Dual power supply ripple question - Page 1
Forum section: electronics / projects

author: MarkF
author_rank: Super Contributor
timestamp: August 26, 2017, 02:45:08 am
content: I built a simple dual +/-15V power supply with a center tapped transformer and 3 terminal regulators. I have a 100mV 4KHz ripple on the +15V output. The -15V output is clean. I believe the 100mV ripple is within spec but I would like to clean it up if possible. Any ideas what is causing it and fix it?Hammond (185D28) 28V C.T. @ 1.5A transformerU1 is MC7915CTGU2 is MC7815CTGC1 & C2 are 6800uF capC3 & C4 are 1uF capD1 thru D8 are 1N5395 diodes

author: jpb
author_rank: Super Contributor
timestamp: August 26, 2017, 12:51:07 pm
content: You could double check the 4kHz isn't coming from somewhere else.One thing I notice is that you've not allowed very much margin for your transformer.Presumably it is 14Vrms each tap which is 14V * sqrt(2) peak ~ 19.8V but you have to allow for two (I think) diode drops and for any ripple and the supply might be down 10% so if you had a 10% low supply and 2V of diode drop then even without any ripple you're down to less than 16V and the drop out voltage of the MC7915CTG is 1.3V.And this isn't accounting for the ripple voltage. With linear supplies a larger margin is normally used even though this makes them less efficient simply because you have to allow for supply variations.Perhaps you keep dropping below dropout or it is just on the edge. So I'd check the voltage across the capacitors with a scope to see if it is staying above 16.3V.

author: MarkF
author_rank: Super Contributor
timestamp: August 26, 2017, 01:43:59 pm
content: This transformer primary is 115VAC instead of 120VAC.My measured voltage at the input of the (+) regulator is 22VDC. That should be plenty of margin.Also, the ripple is 4KHz. Not 60Hz if it was a dropout issue?I have the 4KHz with and without a load and nothing else turned on. I've only tested with a small load. Never checked the ripple when I had a heavy load on.

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2017, 01:48:54 pm
content: I do not see any capacitors directly at the input of the regulators. The data sheets usually require > 0.33 uF, typically an X7R ceramic capacitor, directly between the input and common pins. I have seen ca. 100 kHz oscillations when that was omitted on a 7815.

author: MarkF
author_rank: Super Contributor
timestamp: August 26, 2017, 01:56:57 pm
content: Quote from: TimFox on August 26, 2017, 01:48:54 pmThe6800uFcapacitors are on the inputs and the1uFcapacitors are on the outputs.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2017, 01:58:23 pm
content: But no ceramic capacitors, which have a lower impedance, at high frequencies, than electrolytic capacitors.What load are you using to test this with?

author: MarkF
author_rank: Super Contributor
timestamp: August 26, 2017, 02:04:30 pm
content: I tried adding a couple cermamic capacitors with values of 0.001uF to 0.1uF. They made a small change in freq but no amplitude change.The load I've been testing with is about 100mA.

author: drussell
author_rank: Super Contributor
timestamp: August 26, 2017, 02:09:56 pm
content: Yeah, 100 mV is way, way too high to be "normal." Personally, I would try two things. First, try changing the regulator itself in case it is malfunctioning somehow. You don't want to just band-aid a bad part.Secondly, I always place a small, low impedance capacitor right at the input terminals of the regulator, even on a supply where it works fine, you want to maintain good stability over time as your main supply electrolytics age and ESR increases. Try temporarily sticking a good, low ESR 0.1 to 1 uF cap right across the terminals and see if it works properly.I've just always considered that good practice in this kind of application, even on a linear regulator. It is even usually recommended in the datasheet:QuoteEdit: P.S. Also, make sure that your C4 is good and installed correctly, etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 26, 2017, 02:26:19 pm
content: The 4 kHz seems to be oscillation of the regulator. It is a little unusual to se oscillation with the 7815 - normally the 79xx are more prone to oscillation. Also 4 kHz is rather slow for the oscillation.Usually there should be low impedance caps close to the regulator in the input side. The 6800 µFs are not that good in the MHz range.The DS also calls for something like 300 nF on the output side - a 1 µF electrolytic might have too much ESR to be effective here.I would add ceramic caps on both sides of the regulator.One might consider a bad regulator as the cause too - I have not seen it this way before, but the 7815 are cheap and thus might not be all tested that well.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: August 26, 2017, 02:33:32 pm
content: Quote from: MarkF on August 26, 2017, 01:43:59 pmLine voltage is not constant. Normally equipment is designed for +/-10% variation in input line voltage. In some application I have seen a larger range.Andy

author: MarkF
author_rank: Super Contributor
timestamp: August 26, 2017, 02:33:49 pm
content: I actually built a second complete PCB with all new parts with the same results.Just did some more tests.-With a 100 ohm resistor as the load, the ripple is almost completely gone.-I found the issue powering my AD9834 based function generator. Measured it as only using 3mA.I need to find out what the minimum load current is for stable operation.Also, I will try putting a small ceramic capacitor on the inputs to see if that helps.The wires to the transformer secondary are only 3" long.

author: drussell
author_rank: Super Contributor
timestamp: August 26, 2017, 02:43:34 pm
content: Quote from: MarkF on August 26, 2017, 02:33:49 pmThe fact that it decreases with increased load is interesting... It certainly shouldn't be doing that!QuoteThere is something seriously wrong if it needs load to not oscillate. I wouldn't band-aid it like that.QuoteIt isn't the wires to thetransformerthat are the issue, it is the impedance of the bulk filter cap -> regulator input that is the issue.But, yeah, try adding a good low ESR cap right on the input and the output of the regulator. Even though they shouldn't be strictly necessary, it is good practice.With no load at all it should be stable even without the caps. Are yousureyour 7815s are good? Did the ones you've tried come from the same supplier and batch? Do you have any other ones around you can try, even if you have to steal an old one from another piece of equipment temporarily? It really sounds to me like those regulators are misbehaving.

author: MarkF
author_rank: Super Contributor
timestamp: August 26, 2017, 02:50:38 pm
content: Quote from: drussell on August 26, 2017, 02:43:34 pmThe 7815s are new from Mouser (ordered 5). They are manufactured by ON Semiconductor. Two of them have the same 4KHz ripple. I don't have any others from a different source.

author: nugglix
author_rank: Regular Contributor
timestamp: August 26, 2017, 02:51:43 pm
content: Quote from: MarkF on August 26, 2017, 02:33:49 pmThe datasheets quote something along the line of 5mA minimum load current.A little LED should do it.And of course the capacitors as suggested above.

author: drussell
author_rank: Super Contributor
timestamp: August 26, 2017, 03:53:57 pm
content: Quote from: nugglix on August 26, 2017, 02:51:43 pmHuh?I don't think I've ever seen a "minimum current" on a linear regulator before. That makes no sense.Quiescent current drawn by theregulator itselfwith no load? Sure! That's usually about 3 - 5 mA on a typical 78xx regulator, but there is nominimumoutput current required for proper operation or stability.

author: StillTrying
author_rank: Super Contributor
timestamp: August 26, 2017, 04:09:15 pm
content: "I don't think I've ever seen a "minimum current" on a linear regulator before. That makes no sense."Nearly all of them have a minimum output current, if the output current is below the min. the output voltage tends to rise.

author: nugglix
author_rank: Regular Contributor
timestamp: August 26, 2017, 05:42:42 pm
content: Output voltage defined for at least 5mA of load.Search for " 5.0mA <= IO <= 1.0A,".Btw, some other parameters show the 5mA load.Page 9 for 7815.

author: drussell
author_rank: Super Contributor
timestamp: August 26, 2017, 05:46:33 pm
content: Quote from: StillTrying on August 26, 2017, 04:09:15 pmI don't ever remember seeing an actual "minimum current" specification. Theydousually only specify the load regulation to be maintained within spec from between 5 mA to full output current for that model (0.5, 1, 1.5 or 3 A typically) because obviously there needs to be some current bleed off of some kind for it to be able to regulate within spec, otherwise, yes, it can drift up a little bit so they don't specify it to maintain perfect regulation with no load, but it should still bestable!100 mV of 4 kHz oscillation is not stable, IMHO.They do not specify some minimum load required for stability. There are many applications where they are expected to run unloaded and they certainly shouldn't oscillate like that with no external load. That would be a terrible design and they might as well just increase the quiescent current a bit and stabilize the loop internally if that was actually required. It isn't. You should not need a 5ma load to keep it stable, only to ensure the regulation % spec.I've never seen the behavior the OP is seeing on any 78xx regulator before.

author: drussell
author_rank: Super Contributor
timestamp: August 26, 2017, 05:49:46 pm
content: Quote from: nugglix on August 26, 2017, 05:42:42 pmYes, as I stated above, it isn't specified to guarantee the regulation % at less than 5 mA but I've never seen one bethatunstable at 0 load, just potentially a small bit of drift upwards.Plus, the OP stated that even with 150 mA load, it as "almost" gone. That is still very, very poor! Something is causing that thing to oscillate and it shouldn't be oscillating.In my personal experience, I haveneverseen a 78xx act like that before. Something is amiss.

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2017, 06:01:17 pm
content: Quote from: StillTrying on August 26, 2017, 04:09:15 pmUnlike adjustable regulators like the 317 where the adjustment pin current needs to be low to prevent bias current induced error in the external divider, the fixed 78xx and 79xx series of regulators use their common pin for their quiescent current so they have no minimum output current specification. Their specifications assume 5 milliamps minimum output current but that is not a requirement for proper operation.Quote from: drussell on August 26, 2017, 02:43:34 pmStudy the ON Semiconductor datasheets for their MC7815 and MC7915 carefully.In both cases, the input decoupling capacitor is only required if the input filter capacitor is located at a distance which will not be a problem here. If used, ON recommends a 0.33uF input decoupling capacitor but I have used everything from a 0.1 to 1.0 microfarad ceramic and a 1.0 to 10 microfarad electrolytic. Solid tantalum is probably the most foolproof because of low and controlled Q.The MC7815 does not require an output capacitor but it is a good idea to have one. The datasheet notes that values below 0.1 microfarads may cause instability and this is especially true with low ESR parts. Despite what the datasheet says in the text, the MC7915 *does* require an output capacitor for stability because of its common emitter NPN output stage. I have typically used a 1 microfarad solid tantalum here or a 10 to 100 microfarad aluminum electrolytic with the same on the output of the MC7815.That 4 kHz low level ripple is weird. What I would do as a test is connect the output of a function generator to the output of the regulator being tested through a 50 ohm resistor and a relatively large capacitance. Then with a low frequency square wave applied, the load transient response of the regulator may be measured using an oscilloscope while the input and output capacitor configuring is changed to find the best response.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2017, 06:39:46 pm
content: Quote from: StillTrying on August 26, 2017, 04:09:15 pmYes, this is true, the voltage may rise above the specified level, with no load but I wouldn't expect oscillation, certainly not at such a low frequency.I hate the way the LM78xx datasheets don't directly state a minimum load current of 5mA. All they do is specify the output voltage for >5mA <1.5A.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 27, 2017, 09:08:35 pm
content: I am suspecting that the culprit may be the PCB design!Try to connect a wire to the heat-sink and, with the other end of it, touch the input, output and ground by noting if something changes. Or connect a capacitor of a few pF in parallel with D6 and note the changes, if any.Another way is to construct the circuit on a breadboard or in an "air board" by connecting pin by pin with wire. Connect it as it seems on a circuit diagram:rectifiers --> filter capacitor --> regulator --> outputalso for the 0V (ground)... more tomorrow because it's past midnight and what I am saying may be without a meaning!

author: MarkF
author_rank: Super Contributor
timestamp: August 28, 2017, 01:19:38 am
content: So I'm even more in the dark as to what is going on.Tried 1nF and 100nF capacitors across D6.No change to ripple.I built up the circuit on a breadboard. Just the regulators(U1,U2) with heat sinks, bridge diodes(D1,D2,D3,D4) and input capacitors(C1,C2). Ref. PCB layout in the first post. No load.No ripple.Also, tried PCB without protection diodes(D6,D8).Still had ripple.So, what can be wrong with my PCB layout? It's a single sided PCB with the blue traces on the bottom. The heat sinks are mounted directly to the regulators (no insulators, just heat compound).

author: StillTrying
author_rank: Super Contributor
timestamp: August 28, 2017, 06:52:02 am
content: I think it's a good idea to keep the input and output sides well away from each other on these things, the PCB could be better there.Is the scope probe on X10, it might not like X1 loading.You could temporarily try C4 very close to the reg. on the back of the PCB.You could temporarily try 220nF very close to the reg. on the back of the PCB, output side first, and then maybe one on the input as well.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 28, 2017, 02:02:55 pm
content: In the datasheet, in the section "APPLICATIONS INFORMATION" and the "Design Considerations", there is some text that even if it is not analytic it says something.They (ONsemi) had a book relative to the subject but they are hide it!!! After a search, I found it in the following link:https://web.archive.org/web/20110817001511/http://www.onsemi.com/pub/Collateral/HB206-D.PDFIt contains, among other valuable things, a section about construction and layout. Look also to the troubleshooting section, you have the first symptom that they refer. What is the first thing that they suggest to check?It is a good idea to have (and study) a collection of relative documents, such as application notes, from all the major manufacturers. They provide information, additionally to the datasheet, and analysis/tips/tricks…For example have a look to this one:http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snva558&fileType=pdf&keyMatch=snva558&tisearch=Search-EN-TechDocsSome notes about the board implementation:The output of the rectifiers is pulsing voltage/current that we want to filter it and not to spread it to the entire circuit. This leads to a single line to connect it only to the filter (capacitor).The input of the regulator wants a filtered voltage so take it from the filter: a line from the pin of the capacitor to the regulator.Do not mix the above two current paths.The output should be connected directly to the load (as possible), to avoid voltage drops and other disturbances. Note that the regulator has not a separate sense pin.About the common/0V/Ground pin: because the regulator reads the load voltage between the output pin and this one, it also has to be connected near the load and not to the input or everywhere else. A good solution is to make a "star" point near the output (load). Connect to this point the lines coming from the capacitor(s), the regulator, the load and the output capacitor. If you can separate the two supplies (positive and negative, that needs a transformer with two independent secondary coils) connect them to this point.Again, do not mix the current paths.Have a nice day and enjoy the reading/studying!

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2017, 04:44:46 pm
content: I do find some garbage 78xx regulators (Fairchild) that are really noisy and have the super thin "single gauge" tab.For the PCB, I think the GND pour is not ideal. The transformer CT should not have a path to the output GND, which it presently does through the center of the PCB. Only the filter capacitors GND should have a direct (copper) path to output GND. Compare to your breadboard layout, if this is the confusing difference you are seeing.Even with a PCB GND issue here, it should only be AC mains (120Hz) effects seen, worse at high loads.You line-trigger the scope? even for the 4kHz osc. as it may be modulated by 120Hz hum.I feel your output capacitors are too small, either 10-22uF tantalum or 100uF electrolytic depending on your load for this thing. For audio, I've seen up to 1,000uF used as an incorrect way to lower noise from a noisy regulator.Check your filter capacitors are not fakes (inside has tiny cap, rattles, high ESR, weighs almost nothing etc)I would apply high-frequency design techniques, such as 2x10nF 100V across the transformer secondary to shunt RF and lessen rectifier switching transients. The previously mentioned 0.1uF-0.33uF near the regulator input and output are also good practice. This would not affect the 4kHz problem though.

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2017, 05:29:16 pm
content: Quote from: floobydust on August 28, 2017, 04:44:46 pmThat is a real problem. In the photograph below of a Tektronix PS503A, you can see that there are two separate heavy gauge wires going to each pin of the large input capacitors. (1) That is part of the original design and deliberate and not a mistake! It is done to prevent ground loops.The capacitors are between the rectifiers and the voltage regulator inputs and it is at that point where the grounds connect and only at a single point which becomes the output common connection.(1) The big silver Mallory capacitor is one of the original capacitors and just resting there to show the scale. It was wired in the same way.

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2017, 05:54:16 pm
content: Tek certainly had some masters there. Clear as a bell, but I have this old sketch that might make it easier to understand (see point eight).OP could cut the pcb copper (but I hate to suggest that), or try move transformer CT connection to a V-wire in the air connecting only to filter caps (-).I learned in a recording studio that even 10mVpp of ripple is unacceptable/audible in a mixing console and had to changing grounding to dual-star to fix that.

author: MarkF
author_rank: Super Contributor
timestamp: August 28, 2017, 08:54:06 pm
content: If I understand correctly, the biggest offender cause the ripple is associate with the center ground running through the PCB.I'm assuming the ground is an actual ground and not a pseudo ground. That meaning if only the positive half is being used, the return current path is to the transformer center tap.I've made an attempt to separate the positive and negative supply sections and direct the current from the transformer center tap thru the filter capacitors. I'm not sure I still understand how to handle the grounds. Maybe someone can show a better ground layout.

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2017, 04:28:26 am
content: I'll stick my neck out here.I have no idea this is your 4kHz squeal fix, as that is very unusual; but you say breadboarding this and it's not oscillating.I have done the simplest markups to your Rev. A PCB, which I think you have already built, in order to improve the grounding.Red= a cut or slice in the copper pour.Violet= where the pour could go on a future board spin, next rev.; Copper is free on a PCB so I use it.The first notion is the main filter-capacitors require star connections, for both their (-) and (+) terminals, and both their input side and output side.So the filter cap terminals become the star connection point, current on one side cannot sneak around past the capacitor. Usually thermal-reliefs are present on big parts like these and their geometry has to be considered.The ripple-current peaks with your large 6,800uF capacitors are much higher than your average output current, so the filter-cap charging current can induce pulsing voltage drops on your PCB copper resistance.Downstream of the filter-caps, currents are much smoother, lower and grounding is not so critical. so I would leave the PCB as-is.(ideally the Vreg GND terminal connects to the output terminal block, but I don't see it necessary)The second notion is the main filter-capacitors require star connections, for both their (-) and (+) terminals, and both their input side and output side.Your Rev. B PCB is much better but see we made a lovely loop and lost copper in the center, so I think this more reasonable.You could solder in jumpers for the transformer CT (star) and see if the 4kHz is still there.Usually it's the output capacitor and min. load current that gets a regulator unstable - but 7815/7915 I've never seen oscillate unless noise gets in.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 29, 2017, 07:37:43 am
content: Quote from: MarkF on August 28, 2017, 08:54:06 pmFirst a clarification: the 4 kHz signal is not (remaining) ripple but oscillation. This is produced by the circuit.If you try another brand regulator it may behaving differently. I have seen circuits like this oscillating in frequencies over 30 MHz!The ground is never ideal, even the entire planet Earth is not perfect because it is not a good conductor!You have a good start but not for the entire circuit.Even if your application is not demanding a lot, it is always useful to have in mind some good practices.The line from the rectifiers to the capacitor does not carry more current than the other side of the diodes. It does not need to be wider than you have at the inputs. The same is valid for the line from the capacitor to the regulator. Normally they do not need to be wider than the pad of the component lead (like they are for D5 and D6). By reducing their wide there is less space for unwanted and uncontrolled current paths.The output pin of the regulator has to carry the current to the load and not coupled to everything around! The same as above can be done. Just to "feeling better" the line can be a little wider!For the ground part of the circuit:The lines have to be as described above. There is not any need for a wide area of copper.The separation that you made is a good start but it must be extended up to the ground terminal of the output.The ground pin of the regulator has to be connected near the load and not to the middle of the "return" line (as I tried to describe earlier). There is needed a separate line from this pin to the output. This line carries only the quiescent current of the regulator but it is the reference level (the 0V, common) for it.The protection diodes are normally reverse biased and are for "momentary" operation only. So they do not have any need for "heavy" conduction and can be placed and connected in any convenient way.

author: MarkF
author_rank: Super Contributor
timestamp: August 29, 2017, 10:19:17 am
content: I may be going backwards with this one. Can't seem to get the traces around the heatsinks without making them real long.

author: StillTrying
author_rank: Super Contributor
timestamp: August 29, 2017, 11:41:50 am
content: I keep thinking can't you get D6 nearer to where it says U2, it gets at least some of the noisy input copper side further from the quiet regulated output side, even though the diode's capacitance is quite high.I don't think the empty diamond on the gnd does much.I still think you need some capacitance very near the regs. but there are better expurts than me on that.Is the 4kHz sawtoothed shaped, sometimes you can miss the fact the start of each cycle is actually a very high frequency oscillation, and the sawtooth is just the recovery until the next HF oscillation starts.

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2017, 07:02:31 pm
content: You could move D6/D5 inside the heatsink's gap (where the silk text U1, U2 is; =opposite side of the vreg IC).This gives more room and the board could also be narrower.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 29, 2017, 10:02:35 pm
content: Just a draft sketch to take an idea.

author: MarkF
author_rank: Super Contributor
timestamp: August 30, 2017, 01:46:45 am
content: Quote from: StillTrying on August 29, 2017, 11:41:50 amI'm not too concerned with D6 and D8. I removed them from the PCB without any change in the the oscillation.The 4KHz oscillation looks to be sinusoidal to me. See screen grab and a second one with a 22uF capacitor added to the output.Quote from: Damianos on August 29, 2017, 10:02:35 pmI think I understand your idea. When you start to take the line width into account, the lines all start to run together and you end up with a big copper pour. Especially the ground.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 30, 2017, 07:58:39 am
content: The width of the lines does not need to be more than the pad size of the relative components. There are not any "heavy" currents in this circuit. If there were then it would be needed a wider board or other techniques.It may be a help the use of a "disk" of copper near the output, to be the "ground star point" for the connection of all that lines. It has not to be "huge".There is not a "role" for the diodes in the oscillation as they are reverse biased, in my first comment I mention the D6 only because it was an easy place to connect a small capacitor. Sorry that I leaded you in a misconception!The circuit must be stable and "silent", as you tested it on the breadboard, for any "ohmic" load from minimum current up to the maximum. By adding auxiliary capacitors the problem is not solved but only hided. In this case the results, by the variations of input and output conditions, may be from the reappearance of the oscillation to the overheating of various components (mostly the regulator and the capacitors).Some more general notes:- Check the input voltage with the maximum current that you plan to use it. By the oscilloscope verify that the minimums of the ripple are "enough high" for the regulator.- If you plan to use the maximum current of the regulator, consider for "heavier" rectifier diodes (because of the ripple current) or a bridge rectifier. The other diodes may be the same or even smaller ones.- For the case that the load current goes near zero, consider to place resistors in parallel to each output to maintain the minimum current to be 1 to 5 mA, for better stability. These will also help a little, in "collaboration" with the output capacitors, for the case of long wires to the load.

author: exe
author_rank: Unknown
timestamp: August 30, 2017, 08:50:04 am
content: Quote from: Damianos on August 30, 2017, 07:58:39 amWhy? My 2st1943 has quite tiny leads (TO-264), yet it can handle up to 15A. So, having wide traces would help reduce losses which are noticeable at high currents. Copper clad is quite thin, around 10-20um or so, so I would make it as wide as possible (of course taking potential issues into account).Okay, in this case this might not be essential because average current is not that high. But why not making it as wide as possible?

author: Damianos
author_rank: Frequent Contributor
timestamp: August 30, 2017, 09:13:09 am
content: Because I have already explained that:Quote from: Damianos on August 30, 2017, 07:58:39 amThe goal is to not have uncontrolled current paths and couplings.

author: exe
author_rank: Unknown
timestamp: August 30, 2017, 09:20:50 am
content: Quote from: Damianos link=topic=94100.msg1291100#msg1291100Why not just narrow the trace at the pad and keep the rest wide?Also it's also a common pattern to make a ground plane (yeah, I understand that return current will flow under the trace)... Can we have here a ground plane?(I'm just reasoning, please correct me if I'm wrong. I'm also designing a power supply and routing is one of my biggest concerns)EDIT: fixed formatting

author: StillTrying
author_rank: Super Contributor
timestamp: August 30, 2017, 02:20:10 pm
content: Quote from: MarkF on August 30, 2017, 01:46:45 amAdding the 22uF to the output seems to change the 4.1 kHz sine to a 830 Hz saw, there might be a clue there.I still don't think any oscillation/noise counts until the reg is supplying ~1-2mAAfter reading the MC7815 data sheet, the currents are so low that I think all the PCBsshouldwork fine including the 1st one. Perhaps if the 6800uFs wereverypoor it would explain it.

author: b_force
author_rank: Super Contributor
timestamp: August 30, 2017, 04:15:49 pm
content: What I am missing, is a little 100nF cap close to the output pins of the regulators.Depending on the brand, but some WILL oscillate if you don't decouple them properly.

author: MarkF
author_rank: Super Contributor
timestamp: August 30, 2017, 08:41:57 pm
content: It seems to me that the biggest offender here is not isolating the ground (sense) pins of the regulators. Would cutting the ground on my original PCB to isolate the sense pins be worth trying? (See figure)QuoteAll capacitors are new Nichicon from Mouser.QuoteI tried putting a 100nF capacitor on the regulator output with no effect.

author: b_force
author_rank: Super Contributor
timestamp: August 30, 2017, 09:31:21 pm
content: Quote from: MarkF on August 30, 2017, 08:41:57 pmOk, very odd.Nevertheless, it is very wise to have a 100nF at the output.Some datasheets even will tell you it's mandatory for a stable operation.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2017, 10:12:45 pm
content: It looks like the "motorboating" which sometimes occurs in feedback amplifiers which of course a voltage regulator is. Increasing the output capacitance lowers the frequency.The last layout is not ideal however the 7815 and 7915 are just not that finicky and there should not be any problems even without a minimum load.What happens as the output load is increased from zero up to at least 100 milliamps?Have you tried a different brand of regulator? It is not unheard of for a bad batch of parts to be produced by a major manufacturer.Does the +15 volt output still oscillate if the 7915 is left out?Exactly what kind of output capacitors are you using? The 78xx and 79xx series regulators were designed with aluminum electrolytic and solid tantalum capacitors in mind and lower ESR parts like ceramic, film, polymer tantalum, and polymer aluminum might cause problems. Just as an experiment, try putting 2.2 ohm resistors in series with each output capacitor.Quote from: b_force on August 30, 2017, 09:31:21 pmThis depend on the type of regulator and does not apply to the 78xx and 79xx series. Low dropout regulators, and the 78xx and 79xx are not these, can be very finicky out their output capacitors with restrictions on value and ESR.Low value low ESR decoupling capacitors should be placed across the load where they will do some good instead of at the output of the regulator unless it is a low dropout type which specifically requires this.QuoteThe 78xx and 79xx datasheets will not say this. They will include a warning about including an input decoupling capacitor if the input bulk filtering capacitor is located at a distance and they may mention that using *only* a low value ceramic output capacitor may cause oscillation. They will also mention that the 79xx series *must* have an output capacitor for stability but much larger than 100nF and not a super low ESR part.

author: MarkF
author_rank: Super Contributor
timestamp: August 31, 2017, 12:24:44 am
content: Here are the exact parts used:ON Semi MC7815CTGON Semi MC7915CTGNichicon 6800uFNichicon 1.0uFVishay 1N5395Hammond 185D28I ordered some other regulators from Texas Instruments (UA7815CKCT and LM7915CT) but haven't received them yet.The oscillation occurs with "no load" and a 3mA load from my DDS function generator. The oscillation disappears with a 100 ohm resistor. Maybe I'm needlessly worrying here since I'm not meeting the 5mA minimum load.I have not tried without the negative regulator installed.Quote from: David Hess on August 30, 2017, 10:12:45 pmCan you expand on "not ideal"

author: exe
author_rank: Unknown
timestamp: August 31, 2017, 08:38:17 am
content: Quote from: MarkF on August 31, 2017, 12:24:44 amIn general, with no minimal load don't expect it to work correctly because1) may not be enough current for serial regulator to work (but some have ground terminal that regulator can use for own supply)2) LDO cannot sink current. If for whatever reason it overshoots it cannot reduce output voltage. I think this might be a reason here.3) Feedback loop may be tuned to work only under specific load condition.But these are just my general thoughts on linear regulators. I don't know how many of these applicable in this case.

author: StillTrying
author_rank: Super Contributor
timestamp: August 31, 2017, 11:47:21 am
content: I think there's still a small chance it's a bad batch of MC7815s.With the max. ave. currents only being about 800mA, I don't think you need much more than this amount of copper on the board, - but I'm definitely no expurt on PCB layouts, I can't even decide if making the rest of the area a ground plane is a good idea or not at these currents and speeds. I mostly try to keep the inputs and outputs of any active device as far apart as possible.

author: exe
author_rank: Unknown
timestamp: August 31, 2017, 12:24:19 pm
content: BTW, datasheets suggest placing output caps as close to regulators as possible. They also put requirements on ESR. So, I'd check on these (like, putting different caps or adding more in parallel) before replacing regulators.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 31, 2017, 01:02:39 pm
content: Quote from: exe on August 30, 2017, 09:20:50 amThis is a good idea to "feel better" by lowering the resistance of the trace. I don't say that a wide trace is prohibited but, in this case, it is unnecessary and we have to control the current routes and the coupling between adjacent traces.Quote... if you can manage the current paths, do it.I have already put links to relative documents, as examples, earlier in this discussion.Some more are following.In the snapshots of the AN1849https://web.archive.org/web/20090419113524/http://www.national.com:80/an/AN/AN-1849.pdftake a look at the circuit board and note how they route the currents from the rectifiers up to the output. There is no way for the currents to bypass the capacitors.I took this thread as a general discussion about the board's layout for the "famous" three terminals regulators. My proposals are not directed to solve only the oscillating problem but to get the best from these devices. Of course I have not "invented the wheel" so I present here documents that I have read/study in the past. The mentioned problem was solved (i think so!) by avoiding the coupling between the sections of the circuit.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2017, 11:41:38 pm
content: Quote from: MarkF on August 31, 2017, 12:24:44 amI do not see anything wrong with those capacitors. Sometimes designers figure that if low ESR is good, then even lower ESR is better and use film, ceramic, or other super low ESR capacitors where they are not needed leading to trouble.Quote15 volts at 100 ohms is 150 milliamps which is way more than necessary to solve a problem like this. 5 milliamps should solve it if it exists at all. Personally I have never had a problem with 78xx and 79xx series regulators with no output load for the reasons I gave in my earlier posts. The quiescent current of 317 type regulators flows through their output pin so they are different and absolutely require a minimum load current which is usually provided by the same external feedback divider which sets their output voltage. The quiescent current of a 78xx or 79xx regulator flows out its common pin (1) which is a blessing or curse depending on what you are doing.QuoteThe regulators in your layout share unnecessary ground currents so the current through one modulates the output of the other. I am not sure if this is causing your problem but it is possible. Circuit schematics do not always take layout into account but below is an example which does and gives a good idea of what an optimal layout would look like. One reason this schematic was drawn to reflect the best layout is that the current boosting transistor makes it more sensitive to ground currents.(1) Which is why it is called a common pin instead of an adjustment pin. You can use a 78xx or 79xx regulator just like a 317 or 337 with an external divider however the high quiescent current from the common pin contributes considerable error in the feedback divider so a buffer is required.

author: CopperCone
author_rank: Super Contributor
timestamp: September 04, 2017, 12:01:03 am
content: if you gotta go through this kind of bullshit to implement a linear regulator it means the part is fucked up.I use solid ground planes across the entire circuit with no problems.Unless your building like 5000 of these just use LT parts or something.I can't help but think ' what the fuck ' when I read this thread.The thing only has 60db of rejection too. fuck it. there is no point in using this stone age technology if your spending your own time designing something you can buy for like 5 hours labor in the real world or it does not perform some kind of unique function (noise level, robustness).stray current paths wtf? are you laying shit out on monoatomic copper?if your hell bent on it, this is how you trouble shoot it1) disconnect the transformer, feed it directly with a DC supply. Does it work? vary the input voltage2) remove all the capacitors, see what it does (with a good DC supply). it will probably still work. things don't always jump strait into oscillation3) add parts until it breaks, hopefully you can deduce what happens.Also, my 'default' way of doing it, if I had to do it right now, would be putting SMD ceramic caps right against the pins. Like milimeters away. I would put 100nF on both the input and output of the regulator, right up against the legs, and possibly some capacitance from the adjust pin if its allowed, so it forms a bridge between the regulator pin and the ground plane that you have in the middle.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2017, 12:21:17 am
content: Quote from: CopperCone on September 04, 2017, 12:01:03 amI agree and said this earlier; the 7815 regulator should not be this sensitive to layout problems which may indicate a bad part. I was just responding to MarkF's question about what an ideal layout would look like.I have built combined 7815/7915 power supplies on breadboard without problems so what is going on here seems weird.

author: CopperCone
author_rank: Super Contributor
timestamp: September 04, 2017, 12:52:12 am
content: yea, i see those regulators literally floating on the side of a chassis (used as heat sink), being connected by 6 inch thin gauge wires, then also having their voltage routed to external objects to the chassis, in commercial products that cost north of 5000$ (avtech). i'm saying like 22 or 24 awg.maybe one decoupling capacitor on the thing.i say his regulators are haunted, or that part is extremely bad. it better cost like 40% of the regular if it behaves like that. until I saw this thread i considered them bullet proof. i thought you can jury rig them with cigarette foil and spit.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2017, 03:28:11 am
content: Quote from: CopperCone on September 04, 2017, 12:52:12 amI have heard stories about bad 78xx/79xx designs and bad production lots. I think the only problem I have run across myself are parts which oscillate in the MHz range producing odd voltage shifts in the output and drawing high quiescent current. I have run across the low frequency "motorboating" problem before but only in my own discrete regulator designs when the frequency compensation and/or layout was poor. It also crops up in audio amplifiers sometimes.1. Some integrated regulators designs were initially bad in not implementing hysteresis in the over temperature shutdown leading to not being able to hard start loads. I think Bob Pease mentioned this. These are the ones where the 5 volt output gets stuck at 3.5 volts while the regulator sits there overheating despite the load drawing less than the maximum current.2. The oscillation problem mentioned above crops up occasionally. Those with experience will recognize it by the output DC voltage shifts and excessive power dissipation. Confirmation is with an oscilloscope.3. Some are just drifty with essentially large amounts of low frequency noise. Maybe this is caused by the absence of the stress relieving die encapsulation that linear ICs require? I know this happened to National (or was it Linear Technology?) once but I do not know if it affected their regulators.

author: b_force
author_rank: Super Contributor
timestamp: September 04, 2017, 03:00:13 pm
content: I can confirm that there are bad 78xx/79xx productions.We used to have in a design that was on production.A few times we had to redo a whole batch of board because when testing they also were oscillating.Pretty annoying (especially when costumers are waiting)

author: exe
author_rank: Unknown
timestamp: September 09, 2017, 06:16:17 pm
content: Quote from: b_force on September 04, 2017, 03:00:13 pmWhat sort of output caps was that? I still see pretty experienced engineers put ceramic caps on output, of course this doesn't work well.

author: MarkF
author_rank: Super Contributor
timestamp: September 09, 2017, 06:34:24 pm
content: Quote from: exe on September 09, 2017, 06:16:17 pmI listed the exact parts I used in my last post.Sorry, I haven't been able to work on this lately. I got some new regulators in to try yet.

author: exe
author_rank: Unknown
timestamp: September 09, 2017, 06:59:08 pm
content: Quote from: MarkF on September 09, 2017, 06:34:24 pmI was asking b_force about the case when they needed to redesign the product due to the issue with the regulator.Anyway, guys, you convinced me to make some experiments. I'll try to figure out that thing about "stray currents". I hear a lot about it, but nobody provided any numbers whatsoever. So I'll check by hooking my signal gen to the pcb with a electrolytic cap and let's see. But I wouldn't expect much difference because electrolytics are not very good at high frequency. And I wonder if a few extra inches of copper would add noticeable resistance and inductance... But we'll see.

author: exe
author_rank: Unknown
timestamp: September 29, 2017, 02:11:51 pm
content: I conducted a few experiments with different layouts. Basically, I shunted my signal generator with a cap and measured the residue amplitude with my MicSig TO1104 (with PP-150 probes set to 1X "attenuation"). I think pictures say it all (). I'm pretty sure there are design flaws in this experiment, please let me know about them. And, of course, measurements are imprecise and inaccurate.In general, above 1MHz every cm of wire/pcb affects the result. But only at 1MHz and beyond (I did measurements at 100/200/500/1000/2000 KHz). The best layout was with solid ground plane and straight current path.I'm surprised how well electrolytic capacitor worked at such high frequencies. The 2.8V p-p sine* at 2MHz reduced to 7mV p-p. Not too bad, imho. I also tried to parallel two identical capacitors, the output amplitude almost halved.I also tried a 2700uF capacitor (EEUFR1E272L), but it showed marginally worse at 1MHz. So, at such frequencies capacitance is not a dominating factor. Anyway, comments are welcome.* Because cap is polarised I applied DC offset. See photo of sig gen for actual setup.References:https://photos.app.goo.gl/17rpob2nUHRTyXSa2List of equipment:1. Signal generator: SDG20422. Oscilloscope: MicSig TO1104, probes PP-150 in 1X attenuation3. DUT: capacitor Panasonic EEU-FM1E222L, 2200uF, 25V, 105C4. PCB: 1-sided cheap copper clad, dimensions on photo

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 30, 2017, 04:38:11 pm
content: About the minimum output current some of us thinks it can start from 0 I'll tell you my story:one time I had a NEC 7805 regulator recovered from some board,knowing they are indestructible I just put 9V on input,found 7V on output.throwed out to bin.I took a second one,same 7V on Out pin.started to think...put a load on out 10 mA.output changed to 5 V.Started to test the damn thing in real life with variable load,on scope etc.the thing was perfectly fine.So yes,there are 7805 with minimum load requirements.the NEC one.Of course nothing to do with this case...Imho,he must have some condensator problem,I saw 7805 with bigger routing mistakes without oscillating in industrial environments with thousands of inverters aside (1kW inverters...).Regards PierreEnvoyé de mon iPad en utilisant Tapatalk

author: Damianos
author_rank: Frequent Contributor
timestamp: October 01, 2017, 05:15:47 pm
content: Quote from: exe on September 29, 2017, 02:11:51 pmA few pictures with a capacitor as a load of a signal generator, connected in a few different ways...Is this related with something else and in which way?

author: exe
author_rank: Unknown
timestamp: October 01, 2017, 09:22:55 pm
content: Quote from: Damianos on October 01, 2017, 05:15:47 pmThis was to answer the question how different layouts affect performance of bypassing/filtering caps.

author: Damianos
author_rank: Frequent Contributor
timestamp: October 02, 2017, 05:13:15 am
content: ... and what is the conclusion for what happens at the variousnodesof the circuit? Where are these simulated in your experiment? How anactivecircuit is evaluated by replacing it with a wire? What about the coupling between the parts of the entire circuit? Did you measured anything atvarious points alongthe wire?? ? ?