I was recently asked if I could build a 24 volt AC to DC power supply; something similar to a laptop power adapter. I thought this would be a nice little project and I wondered how much of it could be done with spare parts resourced from other electronics. The only requirements imposed on this project were as followed. Beyond these few requirements I was free do as I pleased:
- The device had to output 24VDC and support 100mA current draw.
- The device had to have a small form factor to make transporting easy.
- The device had to have banana plug post for connecting the external device.
At this point in my electronics experience I have only built the one linear power supply using a transformer as the source. However, I knew to keep the size of this new power supply down I would need something else. I came across a power supply board from an all in one printer that had an output of 31 volts. I figured I could put together a second board that would take in the 31 volts and regulate it down to 24 volts. The following picture shows the original board that I lashed together one weekend. I used Eagle to layout the design and then the toner transfer method to etch the board. Most of the components used were resourced from an old CRT monitor.
After the first test we learned a very important fact; the device that needed to be powered used far more than 100mA. In reality we measured the current draw at just over 600mA, which was far more than the TO-220 package could handle without a decent heat sink. To mitigate this problem, I moved the regulator (LM317) off the board and attached it to a heat sink on the outside of the case. In the end it worked as intended and met the requirements.
I’m not planning on going into a lot of detail about this project here on the blog, so let me know if there is anything you would like to know regarding the design or construction. My hope for this post is to show what can be built with 10 bucks at a radio shack and an old CRT monitor.