Homemade Variable Power Supply Upgrades
It’s been over five years since I’ve designed my Homemade Variable Power Supply. I’ve used it a lot since then, but even in the original post, I knew I wanted to make some upgrades. I was happy with the variable voltage output, but wanted to 3D print a case and add a power switch. I finally made those improvements and added a display to read the voltage right on the equipment.
The first order of business was to improve on the sheet metal case for this project. It worked to contain the electronics but wasn’t very attractive. I designed the case using OpenSCAD. I really like using OpenSCAD when designing enclosures like this because it’s easy to adjust parameters to make it fit. The front metal plate is mounted to four holes in the corners using self-tapping screws. I only made the walls 3 mm thick and the longer walls on the x-axis are flimsy. If I were to adjust anything, I would make them thicker or add another way to add rigidity. The files for the 3D printed case can be found here on Thingiverse.
Next, I replaced the tiny binding posts with these improved banana plug sockets. I ordered some mini grabber test leads so I could easily attach the power supply to any wires on my projects. I bought the cheapest of these items that I could find. I’m actually happy with the test leads, but the sockets are too short and very low quality. I’ll use them for now but will probably change them out with something better at some point in the future.
Originally, I was going to stop the project at this point because I was happy with the changes that I made. But while searching eBay, I found mini panel mount voltage displays and knew they were perfect for this project, so I decided to keep moving forward. I wanted independent power and output switches so I could always leave the power supply plugged in and turn the supply on to view and adjust the voltage, and then turn on the output switch to power whatever is attached. I also wanted the output button to have an indicator so I would know when the output was enabled. I also thought I needed a bleeder resistor to discharge the output capacitance when the output was turned off (the large output capacitor took a while to fully discharge), but then I discovered the circuit below, which is the final configuration. I didn’t need the bleeder resistor for that reason, so I decided to remove it. However, I can think of other reasons why it could be helpful, so I kept the DPDT switch at the input in case I want to add the resistor back in later.
It took me a while to come up with the output indication scheme. Since the DC output voltage is adjustable over a large range, I couldn’t power the indication with the output voltage, and I didn’t have any other DC voltages on the board to use (and I didn’t want to add another regulator just for this). I ended up using a LB26WKW01-01-JC switch since it is SPDT, could handle the AC or DC voltage, and has a mains AC-rated illumination lamp. Unfortunately, the lamp popped the second I powered it, so I had to perform some surgery to replace that bulb with something better. I might have recieved the wrong part, but since I eventually got it working, I didn’t complain.
Now that I had the circuit decided on, I had to design the case. Again I used OpenSCAD to design this part. Originally, I was just designing the faceplate so I could see how things looked, but the second prototype print turned out so nice that I decided to use it in the final product. As you can see, I had to add a shim behind the display to prevent it from moving, but since it’s not visible, it doesn’t affect the case. I also bent the tabs for the input power switch and used small screws to mount it in place. Like I said, this part wasn’t originally meant to be the final case, so I had a harder time designing the remaining support to mount and protect this faceplate to the existing power supply. You can find the files on Thingiverse. Below you can see that design and it looks pretty nice when it’s all together.
I didn’t like how the AC power cable originally came in through the front of the supply, so I drilled a hole in the case and ran the wire in through the side. I used a grommet to protect the wire and placed the clip back into the faceplate to cover the original power cord hole. I also had to cut a hole in the original case so I could wire the new electronics up into the power supply.
The wiring was just a matter of connecting everything according to my schematic. Remember to put the AC power switch on the hot line (the black wire, or the narrow pin on a two prong plug). It was important to add heatshrink to all the wires before soldering and make sure the wire is long enough to connect both sides without too much excess to manage inside the case. I chose to solder everything instead of using connectors because I don’t think I’ll have to change any of these connections in the future.
As I mentioned earlier, I didn’t expect these parts to make it to the final version, so I had to attach them together as best as I could. Right now, the front face shown in the above photos is the main mechanical connection between the meter panel and the original power supply. I have to be careful with it or it may break. If that happens, I’ll have to think of a better connection mechanism.
And as a final special feature I attached a magnet inside so I could mount the power supply to my desk for normal usage, but still easily remove it if I need to. I tried to use old 3D printed scrap to mount this part of the case together, but it wasn’t very effective.
I’ve finally made the improvements to the power supply that I dreamed up years ago. It feels good to have a usable piece of equipment that is exactly to my design. I hope to use it for many years into the future.
