Connect with us

PC power supply as lab power, voltage issue

Discussion in 'Power Electronics' started by power_supply, Nov 12, 2011.

  1. power_supply

    power_supply

    3
    0
    Nov 12, 2011
    Hey, having a little bit of trouble making lab power supply from an old computer power supply. I've connected the Green trigger wire to ground, so I have power.

    My issue is that the yellow +12V wires only seem to be putting out +9.5v.

    What do I need to do so that the yellow wires will put out their constant +12v as they're supposed to.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,192
    2,694
    Jan 21, 2010
    Try loading the 5V rail.
     
  3. power_supply

    power_supply

    3
    0
    Nov 12, 2011
    what do I load it up with, the power supply needs to be run constant so cant have a typical light bulb as would get too hot.
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Why would a light bulb get too hot? Any load will get hot unless it's fan-cooled, and fans wears out too. What lifetime range are you looking to get out of it?
    I'd use a 12V car headlamp bulb (or a complete lamp assembly or something similar) as a long-life load, making a nice, mellow, yellow light.
    Otherwise there are metal clad power resistors made for bolting to heatsinks. Use as many 1 Ohm 25-30W resistors (on heatsinks) as needed to make the 12V steady.
     
  5. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    A 4R7 ohm 10 watt ceramic is all thats normally needed, less resistance will waste power, some supplys will give you 12 + volts with a 10 ohm 10 watt ceramic resistor, ive found a single 4R7 ohm 10 watt works fine, for modest loads on the output.
    Dave. :)
     
    Last edited: Nov 13, 2011
  6. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Dave, you must mean 4R7 (4.7 ohms)? Most PC PSU's have an internal 100 ohm resistor as a base load for the 5V, so adding 4.7k would not be noticed by the PSU.
    Only testing can reveal if 10 ohms (2.5W) or 4.7 ohms (5.5W) will be sufficient loading to bring up the 12V to full voltage. It may depend on the 12V load also.
     
  7. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Yes Resqueline, sorry my mistake 4R7 or written 4.7 ohms, tired when i wrote that post, no excuses my mistake.
    Dave. :)
     
  8. power_supply

    power_supply

    3
    0
    Nov 12, 2011
    The only load that will be running off the supply is approx 4 or 5 Navigation units which accessory sockets reduce the voltage of them to 5V and a reverse camera mirror. wont be much at all.

    So what I need to ask for is a 10ohm resistor and run it between one of the red wires and a black wire on the PSU?
     
  9. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Ask for 4R7 10 watts ceramic high power resistor, or 4.7 ohms 10 watt resistor, the R denotes the value, ie ohms R, kilo ohms K and as the decimal.

    Depending where you are in the world you might have to get a 5 ohm 10 watt ceramic resistor, 4.7 just allows the tolerance + / - resistance in % of the total value.
    Connect one red wire from the main 20 /24 pin out on the atx, also one black try to mount it in a safe place in the air flow from the fan, add a switch from green and black if you want, and an led to any voltage allowing for a series resistor, or use the grey wire and one black wire.

    If any voltage sense wires are coupled with others on any molex connectors, then keep these together, the most common is a slightly smaller gauge orange wire coupled with a thicker orange wire on the main mother board connector, some times the 5 volt rails and 12 volt rails have sense wire as well, but not so common.

    What you get out varys on the psu.
    Dave. :)

    [​IMG]

    These are typical voltages on the output.
     
    Last edited: Nov 19, 2011
  10. DrPinky

    DrPinky

    22
    0
    Nov 7, 2011

    I have also used PC power supply's I just hooked up a old hard drive and that seemed to fix my problem of the varying outputs. Hope this helps.
     
  11. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Hi one and all.
    Often surfing the net i find a good link to projects tutorials, this one stood out for me, there is an atx modding tutorial in there, plus another few projects, i just like the guy's attention to detail, stuff made well.
    Dave. :)


    http://jumperone.com/tag/power-supply/
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-