Variable PSU

Inspiration

Ever plugged in a circuit only to release the magic smoke from some random component? Well I have and it stinks (metaphorically and physically). So I decided that I needed to create a PSU that I could use to change not only the voltage but the current as well. The circuit also needed to be able to have current limiting and automatic shutdown upon over voltage or current. It also had to be cheap and use commonly available components. This is my adventure into creating a functioning variable power supply.

Power Supply Prototype

Before the project went forward I wanted to make sure that the design would be feasible. I breadboarded up the a version of the PSU that used a ACS704 from Allegro to measure the current. The voltage regulation worked well but the current measurement required that a PCB be created with a ground plane in order for the device to function properly. Back to the drawing board. The yellow circuit board at the top of the picture is the ACS704 on a little breakout board.

More Prototype

Close up shot of the LCD displaying measured voltage, measured current and current limit.

Scrap the Fancy

After the failure in using a semiconductor to measure current I went back to the tried and true method of using a current sense resistor. Unfortunately I did not document the construction of the second version of the PSU to you will just have to use you imagination on what it looked like. Above is a picture of the circuit board that houses the microcontroller and all the regulators.

Top Portion of the PSU

I broke the PSU up into two chunks. The top portion contained the LCD and the controls while the bottom portion contained the power supplies. This was done for thermal mitigation purposes.... actually it would not all fit in the bottom box. Above is a picture of the internals of the upper module. The wire with the masking tap on it is the ICSP programming port for the micro in the bottom. It became difficult to cram the programmer into the bottom part.

Bottom Portion of the PSU

Here you can see all of the internals of the bottom portion. It was a little bit on the tight side but it all fit. I used two fans on the bottom portion to keep heat down. The LM317 voltage regulator was mounted on a scrap PC heat sink.

PSU in Action

This is what the PSU looks like when it is turned on.

PSU in Action

This is what the PSU looks like when it has a fault. You have to push the black button on the front to reset the supply. It uses a latching relay to prevent the system from oscillating around a fault condition. The red led is on.. honestly it is.