Power Supply Protection

This article was supplied by John VK3HJQ

Hi there,

Power Supply Problem

I had a slight problem with my shack 13.8V 10A power supply, its output voltage went from 13.8v to 26v, “Oh No”, “Yes No” it took out the 723 voltage regulator, TIP31C & both 2N3055’s in the supply but luckily no radios.

Here is a simple CrowBar circuit to overcome this problem in the future, it’s an over voltage protection circuit for linear power supplies (not switch mode supplies) all it does is put a dead short on the output when the voltage goes over the set Zener voltage, it then blows the fuse on the supply saving your radio gear from smoking.

I have now completed this mod to all my Linear supplies 13.8V 10A, 13.8V 7.5A, 13.8V 30A.

Hope someone finds this useful.

John vk3hjq (01/01/2016)

Power Supply protection


  • Phillip, VK3DCZ

    A very useful circuit. Certainly worth doing to protect your equipment!

  • John vk3hjq

    Hi Phil,

    Thanks for your comment, its a simple circuit that works like a treat.

    John vk3hjq


    Just had a phone call from a local Amateur in the area about this Power Supply Crow Bar circuit, and just to reiterate our conversation here.

    For this Mod to work your Power Supply MUST BE DC FUSED.

    This means it needs to have a DC output Fuse fitted, all my power supplies have both a DC & AC Fuse, but be very careful there are some Branded Power Supplies on the market including Home Brew ones which have NO Fuse fitted at all, this means when the SCR turns on from an over voltage (16V+) you will have a DEAD short on the output of your power supply. not a good look.

    John vk3hjq

  • Hi all,

    Had another phone call about this circuit, to save confusion, i have redrawn it.
    Please see the original post for new diagram.

    Hope this helps.

    John vk3hjq

  • Hi all,

    Make sure you put the Zener into the circuit in reverse, meaning now the Cathode is positive (+) and the Anode is negative (-).

    John vk3hjq

  • Ivelin


    I recently got a TYT UV8000E handheld radio and while I was going through the menu I accidentally pressed the PPT button for 1-2 seconds at high Tx power setting (10W) without the antenna installed. I also did this with my Baofeng UV82. Is it possible that I’ve damaged the radio?

    Thanks in advance for your opinion on this matter.

    Kind regards,

  • Brek Martin

    Hi, Have you actually tested this by putting high voltage through it, and getting it to short? I replaced the zener with 15V (1N5352B), used 40TPS12A SCR suggested here, and can run 20VDC through the circuit, and it’s cold.

  • Brek Martin

    Ok, I had to remove the 690 Ohm resistor to get it to work, and replaced with continuous link.
    Then I tested it to blow a 3A fuse with the circuit powered at 20VDC & at least 3.5A available current.
    One of the caps either side of that resistor is now omitted as well.

    Before this, I measured about 5V where the zener connects to the two resistors,
    but only 0.2V or less where the 690 Ohm resistor connects to the SCR gate pin.

    It’s possible that if there’s more current available from a PSU, then it might trigger, but I’m unable to
    test that scenario without fiddling with voltage adjustment pots in PSUs, which I’d rather not do.

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