Dual Rail LM317/337 PSU - Feedback - Page 1
Forum section: electronics / projects

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 23, 2016, 11:34:44 am
content: HelloI am in the process of making a super simple linear regulator based PSU. This will be powered by a step-down isolation transformer (240Vac to ~16Vac) with the incoming voltage then being rectified and fed into some simple linear regulators (LM317 for +ve rail and LM337 for -ve rail). For protection I plan to have a simple fuse at the mains input and polyswitches at the output. Also the rail voltage will be adjustable via a trim pot and the value will be displayed on a cheapo voltmeter unit. In the end this is all going to go onto a PCB from either seeedstudio or smart-prototyping.With all that said could you fellas:1. Comment on the circuit I've made so far, have I made any simple mistakes?2. Comment if I need to have some sort of transient protection on the input/output stage? Would using a MOV/TVS be a good idea for something this simple?Cheers

author: vasi_ro
author_rank: Unknown
timestamp: March 23, 2016, 12:34:13 pm
content: First you need a double 16v transformer and a full wave rectifier. What is the purpose of "rail buffer for display" circuit ?

author: Audioguru
author_rank: Super Contributor
timestamp: March 23, 2016, 02:55:29 pm
content: Most of the circuits shown in the datasheets of the LM317 and LM337 show the more expensive LM117 and LM137 that use a 240 ohm resistor from the output to the ADJ pin like you have. The LM317 and LM337 must use 120 ohms or the output voltage can rise (and destroy a low current circuit you might be powering) without another load.The details in the datasheet are the "Minimum Load Current" which is 5mA for an LM117 and LM137 but is 10mA for an LM317 and LM337. The resistor from the output to the ADJ pin has 1.25V across it and is the only load if there is no other load. 1.25V across 120 ohms is enough current to prevent the output voltage from rising.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 23, 2016, 10:04:41 pm
content: Quote from: vasi_ro on March 23, 2016, 12:34:13 pmToo late to change the transformer now since I got my current one for free, I think I'll just have to deal with the inefficiency.Also the main purpose of the buffer circuit is to invert the -ve rail voltage (or convert to a +ve value) so that it can be read by something like this voltmeter:http://goo.gl/EgyyG8Quote from: Audioguru on March 23, 2016, 02:55:29 pmGood catch had no idea about the minimum load current, the learning never stopsSince I can't find LM117/137 in a TO-220 package I think I'll just half the original resistor values instead.

author: n3vti
author_rank: Contributor
timestamp: March 24, 2016, 01:51:46 am
content: Is your transformer a center-tap? It looks like there's only one side of the AC going into the input. In looking at the Positive rail, it appears that the rectification is only half-wave, via D1. If you use a full-wave rectification (2 diodes), you'll have about 22.62V coming out of CE2, less the diode drops.You can run both the positive and negative rails off of one transformer, if said transformer can carry enough current.Hope this helps, or hopefully I didn't confuse you

author: Audioguru
author_rank: Super Contributor
timestamp: March 24, 2016, 02:22:21 am
content: Quote from: ANTALIFE on March 23, 2016, 10:04:41 pmYou will also need to halve the values of the pots and resistors in series with the pots.

author: med6753
author_rank: Super Contributor
timestamp: March 24, 2016, 04:48:50 am
content: Your design looks sound. Here's a supply I built 25 years ago to power some police/fire scanner equipment plus some Heathkit test equipment that normally operated from batteries. The supply is still in use today and has been absolutely stable. I utilized a LM317T/LM337T pair just as you propose.

author: Probb
author_rank: Contributor
timestamp: March 24, 2016, 05:25:41 am
content: Just as an after thought - if you're not worried about isolation for your circuitry and want to cut the bulk, toss the transformer and design a simple cap divider circuit to knock down your ac voltage....Also, incorporating any kind of protection circuitry depends on the application. It's good practice to add protection in any kind of power electronics; however, the right size fuse in your situation might do fine.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 24, 2016, 07:16:41 am
content: Quote from: Probb on March 24, 2016, 05:25:41 amWhat? Are you ***ing serious? No!

author: tszaboo
author_rank: Super Contributor
timestamp: March 24, 2016, 08:49:00 am
content: It looks mostly OK for me. You probably dont want to have a fuse on the ground. If that breaks, that will cause more trouble than good. I would not bother with MOVs, TVS maybe. Just size it little bit below the capacitor voltage.Others saying no: Please learn the qarter wave rectifier!

author: ZeTeX
author_rank: Frequent Contributor
timestamp: March 24, 2016, 09:05:57 am
content: Quote from: Probb on March 24, 2016, 05:25:41 amYes, try to draw 1A from a "simple cap divider" without spending half a million for capacitors.also, effiencey would be bad.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 24, 2016, 09:22:53 am
content: Quote from: n3vti on March 24, 2016, 01:51:46 amTransformer is not center tap (model is actually TM50-2, 16V in schematic is peak value), next time I need a tranny will keep the info in mind cheersQuote from: Audioguru on March 24, 2016, 02:22:21 amYuppoQuote from: med6753 on March 24, 2016, 04:48:50 amAww neat, thanks for that always good to have reference schematicsQuote from: NANDBlog on March 24, 2016, 08:49:00 amGood point, now that I think about it having the fuse there does not make it an "accurate" ground

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 25, 2016, 02:15:10 am
content: If you want a little more headroom in the rectifiers, you could replace the diodes with ideal diode-circuits (P-Channel MOSFET + Difference-Amplifier). Depending on the MOSFET you chose, you can reduce the Vdrop in the rectifier to only a few millivolts instead of almost 1V.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 25, 2016, 03:25:26 am
content: Quote from: SaabFAN on March 25, 2016, 02:15:10 amYea did see that Linear Technology have a bunch of cool "ideal diode" IC's, even have one for a full wave rectifier. Super neato

author: TonyD
author_rank: Contributor
timestamp: March 26, 2016, 12:38:56 am
content: Quote from: ANTALIFE on March 25, 2016, 03:25:26 amIs it the LT4320 Ideal Diode Bridge Controller you were speaking about?

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 26, 2016, 01:05:35 am
content: Quote from: TonyD on March 26, 2016, 12:38:56 amYup that's the one, found it on this pagehttp://goo.gl/ZWydnb

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 27, 2016, 11:18:48 am
content: Instead of starting a new topic I'll just post an update here.I've modified the schematic to include a couple of zener diodes for over voltage protection on the outputs, and have also laid out the circuit on a 5cm x 10cm PCB:Running a quick rule check on Altium gives no errors, unless someone can spot a mistake then I'll go ahead and get the PCB's made. Also found a useful website to compare prices between PCB manufacturers:http://pcbshopper.com/

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 11:43:44 am
content: The part number (zener) seems to be a (google check) normal diode ?Will they have enough power dissipation to handle a full on overvoltage ?Will the overvoltage be able to blow any fuses ?

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 27, 2016, 11:49:12 am
content: Quote from: MK14 on March 27, 2016, 11:43:44 amWhoops, forgot to update the library when creating the component, it should be 1N5355B which is also a 18V 5W zener diode.From my rough calculations the zener should be able to handle the overvoltage while it waits for F1/2 to blow, when that happens the output will be completely isolated.

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 11:53:47 am
content: Quote from: ANTALIFE on March 27, 2016, 11:49:12 amWhen you say "overvoltage", do you mean that the LM317/337 have failed short circuit, or do you mean an external overvoltage ?I thought you meant a regulator has failed, short circuit. Hence I could NOT see any fuses.For external overvoltages, I see what you mean by F1/F2.EDIT: Sorry, I somewhat misunderstood, when you said overvoltage. I thought it was regulator failure you were worried about. Rather than external abuse, which is probably your real concern (given your last post).

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 27, 2016, 12:59:18 pm
content: Quote from: MK14 on March 27, 2016, 11:53:47 amYup overvoltage as in I've done something wrong at the output and am feeding too high a voltage into the converter (not at the AC-DC side).

author: AndersJ
author_rank: Frequent Contributor
timestamp: March 27, 2016, 01:16:26 pm
content: What is U3B supposed to do?

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 01:24:04 pm
content: Quote from: AndersJ on March 27, 2016, 01:16:26 pmBest if OP answers/confirms.Presumably it is to invert the negative voltage signal, so that a digital display (or ADC), can show it as a positive value. E.g. One which CAN'T handle negative voltages.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 27, 2016, 01:38:17 pm
content: Quote from: AndersJ on March 27, 2016, 01:16:26 pmExactly as MK14 said, I want to display the rail voltage on a cheap voltmeter likehttp://goo.gl/EgyyG8. Unfortunately these voltmeters can only read +ve voltages, so U3B acts as an inverting buffer (gain = -1) to make the -ve voltage into a +ve one.

author: RiverTown
author_rank: Regular Contributor
timestamp: March 27, 2016, 07:48:38 pm
content: Shouldn't opamp be inverted then, and not like drawn in schematic?I mean inverting amplifier with same resistor values (gain -1), you have positive feedback in your schematic.

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 07:56:35 pm
content: Quote from: RiverTown on March 27, 2016, 07:48:38 pmApparently, the diagram you just produced, is functionally IDENTICAL to the schematic (OP).Where is this difference (between the circuits) or positive feedback ?I CAN'T see it.

author: RiverTown
author_rank: Regular Contributor
timestamp: March 27, 2016, 09:29:29 pm
content: ?

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 09:47:01 pm
content: Quote from: RiverTown on March 27, 2016, 09:29:29 pmSomehow (blame=============>>>OP), different versions of the schematic, have been examined.One WITH, one WITHOUT the issue we have been discussing!

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 09:54:48 pm
content: Correction:I'm NOT sure who is to blame. It could be the forum software, because I apparently only had 2 files, above. But 4 or 5 have appeared.So maybe the forum software is too confusing, when files are edited or something ?

author: RiverTown
author_rank: Regular Contributor
timestamp: March 27, 2016, 09:56:49 pm
content: Weird I still see faulty schematic, maybe it isn't OPs fault but something with forum because there isn't edit mark under the post.Important thing that we are on the same page.EDIT: Yes, could be some forum bug

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 27, 2016, 11:30:33 pm
content: Quote from: RiverTown on March 27, 2016, 07:48:38 pmWhat I've done is exactly the same as in your picture though? Signal goes into -ve input via a R, you feedback the output into the -ve input via R, and +ve input is grounded.EDIT: whoops looks like the very first schematic I posted had that mistake, my latest one has the correct op-amp configuration