Basic LM317-based CV/CC 15V+15V 1.5A split Lab PSU

[fsr]

This Lab PSU is from the following circuit that appeared on LM317's datasheet back in the day it still said "National Semiconductor" in there. I just made some little modifications and added a negative supply, so as to make full use of a 15+15 Vac 3A transformer i already got. So, this is the original circuit from the datasheet:

Positive Side (from datasheet).png


Just don't believe on the claimed max voltage and current there. You would be making a room heater if you go to that values, and will need a serious redesign. Forced air heatsinking it's the first that comes to my mind. Probably a lot more changes required.

Now, before i go further, let me say that i never really tested it, but it seems to work without any issues i could detect. Anyways, i promised i would show the project if anyone was interested, so there it is. Also, this is a simple project with just analog components and one-sided PCB's, that can even be made with the toner transfer method, so it could be an interesting project for beginners.

I made the final version in three little boards, because of limitations in Eagle PCB (i used v6.5). Here are the schematics of every one of the 3 boards (of course, if you don't need the negative board, you don't need to build it, buy you're still going to need the -6v for the positive supply):

B1 - Fixed Voltages SCH.png


B2 - Positive Reg SCH.png


B2 - Negative Reg SCH.png


The power transistors are MJ15003 and MJ15004. Each of them must be mounted on a heatsink of 2,85 °C/W thermal resistance (or lower). I used mica washers and thermal grease. No fan needed if mounted on free air, but take notice that the power transistors have their metal cases connected directly to the collector (in fact, the case is the collector terminal), so... not a good idea to short those. Beware of nearby metals.

The LM3x7's need a small heatsink also. I used a 10 °C/W heatsink, but the theory said you can go up to 25 °C/W.

Well, i think that's all, but i built this like ages ago, so let me know if you need additional information, and check everything. I'll attach the datasheet from which i took this, the PDFs for the PCBs, and the Eagle 6.5 files.

Regards

[David Hess]

It is a clever design which takes advantage of clamping the 301A via its compensation pin.

It can be improved thermally by using the current sharing configuration shown below and mounting the 317 and power transistors on the same heat sink so that the 317's integrated thermal protection can also protect the transistors.