Repair or Rework 30 amp 12 VDC PS

Discussion in 'General Electronics Chat' started by TomW2034, Aug 19, 2015.

  1. TomW2034

    Thread Starter New Member

    Aug 19, 2015
    I need 8 amps of 12 VDC power to operate several reels of LED lights. The only power supply in my toy drawer big enough to do the job has an issue with its regulating circuitry as it is currently outputting 14.2 VDC under load.

    My understanding of proper operation is that Zener diode D3 provides a referencence voltage for SCR1 to do the heavy lifting of regulating the device's current. Last night D3 was replaced with a 12 volt Zener diode. With a small light bulb as load, the output voltage was still high. The Zener got quite warm even though the cathode voltage was reading about 12 volts.

    I assume the SCR must be bad. If nothing else, it never warmed up the least little bit. Mail-order, a new SCR is around $16.

    Out of curiosity, the plan tonight is took hook up a standard, incandescent wall dimmer out of my toybox to the power supply input to see if I can pulse-width-modulate my way into the proper output voltage.

    Does anyone see a long term problem with the dimmer approach? Anyone think the base problem is not the SCR?

  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    That is not a power supply. It is a battery charger. Big difference.

    No filtering, so if you run LED lights from it, they will flicker like crazy...
  3. TomW2034

    Thread Starter New Member

    Aug 19, 2015
    I forgot to mention that I always run a big ole electrolytic on the output.

  4. cornishlad


    Jul 31, 2013
    I would never have considered using a PWM dimmer into a transformer cct let alone the main issue correctly given by MikeLL. PWM control is ideal for resistive or near resistive loads. Others might explain that in more detail - or disagree
  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    While it is a form of PWM, most light dimmers are in fact phase angle modulating a sine wave, not DC as is the usual case for PWM. I've never done it, but my guess is that the transformer will not be happy.

  6. tom_s


    Jun 27, 2014
    personally i'd use a pc atx power supply if you have one lying around.
  7. TomW2034

    Thread Starter New Member

    Aug 19, 2015
    Thanks, Tom_S. But I'm on a mission from... me... right now.

    I just stepped in from the shop where the wall dimmer is doing what I wanted with neither a temperature increase from the Zener, nor magic smoke/unusual heat from anything else.

    My son just now read over my shoulder, and reminded me that we do have an atx ps in the closet. I will call that Plan # 3 right now because eBay sold me a new SCR for a great price. Thanks again for the suggestion.

    Stay tuned... Film at 11:00!

  8. Brevor

    Active Member

    Apr 9, 2011
    I don't think a new SCR will make a difference, It is normal for a "12 Volt" battery charger to put out over 14 volts.
  9. Lestraveled

    Well-Known Member

    May 19, 2014
    This battery charger is an old design, (very old.) It needs a big capacitor on the output, which you have. It also will not regulate well under light loads. You will probably need to load the output with a low resistance to keep the voltage from going too high.

    May I suggest that you look on ebay for a switching step down regulator. For not much money you can get one that can handle a lot of current and will have very good regulation.