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From the Q&A

With TJ Byers


POPS (Plain Old Power Supply)

Question:

I have two regulated power supplies which are not working. One is a Pyramid 30 amp and the other is an Astron 20 amp. The Pyramid was stripped by the previous owner and the Astron burned up the control board. So now I have two good power transformers and rectifiers that put out about 25 to 30 volts unregulated; plus four 2N3055 power transistors on each chassis. All the rest is zapped. If you will, I need a circuit that I can use to put these supplies back in use — just a simple regulator circuit to hold at 13.8 volts would do nicely. Short circuit protection would be nice, but not necessary. Can you help?

Frank Schwartz   
via Internet

 


Answer:

This is an easy order, thanks to the venerable LM723 regulator chip. By itself, the LM723 can only provide 150 mA of output current, but external transistors can be added to provide load currents as high as 10 amps and more. The following circuit is an 8-amp, 13.8-volt power supply using the parts you have on hand.

The power supply is powered by a line transformer with a rectified output of about 28 volts DC (anything between 25VDC and 30VDC will work; but the higher the input voltage, the hotter the 2N3055 transistors will run). The 2N3055 is rated at 115 watts, 15 amps, so I've limited this design to 8 amps. By limited, I mean I've selected the value of the Rsc resistor, that's across the current limit (pin 2) and current sense (pin 3) pins, so that the power supply output voltage drops to zero when the 8-amp limit is exceeded. (If the input voltage is 28 volts, the output voltage is 13.8 volts, and the load current is 8 amps, then the 2N3055 has to dissipate 113.6 watts of heat — which is pushing the power envelope.) The formula is

If you want to lower the output current limit, increase the value of Rsc. For example, a 1-amp load limit calculates out to be 0.65 ohms. To increase the output current, you have to parallel the top pass transistor. Just make sure you include a small 0.1-ohm resistor in series with the emitter leads and a separate base resistor (100 ohms) for each 2N3055 when paralleling them; the collectors can be tied directly together. Here's what that section of the circuit looks like.


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