LM317 in parallel - Answered -> LM2596 circuit question

I'm interested to see what answers you get. I googled around and found several sources that said this was ok. Although 1 design used three of them to ensure he got the 3A he wanted. It's been many years since I did electronics professionally, now it's all hobby. I'm all software now. So it would be prudent to wait on an answer from the pros.

Mike

While I'm not a power supply expert, my understanding is that hooking up the outputs of two linear reguators in parallel is usually a bad idea.  It's very hard to get them to share the load equally, i.e., each put out approximately the same output current.  Usually one dominates, producing most or all of the output current while the other is idle.  I've heard that it's also possible to go into oscillation, with the two regulators fighting each other.

Using linear regulators to convert 14.8V to 9V is a 5.8V drop at 2.5A.  This is 14.5 watts that's going to waste and could make things pretty warm.  You probably want to use a switching regulator, which wastes far less power and runs much cooler.

In the LM 317 datasheet http://www.ti.com/lit/ds/symlink/lm317.pdf (sorry about the pdf) figure 13 on page 12 shows three of them in parallel (of course with supporting circuitry) to create an adjustable 4A regulator. You would probably have to order 2500 of one of the other parts in the diagram.

The regulator they diagram is 4.5 to 25 volts.

Again, though, I defer to the experts.

Mike

Message was edited by: Michael Conners

Hi Michael,

The pdf is moved here:http://www.ti.com/lit/gpn/lm317  via http://www.ti.com/product/lm317

I think you talk about page 12 / fig. 13.

It seems ok.

Hi Christos,

I would use the Webench Designer and choose some switching IC. You can request free samples from TI to try the design.

I used many LM317 and LM317K (TO-3 case) but they generate some heat at high current as I remember.

Regards,

Dan

Hello Folks, I have studied regulators and it's my opinion that if you use three identical regulators you could get away with the triple in parallel circuit, but the best bet is figure 14 of the data sheet. You don't have to slavishly follow the designed semiconductors so if one chip or transistor is unavailable try Googling the original part number e g TL080 and you will get access to a data sheet, decide what are the important parameters, in this case its an FET op-amp if I'm not mistaken (look it up) and try to Google for equivalence tables which should be available. The more time you spend in the design stage the better since you will accumulate knowledge and become adept at scanning data sheets. A lot of info on data sheets is superfluous or unnecessarily accurate. Then make up your 1st stab as a breadboard and put it through its paces. You can tell if a device is overheating with your fingers, but I can recommend getting a non-contact thermometer/pyrometer (digital) which are available for under £20. Then you can do some graph plotting. It's a wonderful hobby and also profession, but to be professional you have to surrender the serendipitous side of things and crack your problems. Happy hunting.

PS the reference M Conners gave for the data sheet was wrong, change 117 to 317 in the URL.

Interesting, that link was there yesterday. It was the datasheet for the LM117 and the LM317. And yes, now it is the circuit on page 12 , figure 13.

Thanks.

Mike

I updated the link and the reference.

Mike

One thing to keep in mind is that you said you will be running this off of a battery. I think John Beetem is correct in asserting that you will be generating quite a bit of heat with this circuit. Heat means power and I would be afraid that you might drain your battery pretty rapidly with a lot of unused potential.

Hopefully we have given you enough ammunition and information to help you decide whether this is still the direction you want to go.

Good Luck,

Mike

the lm2596 is a good choice for a simple buck converter http://www.ti.com/lit/ds/symlink/lm2596.pdf

it is avalable in through hole, and needs minimal heat sinking. and the math is straight forward right on data sheet