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Gutless, not bloated, killer, and should be hall of shame POS

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    Gutless, not bloated, killer, and should be hall of shame POS

    Introducing... the POS-124Z which really is a POS:

    Yep it's a POS allright.

    I tried powering it up. It's a KILLER - not fried, but sure will fry things! I got 21V out of it unloaded according to my DMM!

    I tried 21W load (car lamp) - got 16V, which is really pushing that lamp.

    Then I tried a 50W load (car headlight) - got 13.2V. Not too bad but this is with the heavy load...except the circuit breaker kept tripping.

    So I had to take a peek:



    EIEW. Looks like a Darlington emitter follower + Zener device. Heatsinking? What heatsinking? Oh that little heatsinking...

    I suspect this is a 16VAC RMS transformer though I didn't check. So peak voltage is like 22.6V though should get 16VDC continuous. At the full 4A and 13.2V output it's dissipating more than 10W and I would expect some finned radiator here.

    hmm... what to do with this...makeover with some other regulation circuitry?

    BTW, schematic: nothing much to it. Just two NPN transistors (TO-3 outside, and a TO-220 attached funny on the inside) wired Darlington. A 12V (assumed) zener with a 1K dropper resistor, and a 200uF filter across the zener. LED has a 1KΩ dropper resistor too. Main filter cap is 4.7mF. Not much else is in this PSU...
    Attached Files
    Last edited by eccerr0r; 10-05-2022, 01:35 AM.

    #2
    Re: Gutless, not bloated, killer, and should be hall of shame POS

    Would be ideal for a vintage model train layout (Athearn blue box etc.) if it had DC output, a variable resistor and a way to switch polarity. Wouldn't put it anywhere near a modern DCC layout though.

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      #3
      Re: Gutless, not bloated, killer, and should be hall of shame POS

      hmm...wonder if I changed the topology a bit, it would make it a bit better regulated...

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        #4
        Re: Gutless, not bloated, killer, and should be hall of shame POS

        Never did I ever assume this PSU was broken. While changing topologies I realized the series pass transistor was bad. It had no markings on it, but of course it's NPN so I swapped it with a junk 2N3771 and reverted to the original layout. It works much better now, though 14.8V OCV, it does get down to 13.8V fairly quickly when loaded.

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