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using a pc powersupply to feed led strips

Discussion in 'LEDs and Optoelectronics' started by camilozk, Apr 20, 2014.

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  1. camilozk

    camilozk

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    Apr 20, 2014
    Hi People!!

    this is my first post here. I am also not an expert so please be patient!!!

    I would like to feed several led strips with a computer power supply. I got myself a Compaq pdp-123p, and I have different led strips, some that require 12 Volts and some others 5 Volts.

    I turned on the power supply using the procedure illustrated here (http://www.instructables.com/id/How-to-power-up-an-ATX-Power-Supply-without-a-PC/) and I measured the voltage coming from the power supply to be sure everything works ok. What I got from the cabels that are supposed to deliver 12 Volts, was 9.05 Volts, and from the ones that deliver 5 Volts, 3.98 Volts.

    Is this normal?
    Am I safe to connect the led strips anyway?
    And why do I obtain this values and not 5 or 12 Volts?

    thanks in advance!!!!
     
    KhevAun likes this.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Some older PC power supplies require a load to bring the rails into regulation. Try connecting the 5V strip to the 5V output an see what voltage you get.

    The LED strips won't be damaged by low voltages, but they won't be very bright either.
     
  3. Kiwi

    Kiwi

    336
    84
    Jan 28, 2013
    As Steve said, some power supplies need a load to work correctly. Suggest that you connect a 10ohm 10watt resistor between red and black(+5v) to load up the power supply. Voltages should then be stable.
     
    KhevAun likes this.
  4. camilozk

    camilozk

    116
    4
    Apr 20, 2014
    Hi!

    Thanks for the answers.

    I had just connected a 12Volts led strip to a yellow 12Volts cable from the power supply.

    While the led strip is off, I continue measuting 9.05Volts, and as soon as I turn it on, I measure 8.72Volts.

    So instead of reaching the desired 12Volts, I am actually getting less...

    What could be happening?

    thanks!
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Do as I suggested and try loading the 5V rail. Place the 5V LEDs on the 5V rail then check both the 5V and 12V rails.
     
  6. camilozk

    camilozk

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    Apr 20, 2014
    5V led strip connected.

    I continue measuring 3.98V in the 5V rail, and 9.05V in the 12V rail.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    There are several possible reasons:

    1) The power supply is faulty.
    2) Some other rail needs to be loaded.
    3) You're using the wrong 5V supply (it may have a 5V standby as well as a main 5V supply)

    When operating correctly (and appropriately loaded in some cases) the voltage should be within 10% -- and is usually significantly closer -- to the nominal value.
     
  8. camilozk

    camilozk

    116
    4
    Apr 20, 2014
    I tried the past days to load different rails, with different loads (the most reliable was a computer cdrom), always obtaining the same results, so at this point I have to be convinced that the power supply is faulty...

    thanks for your support steve
     
  9. camilozk

    camilozk

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    Apr 20, 2014
    Ey again!

    I have to insist a bit, because I tried already 3 power supplies, with different loads (being the most reliable was a computer cdrom) in different rails, and I still havent got 12V... I did read 5V in the 5V rail, but not 12V in the 12V rail. I am thinking that I have to do something else to get the 12V??
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What was the maximum load you placed on the 5V rail?
     
  11. camilozk

    camilozk

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    Apr 20, 2014
    I didnt load the 5V rail. I simply measured it with the multimeter and I read 5V

    I loaded the 12V rail with a cd rom and a led strip.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    As suggested in posts 2, 3, and 5 -- load the 5V supply.

    Tell us what the load is (500mA to 1A should be sufficient) and then tell us what the 12V rail measures under this load.

    Beware that you need to load the proper 5V rail. Do not lace this load on the +5sby output.
     
  13. davelectronic

    davelectronic

    1,087
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    Dec 13, 2010
    After having modded a few ATX units now and using them to power all sorts of stuff a 4.7 ohm resistor of 10 watts works best on the 5 volts rail gives best stability. I did try the 10 ohm 10 watt resistor but found the voltage barely reached 12 volts or 5 volts. A guy called Phil on his site called jumper one has an excellent tutorial.

    http://jumperone.com/2011/06/atx-power-supply-tutorial/

    I power audio amps radio amps power 12 volt tools even charge multiple smart phones with no problem. another good hack is the xbox power brick as its known. but the above link is worth a look.
     
  14. camilozk

    camilozk

    116
    4
    Apr 20, 2014
    Indeed an excellent tutorial.

    I got myself 3 different power supplies (1x King year KYP-200 WT and 2x Compaq PDP-123P) and I have been trying to load them with cdroms and hard drives. (didnt have yet the chance of buying resistors).

    The King year KYP-200 WT was able to make the cdroms and hard drives work, but I still read no more than 11V in the 12V rail.

    Both Compaq PDP-123P gave me no more than 8.5V and they were not even able to open the cdrom tray.


    I will definitely try the resistors, but I am a bit skeptic. Shouldnt they work with cdroms and hard drives?
     
  15. camilozk

    camilozk

    116
    4
    Apr 20, 2014
    I wasnt able to load them either with 5V led strips, because simply I dont have. I couldnt figure out about any device that I have in my house that works with 5V
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    A 12V 12W light globe (anything between 10 and 50W should be fine) is an excellent choice. It will work for any rail 12V or less, is cheap, and it is designed to get hot. Even if it doesn't do more than glow, that's fine.

    The current drawn will be approximately proportional to voltage, but at lower voltages it will actually draw more than the proportional amount as the filament has a resistance which increases with temperature.

    So a 12V 12W lamp draws 1A at 12V, at 5V you might expect it to draw (5/12) * 1 = 416mA, but it may draw in excess of 500mA
     
  17. davelectronic

    davelectronic

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    Dec 13, 2010
    These switch mode supplys really need a minimum load to function well. Like Steve said a lamp will do the trick ! halogeon lamps 12 volt units or automotive lamps. Occasions do come up where you get an SMP ATX that just wont play ball, but its rare and most will respond well to a lite load on the 5 or 12 volt rail depending on the technology used in a given psu. A few multimeter tests on current and voltage drop will soon tell if you got it right and on the correct rail for that supply. Anyway good luck with it, play safe if you open any units up (-;
     
  18. Den095

    Den095

    2
    2
    May 30, 2016
    Hello, a bit late but better than never. I had the same problem as you with PDP-123P. So I tried everything, in the end I loaded the 3.3V with 10ohms 7W resistor and it's just working as it should, 5.09V at output...

    (the 3.3 is rated 19A !)
     
  19. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Please be careful which rail you actually load...
    The PSU should have a table indicating the wire color for each voltage rail.
    You don't want to drive a load off the standby wire, or the status wire.
    +12V rail is commonly Yellow,
    +5V rail is commonly Red,
    +3.3V rail is commonly Orange.

    Make sure you draw from one of these for the LED strips and the Resistor.
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm not sure I realized the power supply had a 3v3 rail.

    does the power supply mention minimum loads anywhere?
     
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