LM317 Variable supply - Feedback/Suggestions appreciated. - Page 1
Forum section: electronics / beginners

author: Anonmis
author_rank: Contributor
timestamp: April 12, 2013, 07:10:00 am
content: So I have decided to attempt in making my own variable power supply. I would like to start off with something a little more "simpler" and work on more advanced ones in the future. I have watched the power supply videos on here as well as several different ones on youtube. I have also done a search for LM317 on the forums to see what has already been covered. I have drawn a design and would appreciate if I could get feedback whether or not this type of design would work. (Please see attached).Questions that I would appreciate feedback on:- Am I able to use the same LM317 input voltage on the Voltage inverter input? (same node).- Would this design work or what are some issues that it can possibly have?- Is a center tapped transformer better, why or why not?- What are somethings I should pay closer attention to?- Verifying that I am connecting this transformer right..... My reference in this case would be the - of the rectified voltage.Feel free to answer any questions or provide any feedback that may be helpful. Thanks everyone.

author: Strada916
author_rank: Frequent Contributor
timestamp: April 12, 2013, 10:17:24 am
content: See app notes on LM317 I'm sure I have seen two diodes in series between ground and adjust variable resistor. To make it flot 1.2v above.

author: Anonmis
author_rank: Contributor
timestamp: April 12, 2013, 03:41:45 pm
content: I have checked the app notes before, but I checked them again now just to make sure. I am not seeing the configuration you mentioned in the datasheet, unless I missed it somewhere lol. They do have a 0-30V regulator design, but this uses a negative supply which in my design I am using the voltage inverter to create that.That is my quick look over though, I will go back and look at the datasheet a little closer.

author: Thor-Arne
author_rank: Unknown
timestamp: April 12, 2013, 05:35:40 pm
content: Strada916: If my memory serves me well; the diodes on the adj pin is used to compensate for the drop in a pass transistor, usually used with fixed voltage regulators like the 78XX series.Anonmis: Your charge pump should be connected to the AC side of the bridge like they've donehere.

author: stratogazer
author_rank: Contributor
timestamp: April 15, 2013, 02:00:10 am
content: Which charge pump are you using? The 7660 family would work but watch the input voltages. My design is similar using 3 charge pumps in parallel to reduce output resistance and increase output current. I'm using the ICL7662 by maxim.

author: Anonmis
author_rank: Contributor
timestamp: April 17, 2013, 04:48:36 am
content: Thanks for the suggestions, I am still working out a couple of different designs to see which works best. I might try the one you mentioned.

author: Paul Price
author_rank: Super Contributor
timestamp: May 02, 2013, 05:07:33 pm
content: -2V bias for LM117 the hard way. If your power supply range is below 15V you don't need the 12V zener and 220 ohm resistor.

author: kt315
author_rank: Regular Contributor
timestamp: May 03, 2013, 01:20:30 am
content: Paul, could you explain please, how do you get -2 V out of it?

author: ELEKTRICK GASCHIEF
author_rank: Newbie
timestamp: May 03, 2013, 08:53:30 pm
content: Quote from: kt315 on May 03, 2013, 01:20:30 amThe red LED is acting as a 2 volt zener diode!My take on getting the lm317 down to zero volts was to use the ICL7660 to generate the negative supply, though I think the output impedance might be too high, I think the 555 and charge pump might be better.I thought that maybe a -ve lm337l regulator could be used post charge pump to get a more stable negative reference.Another concern with charge pumps is the amount of ripple on their outputs, of course a regulator like the lm337l or something with a TL431 should clean that up, it would be a shame to mire the lm317's otherwise excellent ripple performance with a noisy negative reference.

author: kt315
author_rank: Regular Contributor
timestamp: May 04, 2013, 04:15:45 am
content: Curious. Thanks.

author: c4757p
author_rank: Super Contributor
timestamp: May 04, 2013, 04:39:53 am
content: Whatever happened to just grabbing a negative rail off the transformer? Use a center-tapped transformer, two-diode full wave rectifier for positive and another diode for negative. No need for a center tap either if you don't mind half wave. That 555 circuit is awfully complicated just to drop 1.25V off the output...

author: digsys
author_rank: Unknown
timestamp: May 04, 2013, 06:48:15 am
content: Optionhttps://www.eevblog.com/forum/beginners/zero-volts-from-a-lm-317/msg227118/#msg227118

author: rdl
author_rank: Super Contributor
timestamp: May 04, 2013, 01:31:02 pm
content: Quote from: c4757p on May 04, 2013, 04:39:53 amPlease excuse my lack of knowledge, but doesn't this basically waste half the output capacity of the transformer for no reason other than generating a couple of milliamps for the negative reference?

author: codeboy2k
author_rank: Super Contributor
timestamp: May 05, 2013, 03:25:50 am
content: Your knowledge is correct, it will waste half the transformer's output capacity if the only reason for using a centertap is for the purpose of generating a small -1.25 V bias for the negative supply.The center tap as described by that post is best used for an equal, bi-polar supply of + and - voltages. It gives a full wave rectified voltage at + and - the transformer's half-winding rating.If what is wanted is a single output positive supply capable of being adjusted down to zero, then it's much better to generate a small -1.25V bias for the Adj pin using one of the many methods already described (half-wave, charge pump, zener, shunt-regulator, etc).

author: c4757p
author_rank: Super Contributor
timestamp: May 05, 2013, 03:50:55 am
content: Quote from: rdl on May 04, 2013, 01:31:02 pmYou're both misunderstanding what I was suggesting - more like this. No capacity is wasted - both half-windings are supplying positive current. One winding is also supplying some negative.

author: mariush
author_rank: Super Contributor
timestamp: May 05, 2013, 04:03:37 am
content: Quote from: rdl on May 04, 2013, 01:31:02 pmI never claimed it would be very power efficient, or cheap, just that it's possible.But c4757p is right, it's possible to use a bridge rectifier and a diode to have both full current dc and a negative rail ..

author: codeboy2k
author_rank: Super Contributor
timestamp: May 05, 2013, 09:15:28 am
content: Quote from: c4757p on May 05, 2013, 03:50:55 amyes, I misunderstood. I somehow thought you were talking abot a 4-diode full wave bridge and a center tap, thus bipolar. The 2 diode full wave and the half-wave negative rail is really all that's needed; there's plenty of info in this post for anyone interested to do it right now, in many different ways, and when they do or do not have a center tap.That negative rail in your example can be filtered, zenered to -5.1V or so, LM337 to -1.25 volts, output filtered, and bob's ur uncle.This thread should be sticky. Or at least in the wiki.

author: Anonmis
author_rank: Contributor
timestamp: May 07, 2013, 04:56:47 am
content: Everyone has been giving very valuable information which I will definitely be trying out