PC power supply unexpected failure

Discussion in 'General Electronics Chat' started by jeka616, Nov 8, 2012.

  1. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    Good day all

    Trying to use PC power supply, which says it is rated to 500W to power my general house video cameras, 4 of them, each rated to 1amp max. I connected green wire to black, and it is working.


    The supply is (at least written on label) rated to give 20A @12V (yellow cables),but it shuts down at 4amp.

    The protection circuit works for sure. Another thing is, I connected 12V 5A dc motor to it shortly, and I think it did cause the problem, damaging something.

    Anyways, is there any particular reason what causes the supply to shut down not reaching even the 1/5th of its rating and solution ahead, that I can do to make the power supply to put out even not its full rating, but at least 6-8 amps?

    thanks ahead
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    open up the psu and see what the ic numbers are.
  3. ScottWang


    Aug 23, 2012
    How is the power's situation as you knew?

    Probably the filter capacitor was damaged or the capacitance is decreased too much.

    You can open the case of the power, to check does any capacitor has convex top?

    Sometimes you have to De-soldering the cap from the PCB, and check the pins of cap, and use the cap meter to measuring the capacitance.
  4. tindel

    Active Member

    Sep 16, 2012
    A lot of times the 12V supply won't regulate correctly if you don't have a dummy load on the 5V supply. I usually use a 22ohm power resistor or two that I mount inside the PSU with a couple zip ties.
  5. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    I love this forum man:)

    My guess also was regarding the power caps, as when I used to open up older models, they were "physically" looking big and tough! With A LOT OF power caps inside....when you open now days the chinese made ones you hardly find any big caps......

    So today I will open it up, try to add some caps inside.

    Another good note is to add resistor to 5v rail. Will do that too. Will post results here soon.

    Appreciate a lot folks,


  6. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    Opened the PSU. caps seem ok. Also connected 25v 10,000mf paralel to 12v. Did not work..

    connected 22ohm/5W resistor to +5v rail. non of these helped. Still shuts down.

    Can I damage the psu by connecting heavy load motor? 12v...

    I do remember, about 2 years ago when I was building similar type project, and used PC PSU, I accidentally shorted the output, and it didi behave like this. So it comes on, but when load is connected, it shuts down.

    Now on this one, I did short the outputs with motor.

    should i bother repairing it or just buy another, but better made brand?
  7. takao21203

    Distinguished Member

    Apr 28, 2012
    Large motors are especially bad for PC power supply.

    -Inrush current
    -Back EMF

    They are NOT designed for discrete electronic circuits, motors, lamps, batteries etc. ONLY for personal computers.

    A PC PSU may work for a while but it is quite easy to destroy them. I had even one older PS/2 PSU module catching fire...

    Drawing a few Amps is not a problem usually. But you can NOT draw the rated Amps. For that, the load needs to be balanced pretty much like a PC mainboard. At least you need a good amount of current on the 5V and 3.3V rails. You also can not use large motors because of said inrush currents.

    Back EMF from Tesla coil circuits, DC/DC converters, etc. will easily destroy such a PSU, at least the frontend schottky diodes, which have a very low voltage rating only.
  8. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    Where will be good start point to start repair?

    Im planning to by a new one anyways. Only concern is, I'm planning it to power 4 CCTV cameras, with max current 1 A each, so max 4-5 Amps. Wonder if it will be capable to drive them..

    anyone can suggest robust regulator schematic for 12V and around 6-7 A?

    p.s. I've read in some electronic forum that PC power supplies are indestructible...wrong thought..
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Like i said previously, What are the numbers on the ic's inside, as we can get the datasheets.