This messy bunch of parts is a 0 to 9V adjustable power supply. The original power supply board connected the two transistors in the Darlington Configuration. The unregulated 12V input goes through a 187 ohm resistor to a 10V zener diode to give a stable reference voltage. This voltage is fed to a 2.5k linear potentiometer, and the knob taps the pot to give zero to 9.8 Volts. This was fed to the base of the first transistor.
But this circuit didn’t have very good regulation. A few hundred milliamps load caused the voltage to drop several hundred millivolts. I didn’t find that acceptable so I cut traces and rewired the two transistors in the Sziklai configuration. Now a few hundred mA load causes the voltage to drop only 20 to 30 millivolts – that’s hardly noticeable, much better regulation.
So I assembled the parts on the PC board and instead of using a low current transistor, I replaced it with a BC337-40. I mounted the parts on the piece of wood and powered it up. The maximum voltage was about 9.25V. And the really great thing about this circuit is it can go all the way down to zero volts. So I turned down the knob and it went down to 2.5V, but when I turned it down below 2.5V, the output went all the way back up to 12 volts! What?!? It’s not supposed to do that!! It worked fine all the way down to 2.5V, but then went crazy when I turned the knob below that point! There was something really wrong with this circuit. I took some voltage measurements but I couldn’t find anything wrong, so I gave up and hit the sack and decided to start fresh and work on the problem tomorrow.
Well, I finally found the problem. The original transistor’s pinout was Emitter, Base, Collector. But the new BC337 pinout is Collector, Base, Emitter – the reverse of the original. But I soldered the BC337 into the board in the same direction as the original so the emitter and collector were swapped. I unsoldered it and put it back in the correct way and the knob adjusted the output voltage clear down to zero.
So I put a 16.67 ohm load on the output and adjusted the voltage to 5VDC. Without the load the output voltage went up about 25 millivolts, that’s hardly noticeable. But the 0.3 amp load made the heatsink very warm. I need to use a bigger heatsink and/or add more heatsinks. I also need to add a current limiter so a short circuit doesn’t fry the output transistor.
