resistors on the 5V rail or a couple of R22 100W resistors on the 3.3V rail. Either way you are going to dump a lot of wasted energy.

the above load resistors were chosen to make the regulated load ( ~100W) close to the the unregulated load(~120W). you may find it doesn't need to be so high, but that will depend of the specifics of your PS.

If it was me, I would buy a compact SMPS designed for 12V at 10A for about US$25 or 20A, for about US$40.

Cheers
Matt

[D_E_H0987]

There is more than one twelve volt power leg on atx power supplys, one is a low current one for the serial ports and reset on the microprocessor it only carrys like 0.5 amp or so and as its just a rectified fliltered out put on most supplies, you need to use the main twelve volt leg on the drive connections (total current on this leg varries by power supply size and output), the yellow side is 12volt, also you will need to either mod the power supply and replace the power output leeds or seperate your LEDs to seperate 12 volt legs as the wireing will not carry that kind of current. There is also a seperate 12volt connection on the 4 or 6 pin plug its around 4 amps and up I think. There is also a -12v at around 0.2 amp for serial ports and processor reset.


There are at least three 5 volt legs on an atx power supply, 5v for the power switch and power on that is always on even when the system is off, its low current like 1 amp or less.
Then there is the processor 5 volt and on older power supplys sub regulated to 3v at around 4 amps and up I think, on newer power supplys the 3volt is a seperate output, then there is the main 5 volt leg (current depends on power supply size, etc.) on the drive connectors and on the main board power connector, this is the high current output, you would need to mod the power supply to get its full current as the wiring will not carry the full current, or you could split your led's to seperate legs.

Also you may need a load on the 5v/3v processor leg on most ATX power suplies, the new 24 pin ones also have another sense line as I recall, that needs connected back to the 5v, and there is a power good line that needs to be connected back to the 5v on some power supplys.
As too that power supply you got from a freind, it was probably a dell, etc. with a non-standard pinout, so you have to buy a new motherboard from dell to upgrade or a new power supply from dell if one fails, its just a scam to make people spend more money if you ask me.