Making a quick and dirty adjustable PSU.

An adjustable PSU is easily one of the most important pieces of equipment in any electronics related workshop. It just so happens that I don't have a functional PSU and without one not much is possible. So I sat down at my messy workbench and decided to take a crappy old adjustable PSU that I had made long ago and improve it by quite a margin. The crappy little PSU consists of a small linear transformer arrangement from a TV where it served as a stand-by power supply, I just added a LM317T to it. The PSU was crappy because it could only output very low current, 200mA max if I remember right. To put that into perspective it could barely just about blow up an LED, that's how weak it was. This "feature", let's call it, was often used for charging lithium batteries. Of course I need more power and I need it now so I gathered some scraps from other failed projects to upgrade the crappy PSU at hand. I had a laptop power brick that I modified to work at 24V that I would use instead of the linear transformer. One problem, though, is that if I wanted to supply something with, say, 1A at 4V my LM317 would be dissipating about 20W, granted it is on a heatsink but it's too small for 20W of heat and I don't feel like adding a bigger heatsink. But fear not for I have a smart solution to this conundrum. The laptop power brick is a regulated power supply and my plan is to modify it so that it tracks the output of my LM317 but is about 4 - 5V higher. Here's a quick sketch of what I'll be doing to achieve that.

The TL431 will be connected in a configuration where it acts just like a regular 2,4V zener and this combined with about 1,2V LED forward voltage will give my LM317 about 3,6V of headroom. Now to modifying the laptop brick I have. First I changed the series current limiting resistor of the opto LED from a 330 Ohm to 10 Ohm.

The TL431 uses a resistor divider to set the voltage and to turn it into a zener I only needed to remove the resistors that were connected to ground.

I then carefully cut around the pin 2 of the TL431 to isolate it from ground and attached a wire to later connect to the output of the LM317.

The power brick was now ready, not to the LM317 side of things. I thought about reusing the original potentiometer I had in the PSU but it was a crusty old single-turn and for my PSU I had some better things in mind, which involve a plastic roller, a knob and, most importantly, a 10-turn trimmer. Yes I'll be using a trimmer mainly because I don't have a 10-turn pot, after all the trimmer is an order of magnitude cheaper and for the purpose of this PSU the effect will be the same.

The nylon roller was epoxied on the trimmer adjuster and with some electrical tape the knob should be a snug fit on it. And so with everything ready I quickly threw together the PSU and gave it a test.

First problem I encountered was that the PSU was too unstable to operate below 4V and from 4 to 10V the output had some random 200mV ripple - not good. Luckily all was fixed with a 3V3 zener between the TL431 and my output. I also added a 2K resistor across the output just to give my LM317 some idle load. Now testing my PSU with a 6R8 resistor I noticed that it's still unstable below 6V under load but I can live with that. Above 6V the PSU handled the resistor very well, I kept the resistor on at 12V until it got too hot and I had to disconnect it to keep it from self destructing. The LM317 was barely warm after I took the resistor off. Last thing to do was to bodge the front panel on the box and the PSY was done.

As advertised this thing is quick and dirty but it works well and the 10-turn pot makes adjusting the voltage quite easy. As of now I have it charging a tiny Li-Ion cell through a resistor. Also here's a shot of the heatsink on the back of the PSU.

You can see it's quite small, only about 50x50x10 mm in size but is enough to keep the LM317 cool and since my power brick tracks the output the LM317 for a fixed output current dissipates the same amount of power at all voltages unlike if I had a fixed 24V going to the LM317. All in all, I'm quite pleased with how this PSU turned out and I'll be using it quite a bit in the future.
That's all for now, thanks for reading.