I have a Pyramid PS14KX 13.8 VDC 12 Amp Regulated Power Supply that I'd like to modify into a variable 0-14V 12A supply. Has anyone done this before?

Looking at a similar, variable power supply below that uses the LM723 and 2N3055 transistors, it looks like it might be possible to put a 5K pot between pins 5 and 6 on the LM723 with a 10uF cap between them for voltage adjustment and another 5K pot between pins 2 and three for current adjustment. Would this work, or will I need to make other changes?

Pyramid PS14KX 13.8 VDC 12 Amp Regulated Power Supply

0-30V 10A power supply

 

Certainly that modification is possible, BUT probably there will be a serious heat problem with the pas transistors as they will dissipate a lot more power with the higher voltage drops at lower output voltages.

 

Thank you for the quick reply! I considered adding at least 2 more 2N3055 transistors with 2.2 ohm resistors (increasing them to 10 watts), and a fan. I could also increase the size of the heat sink as well. The current adjustment pot should be 500 ohms, not 5K. Is there anything else I could do to reduce the heat problem?

 

On some wide range variable supplies I have seen tap switching on the transformers, done, based on the output voltage selected. THAT is a very big deal. So reducing the voltage supplied to the pass transistors is an option that is probably not practical. If it is convenient yo put a thermometer on the heat sink that would allow you to see if the temperature rise was excessive.

 

On some wide range variable supplies I have seen tap switching on the transformers, done, based on the output voltage selected. THAT is a very big deal. So reducing the voltage supplied to the pass transistors is an option that is probably not practical. If it is convenient yo put a thermometer on the heat sink that would allow you to see if the temperature rise was excessive.

Thanks, MisterBill2. I can't tap the transformer without damaging it but, I like the idea of putting a temperature sensor on the heat sink. I can tie it to the fan and the buzzer alarm.

 

Thank you for the quick reply! I considered adding at least 2 more 2N3055 transistors with 2.2 ohm resistors (increasing them to 10 watts), and a fan. I could also increase the size of the heat sink as well. The current adjustment pot should be 500 ohms, not 5K. Is there anything else I could do to reduce the heat problem?

You have to put in equalizing resistors on the emitter outputs, just like in the diagram that has adjustable voltage. Your original diagram with the 2 transistors is a poor design as it does not have equalizing resistors on the emitters. They are necessary to prevent one transistor from handling too much of the load vs other pass transistors.

 

Thanks, MisterBill2. I can't tap the transformer without damaging it but, I like the idea of putting a temperature sensor on the heat sink. I can tie it to the fan and the buzzer alarm.

I did not anticipate changing the transformer, but some of them have a 120/220 switch and selecting the 220 position will reduce the voltage when using a lower output voltage.