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

[eccerr0r]

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...

[Heihachi_73]

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.

[eccerr0r]

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...

[eccerr0r]

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.