Building Your Own Bench Power Supply

After looking around online at expensive bench power supplies, I decided they were often much too costly, and cheaper ones used boring resistor voltage dividers. I decided to set out on my own journey... I probably should have just bought one...

I decided that I would build a bench supply with DC 5v and a variable output as well. I am trying to use as many parts that I already own as possible to reduce cost. I dug through my pile of voltage regulators, and pulled out the trusty old LM7805 for 5v, and the LM317 for Variable. Those regulators are classic and tried and true. The data sheets also contain diagrams for building power supplies, it couldn't get easier! well... no.

It looks like I know what I'm doing, eh? HA!

 

It looks like it's fine, 15V no ripple...

So, this thread really isn't complete, I'm only half way into the project. Instead of an AC transformer, and bridge rectifier, I tried to fast track by re-purposing an old laptop 18.5v supply. (FAIL). Stepping down directly from that high of a voltage down to 5v produced an insane amount of heat on the LM7805 - it could actually only run with a load on it for a few minutes (1/3 of an amp). On the other side, with the LM317, if I set the voltage to 15v I could hold a full amp for 5-10 minutes without getting some voltage drop and thermal shut down issues.

 Ooops! Melted breadboard and resistor!

(Note the added heat sink on regulator)

This was actually my fault and not the resistor. I was using this 5 watt resistor as a test load. It's 15 ohms, and I was running it at 15 volts as a test load. That's 1 amp and 15 watts... 10 watts more than the ceramic resistor rating - after 20 minutes it became really really hot, I put a heat sensor on it, and it exceeded 300 degrees Celsius! This load had nothing to really do with the operation of the circuit, but it entertained me anyway. 

I still do have heat issues with both regulators going into shut down. I added small heat sinks that helped, but they aren't making the cut. I also need to consider perhaps using two transformers, one dedicated to the 5V rail that is maybe only 9-12 volts. Heat issues with the variable side are also concerning, I'll try tracking down a better heat sink and consider a cooling fan as well. I suspect most the variable supplies you see people make online are not handling much load, or aren't pushed very hard or for very long. The ideal simplistic versions of LM317 supplies don't seem to deal with the issues that arise when you test them. I am very glad I bread boarded this and am working out the kinks slowly.

The main project goal is to have the finished project on a solder board and in an enclosure. I posted this prematurely, but I think posting this will help to prevent me from shelving the project.

Once I have what I feel is the best configuration, I will post a schematic and some pictures.
Stay tuned!