24 pin pc power supply 24v output is pulsing

Discussion in 'The Projects Forum' started by psreynol, Dec 21, 2010.

  1. psreynol

    Thread Starter New Member

    Dec 21, 2010
    hello this is my first post and though I've tinkered with electronics and built a few boards I really have no idea how to troubleshoot electronics.

    I had a extra power supply from a old dell computer and i want to use it to power my G gauge train since I've out grown my old mrc tech II 1500. Ive read online about people using one or 2 computer power supplies as power so I thought it would be educational and a way to save some money.

    I used the following link to make some small modifications and get rid of the extra wires and I purchased a maxx tronic mxa033 dc motor speed control


    so my question is as follows: the switch i used to power on/off has a led and resistor built in and it blinks when i turn it on. the light i used to test the power output at 12v/24v and with and without the speed control the light also blinks. as I turn up the speed control the blinking seems to be more prevalent. however when I use the voltmeter the voltage is stable so is this normal for this type of pc power supply?

    the speed control is a pulse with modulation unit so perhaps this is normal since voltage for a motor is a bunch of pulses and the pulses control the speed in a series of on/off pulses. Im concerned that the lights are visibly blinking and i did not expect to see it. my existing mrc train power supply/speed control transformer does not behave like this so I wanted to find out if this sounds normal or if something is wrong with my power supply or perhaps the way i hooked it up.

    is there a way to make the power light not blink?

    I would also like to change the direction of my train so I was going to use 2 relays to switch the +/-. I could also just use a DPDT switch to do the same thing with less wiring. any thoughts? thank you.
    Last edited: Dec 21, 2010
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    There is no 24VDC output from a PC power supply.

    There are several very high current +12V, +5V, and +3.3V outputs, and ONE -12V output than can only supply 1A-1.5A Max typically (using a 7905 or similar). Newer supplies do not even provide the -5V.

    The 24V can only supply current to the weakest link, which is the -12V line, at that point, the -12V shuts down from overload and you have no power.

    If you need a 24VDC supply, you can order a 24V SMPS with High Current.

    Trying to change a computer power supply that supports negative voltages for legacy peripherals only isn't the way to get it. The cases and grounds of the power supplies are often tied to the AC Ground, so they cannot be connected in series.
  3. psreynol

    Thread Starter New Member

    Dec 21, 2010
    thank you for the helpful reply. so the cause of the pulsing is drawing to much current from -12v line? I can try the configuration with 17v (it is a older power supply) and see what happens but I am suspicious the power supply will behave similarly.
  4. SgtWookie


    Jul 17, 2007
    Both the -5v and -12v are typically limited to 0.8A to 1.5A as Thatoneguy suggested. If you exceed the rated current for the -5v or -12v supply, the power supply will cycle on and off.