Home made lab PSU from ATX PSU, negative V A and resistor question.

Discussion in 'General Electronics Chat' started by Barry M, Feb 11, 2009.

  1. Barry M

    Thread Starter Active Member

    Feb 11, 2009
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    Hi, I am converting an old ATX PSU into a lab PSU. As I understand it I can get 24V out it by using the -12 and +12, giving me the range, or close to it.

    Because -12 will be treated as ground does the max amps for the specs still stand or are the specs only rated against ground?

    Also it's recommended that you put at least a 10W 10 Ohm resistor between a +5 wire and a ground wire 'or else your PSU you may not stay on'. Ive never had this problem, never used a resistor and been using computer PSUs for a few years to run other things, for example router, modem, phone charger and additional fans as a single source. I just short the green + a ground. A PSU I am about to be experimenting on though says its 5V 2.5A-40A. Does that mean I should get a 15W 5 Ohm res to bring it up? 12.5 with 2 Ohms being the exact values needed, I think ... I get the feeling Im looking at this resistor problem wrong, once I pick it up though I should be fine, so please any input at all that will help.

    Thanks.
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    If you use the -12V and +12V to get 24V, remember that the original ground is not the 24V ground anymore. Take care not to mix the original ground and the 24V ground in a common ground system.

    The resistor is to there to set the minimum current. In most PSU, the minimum current specified is necessary to achieve good voltage regulation, ripple specs, EMF, etc. It might work without the resistor, but it would be outside the specs. Some newer PSU might sense this current, and shut down if not met.
     
  3. Barry M

    Thread Starter Active Member

    Feb 11, 2009
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    How do I select the best resistor though? The PSU I will be using is a lot more powerfull than the tutorial ones and I dont know if I have to factor in max amps. Im unsure about which equation to apply where, I think Im getting them mixed up. On a 5V to ground how would I work out what resistor to use to produce 2.5A?

    I learn better with practical anyway, so this is a good start :)
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    I don't think you need to waste all that power, but Ohm's law will figure it - R = E/I.

    By the way, the spec on the -12 volt output is usually significantly less than for the +12 output. You can only use the combined +/- 12 volt supplies up to the - 12 volt current limit.
     
  5. Barry M

    Thread Starter Active Member

    Feb 11, 2009
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    I'll scrub the 24 volt attempt then an just max out at 12, I want the extra amps for a cooling experiment and theres absolutely no way a max of 0.8A is going to support a peltier or two.

    With the resistor though, do I have my thinking right that in order to get 2.5A through the 5 volt I need a 12.5W 2Ω resistor or am I working that out wrong? If I have got that wrong can you point me in the right direction please?
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    I always use a 10Ω 10 w resistor between 5v and neg and never have a problem.
     
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