View Full Version : Variable power supply circuit question

09-10-2006, 04:16 AM
I have a bunch of wall adapters that output around 15v-18v DC. I'd like to have a variable power supply that can output 2v-24v at about 1A at least and I found some schematics that uses the LM317 regulator.

The max that the LM317 can regulate to is 12v output though, I was wondering if I could build two of these regulating circuits and put them in series to create a variable 24v power supply? Would this be a good?

If it's a bad idea, should I series the wall adapters first and then run it thru a regulator that can regulate up to 24v?

PJ Allen
09-10-2006, 05:23 AM
Excuse me, but, the LM317 can regulate voltages higher than 12V -- check the datasheet.

"Series the wall adapters"? Bad idea.
Putting the outputs of two 78xx-based circuits: I have trouble just saying "OK" to this one.

Post Edited (PJ Allen) : 9/9/2006 10:27:58 PM GMT

Mike Green
09-10-2006, 05:34 AM
The LM317 can take an input voltage up to 30V or so, but has to dissipate a lot of power if you're drawing 1A at 2-3V output. If your series wall adapters are putting out 18V each, that's 36V - 2V = 34V x 1A = 34W of power to dissipate. I don't think the TO-220 package will do this. If you can find a TO-3 packaged LM317 with a really good heatsink, you might make it. You could certainly make two identical power supplies putting out 2-12V and connect them in series. That would require the LM317 to dissipate 18V - 2V = 16V x 1A = 16W. You'd still probably need the TO-3 package and a really good heatsink for each regulator, but that's more likely to work. You'd also have a second power supply if you only need 0-12V.

For this kind of usage, I'd recommend looking at a switching regulator. Their power dissipation doesn't significantly go up with increasing difference between unregulated voltage input and regulated output voltage.

Post Edited (Mike Green) : 9/9/2006 10:37:49 PM GMT

09-12-2006, 12:31 AM
*slaps forehead* Doh! Thanks for the correction PJ, I guess I was too quick at looking at the schematic's spec to realize he was just using a 12v source to be regulated.

I'll look into the TO-3 package, I don't think I'll need to run anything higher than, say, 18v but I was just wondering if adding the voltages together was a good idea. Can you recommend a switching regulator circuit or IC I should look at?

Thanks guys.

Mike Green
09-12-2006, 01:23 AM
Check out <www.st.com> for a whole line of switching regulators. Their L4973 can have outputs from 3.3V to 24V at a current of up to 3.5A. The datasheet shows a reference design complete with a PCB layout, schematic, and parts list. There may be others equally useful.

Steve Andrues
09-24-2006, 10:52 PM
You might look at the Linear Technology p/n's LT1070 and LT1074 switchmode regulators. One is a step-up regulator and the other is a step-down. They are easy to use, with a minimum number passive parts. I cannot say enough good things about these parts! They are used exclusively in all of our in-house test equipment. So far, the equipment has accumulated 100k hours of operation with no failures! Linear Tech's on line support is almost as good as Parallax!

Give them a try.