Oscillations on the output of power supply (LM317/LM337) - Page 1
Forum section: electronics / repair

author: Ahura Mazda
author_rank: Contributor
timestamp: October 20, 2020, 11:24:25 am
content: Hi everybody, I've spent a few last days building linear supply with adjustable voltage using LM317 and LM337 combo. But I noticed some strange behavior.Before changing Cout (2200uf):Thinking it would be nice to add as much capacitance as possible, I firstly put 2200uf on the output. I missed that datasheet says 1uf recommended for better ripple rejection at low frequencies, but not needed.Either way, the circuit seemed to work until I AC coupled the signal and saw some strange behavior I still don't understand. Random spikes.Another example, but here I understand LM317 is no longer regulating since headroom of 3V is lost.Removed caps / 1 uF / 10 uF:Signal is oscillating, frequency of arounf4 Mhz,with or without load. The difference is, as higher in capacitance I go, the higher the Vout until the oscillation occurs. Also, both outputs are oscillating.------Steps taken:Pretty certain all connections are good on the PCB.Disconnected Q2 and Q6 thus also eliminating Q5 and Q5. Still oscillating.Cut open connection on R8, eliminating Q1 (so Q3, too). Still oscillating.Any help or suggestion for the oscillation would be greatly appreciated. Thank you guys.

author: fcb
author_rank: Super Contributor
timestamp: October 20, 2020, 11:41:13 am
content: Layout could be a contributor. How close are the caps to the regulator?Also have you tried removing the cap on the ADJ pin and/or bypassing it with perhaps 100nF?What's your static load? They like to be loaded a bit (10mA?) - although you indicate it's the same loaded and unloaded, your load might be quite inductive. Try a small load right up close to the regulator.Have you tried a zobel network on the output (perhaps 2R2 in series with 220nF-pure guess).

author: Ahura Mazda
author_rank: Contributor
timestamp: October 20, 2020, 05:08:16 pm
content: Well, it turns out it's the pots. Brand new, yet obviously not made for this purpose. I've swapped them with precise trimmers just to test whether they were the problem, and they were. Now the signal looks quite clean, would say under 1mVpp noise. Just need to figure out what potentiometer to buy so I can actually mount them on the case. Am worried they might do the same thing.I thought the signal is common for both outputs, but apparently it's not. After changing one, I checked the outputs, and one was crisp clean, while the other was oscillation.QuoteAs a matter of fact I did with another 1uF. 100nF probably wouldn't change much since it would be 10uF in parallel with 100nF, right though, and the freq?QuoteNo, but now I must research this. I don't know this stuff.I appreciate you responding. Thank you.Edit:Oh my, I forgot to write the most important thing. I couldn't get the signal not to have huge ripple without Cout being more than 470uf. With pots changed, the ripple was still present.

author: magic
author_rank: Super Contributor
timestamp: October 20, 2020, 06:01:09 pm
content: Unlike LM317, negative regulators tend to have some minimum requirements regarding output capacitance and possibly ESR. Check your LM337 datasheet.The second photograph looks like the positive regulator might be dropping out due to insufficient input voltage, if the bottom channel is scaled vertically more than the top. Not sure how pots would affect that.

author: Ahura Mazda
author_rank: Contributor
timestamp: October 20, 2020, 08:12:17 pm
content: Thanks for replying.Yeah, I thought the same thing about the insufficient headroom.The pots are noisy. I touch the lead, signal jumps up and down. It looks like I had problem with both pots and Cout. When switching the pots, no signal was jumping around, and when I increased Cout, there was no ripple.Now, I might have made a mistake confusing the videos. In the last one, there might have been load connected, I'm just not sure now.The question I have is, should there be a ripple on LM137 when resistive load is connected, but with Cout<470uF in my case? Looking by datasheet, Darlington pair is always on, supplying current as needed, so I in fact I see no reason for the filter cap. I've connected 680 Ohms as load, drawing around 17 mA at 12V, and yet there was that ripple.I'm just not knowledgeable yet to answer precisely, but switching pots and inserting high value cap solved my problem.

author: Labrat101
author_rank: Unknown
timestamp: October 20, 2020, 10:03:53 pm
content: Remove the Diodes D7 & D8 that sits across the in and out of the lm337 & 317and put them between the out to centre ground .The lm337 & Lm317 are not matched so there is a voltage swing .So causing an unbalance . what you are seeing is the higher voltage pulsing in reverse .To get a perfect match between the plus 0 Negative will always be slightly higher on the Lm317 side.so placing the diodes across the outputs in reverse polarity will stop the feed back. ie across the two C 470ufThis is showing on the funny ramp on your scope .This is also recommended by the manufactures as wellThe diodes as they are now are causing the internal overload to triggeron reverse voltage .once those diode are moved to the right place the circuit will function correctly .D4 & D6 are also not necessary and should be moved as per picture& MUST remove D7 & D8Sorry about the bad paint over . did it by handmake sure they are in the right direction .

author: Ahura Mazda
author_rank: Contributor
timestamp: October 20, 2020, 11:01:04 pm
content: Hey, thank you for replying.Are you sure? I've browsed a lot of LM317-LM337 combo psus and not once have i came across that. Where can I see the recommended schematics?QuoteI guess I can add additional diodes and try, but for example, when set the output of LM337 to the lowest, the signal on Vout of LM317 will look the same. Shouldn't the signal change if what you are saying is true?Also, I see that both signals are clear now and only when I set the output Voltage too high do I see the funny ramp, as you say.Quote from TI:QuoteONSEMI:QuoteEdit, I'll definitely try now. Won't hurt to try either way. Thank you again.

author: Labrat101
author_rank: Unknown
timestamp: October 20, 2020, 11:04:21 pm
content: Quote from: Ahura Mazda on October 20, 2020, 11:01:04 pmIt will work I made similar circuits and they work fine . change as per my drawing and it will work . the diode across is only for single . not duel output the diode I moved will protect the circuit just make sure they arelarge enough to take a surge .

author: Ahura Mazda
author_rank: Contributor
timestamp: October 20, 2020, 11:38:40 pm
content: Nope, same thing happening.At 15V on output, this starts happening. Vin was 18.18V, so that is that 3V headroom needed for regulation. Thank you for suggestion though.I think the circuit is working as it is supposed to.Edit: The noise literally doubled to around 1,3-1,4 mV. Though I can't see the spiked nicely. It's even greater.

author: Labrat101
author_rank: Unknown
timestamp: October 21, 2020, 08:30:31 am
content: Quote from: Ahura Mazda on October 20, 2020, 11:38:40 pmHi sorry it was very late when I looked at the schematic .you have other errors in the drawing R3 should be at-least 240 ohm and the cap C1 & C3 should beceramic Not electrolytic try 0.1uf .. What is the value of your Pot ??Also use this link to do the calculations it will make life easer .https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator.Also why did you put R11 & R12 . they are too low a value and will draw a lot of current .I am not sure were you found this circuit from but its very power hungry and properly only 60% efficient.What sort of load are you aming for ie Amp and voltage output required ?? .You have also put a current limiting and voltage . the Q4 & Q6 Will switch on hard duepower supply connection being wrongAlso the circuit shows the bridge 4 diodes .Where is the power coming from for the LM337 ?? I guess you stuck it to the neg of thebridge ..Please show the complete Power source connectionsFor Dual power supply to work correctly . You must use a centre tap transformerie 15v 0 15v . having a floating ground can work fine .But Not as you have done it ..THIS TYPE OF CIRCUIT MUST HAVE ITS GROUND CONNECTEDto the transformer centre TAPThis is an example I found as I was to lazy to draw it out . That is correctly PoweredBTWQuoteConnect the power supply as per the drawing and all will become clear.Plus the other recommendations

author: Ahura Mazda
author_rank: Contributor
timestamp: October 21, 2020, 02:41:17 pm
content: QuoteWhat, no. Value is to supply minimal load current for maintaining regulation. 1.25 V / 120 Ohms is around 10mA. Datasheet says, 3.5 mA is typical, yet I've read that it's for more expensive version of LM317. Either way, for 240 Ohms, it would draw around 5 mA and I'm not saying it won't regulate properly, but judging from other's experiences, 120 Ohms is a better choice.QuoteAll datasheets suggest putting 1/10uF electrolytic capacitor.Quote from ONISEMI: "A 10uF capacitor should improve ripple rejection about 15 dB at 120 Hz in a 10 V application."Quote5k.QuoteTo discharge the caps after power is switched off, they are too big actually, takes ages since they almost draw no current. Since neither output voltages are high, and I'm the only one using this, I just put big value.Don't know about efficiency, but it depends on many factors, one being Vin-Vout and Iload. I'll be using for many things, don't know.QuoteYou got something wrong. Q6 will be fed through only Q1, and Q4 won't turn on all until around 7 A are passing through the R9.QuoteIt is center tapped transformer. You can see all the lines on the schematics.AC+andAC-are from the output of the transformer, andGNDTAPis from center tap.It is the same circuit with added current boost circuit, short circuit protection for power transistors, improved ripple rejection and transient response, and plus the diode protection for the ICs. Absolutely the same starting point.I ask you to tell me why what the circuit is now doing is not as it should be?1. No signal bouncing due to new pots and no ripple on the output.2. After Vin-Vout falls below the headroom, IC doesn't regulate.I see it's all good.

author: rdl
author_rank: Super Contributor
timestamp: October 21, 2020, 06:37:01 pm
content: R3 and R5 are fine at 100-120 ohms, higher will usually work but is only in spec with LM117. I have never seen a datasheet with anything but electrolytic for the adjust pin bypass cap.Do you have the wipers of the adjust pots connect to one end or the other? I saw a web page not too long ago which showed a better way to connect the adjust pot, but I can't find the link now. I will look more.As for making the LM337 fixed, I fail to see the point but maybe I'm misunderstanding. AoE 3 has a schematic for a dual tracking lab supply which uses just one pot to adjust both regulators, maybe something like that is what is meant.Link to free Art of Electronics 3 chapter 9. Schematic is on page 608. Anybody thinking of building a power supply should check this out.http://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdf

author: Labrat101
author_rank: Unknown
timestamp: October 21, 2020, 08:19:07 pm
content: Quote from: rdl on October 21, 2020, 06:37:01 pmWhat you say is correct . But one has to take into account that the details you are referring toare for the Texas OEM's which are slightly different . In this manual which is now 20 ago.In the real world most of the LM's are not OEM and are of another origin .From my past experience of power supplies using a ceramic works far better . on the sense pinupto 10uf will work even on a NON OEM .And Using op amp's is a far better way to go.One of his scope reading looked like the clipping of the internal LDO which tells me that theseare not the t series . and maybe rebranded which will work OK but they need bigger heat sinks.That why I suggested the 240 ohm . And the calc link .I have made so many power supplies I have lost count over the last 50yrs.I used a LM317t a few weeks ago to make a precision 3.3000v that had to be drift free.electrolytic caps 1uf to 10uf have a high ESR . Ceramic have a better characteristics.This type of duel power supply has been around for yonks .But today many of the LM317 & 337 are not equal and when paired there can be a noticeabledifference . ie Example >>The Pos. +14.90v & Neg. -14.45v with identical parts and layout.The error may not bother you but its going in wasted heat & power loss efficiency .RNS

author: fcb
author_rank: Super Contributor
timestamp: October 22, 2020, 10:37:15 am
content: Quote from: Labrat101 on October 21, 2020, 08:19:07 pmLM317 and LM337 work by servoing the output to keep 1.25V between the ADJ and OUT pins.What you describe (14.90-14.45=0.45V) is likely not the fault of the LM3x7 but whatever you have hanging on the ADJ pin.

author: Labrat101
author_rank: Unknown
timestamp: October 22, 2020, 12:29:15 pm
content: I did write 'Example' ...QuoteThat was the funniest thing I have hear all day((Personally my ADJ pin works just fine Thanks for asking ))Some how no one has read correctly anything that has been written.I / we don't have any such problems with power supplies . Building or repair. etc.Its the other people that have trouble .First He has Never supplied adequate info.1 .. was it build on a push board ?2 .. Are the components new or from junk S/H parts ?3.. Was that scope ever Calibrated & reset before the test ?4 .. Have the probes been checked .. were they set to 1 or X105.. what were the exact setting of the scope during the test .6.. Only a drawing of the schematic was shown .. Not the Actual Built assembly . !!How do we Know, That it was wired up correctly ?SO Far it's only been hear Say ..He has ask for help and not given enough info .. The schematic should work Maybe not perfectly .So Far the Info shows some images of a scope screen which could rightly displayinga Faulty Scope Problem ...or Human error more likely .There is no clear pictures of this layout complete with showing all the connection .And he is assuming that we Here on this Forum all HAVE a magic crystal Balls showingwhat's on his desk /Bench .

author: Ahura Mazda
author_rank: Contributor
timestamp: October 22, 2020, 04:33:57 pm
content: Hi, I suggest you repost your condescending comment from day earlier. For a man with such integrity, given 48 years of work in power supplies, I expected so. I certainly appreciate your time given into this writing and redrawing the schematics to help me rewire the thing. And of course I tried, you tried to help. Yet, you've lost credibility when, apart from not following datasheet guidelines, to telling me the circuit won't work (when it did even before modifications) and Q4 and Q6 would burn up because improperly wired. Also giving me far less complicated schematics of how it "should be wired up". I saw appeal to authority fallacy.Also, I think schematics was pretty straight forward.Though you are absolutely right I haven't given explicit information whether my scope is calibrated or not, nor have I given exact ripple voltages and my oscilloscope settings, I expected one knowledgeable enough to know whether the signals output was in accordance to what it should be. And you are right, I apologize. I will learn from this, since it indeed might be confusing to many.I appreciate you reading this. Thank you again and have a great day.

author: Labrat101
author_rank: Unknown
timestamp: October 22, 2020, 05:36:51 pm
content: All is Ok .BTW I never saidQuoteI said they would switch hard .. As opposed to smooth. ( linear )Enjoy Have aRNS101

author: rdl
author_rank: Super Contributor
timestamp: October 22, 2020, 08:16:45 pm
content: How is the circuit currently assembled? Is it on a breadboard or point to point wired so that changes can easily be made? Do you believe that the regulator chips are genuine?

author: Labrat101
author_rank: Unknown
timestamp: October 22, 2020, 10:20:27 pm
content: Hi ,I am not sure why I am doing this But here it is.I ran LM317 under a stress test .15v @ 1.5 amp as MAX ratingCh 1 set 200mv @ 2msto show the shutting down . or over heatingwith your values .is this same as you photo .??Which emulator did you use to draw your circuit ??

author: Ahura Mazda
author_rank: Contributor
timestamp: October 23, 2020, 05:50:54 am
content: Thank you for replying.QuoteIt's PCB, was since I post one. Chips are genuine. Bought from credible shop here. I've attached my assembly, and all connections are 100% made even though it looks quite ugly due to my inexperience. PCB layout is debatable and rookie in itself; I later saw I could have made regulators more distant. I've added ferrite beads on the inputs and outputs, as for Jim Williams application note I saw, on how to reduce noise on voltage regulators.QuoteThank you, I appreciate it. As you can see, I've added heatsinks. When drawing 1,5 amps, or any higher than a few hundred mA, transistors take the hit. Even when drawing 2 A from both sides both, LMs are cold, and transistors get nicely warm. So , I don't think that any of the LMs are getting shut down. I think it's from not sufficient headroom to regulate. Also note that NPN power can take 7A, that's why R9 is 0.1 Ohms, so it will automatically turn on Q4 an make LM317 supply more current, thus not burning the transistor. Same goes for the other side, with PNP's maximum draw being 3 A.My reply #8 shows when I just make the output 15V, with input being 18.18V, and losing headroom. Then the second one is me increasing Vout.I did say that even when drawing high currents, with enough headroom, output voltage is perfectly stable. I'll post some photos later today when I get back from university, giving more on current draw from the chips.Edit: I've forgot to mirror the layout, so I got into some trouble later with LMs. Had to mount them upside down on the heatsink.

author: Labrat101
author_rank: Unknown
timestamp: October 23, 2020, 09:36:42 am
content: OKFirst The top board layout looks nice . I can see you have put a lot of work into this First attempt .The soldering side . Well . First I will make a Friendly suggestion do not power it up again untilYou have removed some of the Solder with a solder pump or Solder wick.You have used enough solder for 10 such boards.I zoomed in on your soldering photo . there are at least 3 near shorted tracks and several bad joints .yes I know you said that you were new at this.I don't know what type of soldering iron you are using . But by looking the solder wire that you are workingwith is properly too thick for your iron . maybe your working with 1.5mmif so find some 0.8 mm 60/40 Multicore CF10 flux 2% not the lead free stuff .I recommend that you use some flux and remove all the solder and re do it ..Sorry . yes its a bummer . (look at it as a learning curve and not a punishment )when complete clean the entire board with a toothbrush (old) and Alcohol to remove any stray blobs etc.90% of all electronic problems are due to bad connections & dry joints .I think after you have done that. Things will look different electronically.Reducing Risks of any project ''The Dreaded Magic Smoke ''& the First Law of Murphy.1 Question what are you going to use this power supply For ?? or to power ? .Is it by any chance A bench power supply ??UPDATE .. on one of your earlier post you ask were did I find the 240 ohm value .A lot of the data sheets posted on the web had a typo and was updated a long time back but most of Google searchesdid not include the newer revisions . Photo .