Noisy power supply, critique PCB design - Page 1
Forum section: electronics / beginners

author: mike-k
author_rank: Contributor
timestamp: January 30, 2021, 07:19:49 pm
content: Hi,I got a PCB made up for a dual power supply for an old synthesizer project of mine which needed repaired. I used a circuit I found on the web, and it works but it's noticeably noisier on the scope than my commercial dual power supply for Eurorack synths. I can also hear the noise in the oscillators of the synth when using it, as compared to the commercial power supply I have.I'm wondering if I made some beginner's mistake in laying out the board in Kicad, as it seems to be a fairly standard circuit design. It uses a 12V AC wall wart and does half-wave rectification as documented here:https://wired.chillibasket.com/2020/06/dual-power-supply/It's a two-layer design and I used a ground plane in the bottom layer. Schematic and PCB images attached. Thanks for any advice.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 08:32:18 pm
content: If not talking about details of schematics and PCB then I can say in general that 12VAC is probably too low to have a well-regulated 12VDC with a conventional (but not low-drop) IC regulator. Moreover, you use single-phase rectifiers. It is much better to use a 15VAC (or even 18VAC) source to have enough headroom.

author: mike-k
author_rank: Contributor
timestamp: January 30, 2021, 08:55:39 pm
content: Thanks I will keep that in mind. Aside from that though there seems to be something amiss in my circuit because I am powering my commercial eurorack power supply with the same 12V AC wall wart and getting much better results.One thing I am unsure about is C1 / C2. I used tantalums at first but blew the one at C2, presumably because it was put in in reverse. I have non-polarized caps in there now. Perhaps I should put tantalums back in. However I am unsure if the polarity is properly accounted for in the circuit. I didn't design with polarity in mind (even though the Kicad footprint has polarity - that should have been a clue). Which way round should they go?

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 09:13:24 pm
content: You don't need C1, C2 at all. There is no need for them.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 09:27:32 pm
content: Wired in the way I show on the edited picture it can be a bit less noisy (and you have to rewire in the same way a negative part of a PCB too). But with a half-wave rectification and at the same time too low input AC voltage, it is hard to talk about real low-noise design.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 09:35:20 pm
content: If you know that power (current) consumption is low enough then you may use not half-wave rectification but a voltage-doubler design of a rectifier. So you'll have enough DC voltage headroom for any IC regulator. (You need at least 3.5 V drop usually for IC regulator under load).

author: srb1954
author_rank: Super Contributor
timestamp: January 31, 2021, 12:14:51 pm
content: Quote from: mike-k on January 30, 2021, 08:55:39 pmYou shouldn't use tantalum capacitors on the input side of a regulator as they don't withstand overvoltage transients or surge currents too well and can blow up even if they are wired with the correct polarity. You don't really need capacitors C1 and C2 in this circuit provided the main smoothing capacitors C3-C6 do not have long wires to their respective regulators.Tantalums are safer on the output side of the regulator as they protected from overvoltages and surge currents by the regulator. Nevertheless, to achieve acceptable reliability, the tantalum caps should have a voltage rating of at least twice, preferably three times, the normal operating voltage expected in the circuit.

author: mike-k
author_rank: Contributor
timestamp: January 31, 2021, 01:09:27 pm
content: Regarding tantalums on the input side: @srb1954 interesting to hear your point about avoiding using them there. Just looking at another thread on here about a very similar power supply,https://www.eevblog.com/forum/beginners/is-this-circuit-modification-right/, I see they have tantalums specified before and after the regulators. The article I linked to in the first post says:QuoteSo there seems to be some variance of opinion on this. My tantalums are 35V, so perhaps that's not enough?@Vovk_Z thanks for the layout suggestions. I wondered if my PCB layout should be structured more like the schematic, but as Kicad was happy that the electrical circuit was equivalent I presumed it was OK the way I had it. (My layout was mostly influenced by the space requirement and fitting everything in.) What is the thinking behind the changes you suggested?

author: mike-k
author_rank: Contributor
timestamp: January 31, 2021, 02:16:49 pm
content: I could try a layout like this by cutting a few traces and adding some manual wiring, if you think it will make a difference. I have a few of these boards to experiment with.

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2021, 03:33:59 pm
content: What sort of noise do you hear?

author: mike-k
author_rank: Contributor
timestamp: January 31, 2021, 04:10:37 pm
content: When I say noisy I mean the line on the scope is not steady and straight like on the commercial power supply. It breaks up slightly, though not with significant big spikes.The effect on the synthesizer is that the tones do not sound 'pure' as they do with a good power supply - they sound a bit fuzzy, like they are being modulated at high speed, for example with a square LFO.Another symptom is that I am also powering a MIDI-CV circuit which is part of the synth. When I use my home-made power supply and play notes on the keyboard, the notes trigger OK but they fall in pitch on key down and rise on key up. This doesn't happen when I use the 'good' supply – I just get the steady note.

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2021, 04:19:24 pm
content: So put that scope on the input side of the regulators and tell us what you see when you press a key on your keyboard. I would speculate that your input side is dropping low enough that the regulators are starting to drop out. Also, it would be better to have C1 and C2 on the output side of the regulators and use a 10uF cap in parallel, not 100--but that's a minor issue at this point.

author: mike-k
author_rank: Contributor
timestamp: January 31, 2021, 04:29:33 pm
content: Hi, yes if I do that the input at the positive rail drops about 1v. Nothing obvious happens on the negative side.So you suggest having C1 and C2 as well as C7 and C8 on the output side?For info - I'm powering the 'good' commercial supply I have with the same wall wart, and in terms of capacitance it uses four 2200uF 25V caps. I'm using four 4700uF 35V caps. It looks as though the 'good' supply has Zener diodes between the pairs of caps, though they may be the equivalent of D3 and D4 in my circuit, it's hard to tell without a schematic.

author: MarkF
author_rank: Super Contributor
timestamp: January 31, 2021, 04:37:26 pm
content: I would suggest a full-wave bridge. Your voltage drop would be smaller with the same caps.Here are two layouts that I did. (Never built the 2nd one with the small heat sinks.)

author: ledtester
author_rank: Super Contributor
timestamp: January 31, 2021, 04:49:20 pm
content: Quote from: mike-k on January 31, 2021, 04:29:33 pmYou can tell if they are regular diodes with a multimeter that has a diode test function. Regular diodes will have a forward voltage around 0.6-ish V and Zeners will be a lot higher (or measure "overload" in both directions).If might be worth it to reverse engineer the schematic - if it's not too involved.

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2021, 05:32:16 pm
content: Quote from: mike-k on January 31, 2021, 04:29:33 pmYou could post more detail on what you see on the scope. Set it to a timebase of 10mS/div or so and observe:1) The actual voltage with and without a load on the scope.2) The actual voltage on the output at the same time, if possible (what scope you have would be good info)3) The shape of any distortion on either signal.I suggest using a 0.1uF film cap and a 10uF electrolytic on the output side. You shouldn't need decoupling on the input side for this.The obvious conclusion here is that your output voltage is dropping under load, as well as the voltage across the regulators. If Vin- Voutgoes below about 2 volts, the quality of the regulation declines rapidly. If the input is marginal (say 13 volts minimum) as opposed to below the regulated target (12 volts), you might be able to salvage the design by substituting LDO fixed regulators like the LM2940-12/LM2990-12. You also might scrape a few hundred millivolts back by using Schottky diodes for the input rectifiers. Or you could use a slightly higher voltage wall wart. Do you know what the maximum load current is here?

author: mike-k
author_rank: Contributor
timestamp: January 31, 2021, 06:07:10 pm
content: Thanks. I have to hold my hand up here and apologise. I looked into the spec of the 'good' supply in more detail and realised it is using a 9VAC supply. The plug socket is not visible from my desk but I checked and I've been using a 9VAC supply. The 12VAC supply was sitting unused alongside.Unfortunately it's not a happy ending as when I switch to the 12VAC supply I am getting 0V at the input and output of the 7912 and the 7812 seems to be outputting about +10V instead of +12V.

author: MarkF
author_rank: Super Contributor
timestamp: January 31, 2021, 08:27:55 pm
content: Are you sure the 12V supply is AC and not DC.Which would account for no negative output.Here is a link to a ripple issue I had (which turned out to be no minimum load):https://www.eevblog.com/forum/projects/dual-power-supply-ripple-question/?allIf you make a new PCB, the main take-away for me was to create a star ground at the output.It's most clear on the 1st layout with the large heat sinks.The small square copper pour at the output ground.The reference pin (or adjustment pin for variable regulators) is connected close to theoutput pins of the PCB. You can see the separation in the traces closer to the output.

author: tooki
author_rank: Super Contributor
timestamp: January 31, 2021, 08:35:00 pm
content: Quote from: ledtester on January 31, 2021, 04:49:20 pmUmmm no, Zeners measure 0.6ish forward, just like any ordinary diode. It’s theirreversevoltage that is defined (hence why it’s sometimes known as the Zener voltage.)

author: mike-k
author_rank: Contributor
timestamp: January 31, 2021, 09:17:34 pm
content: QuoteThank you! I'm not doing too well on the details today. I'm getting different projects and power supplies confused. I switched to an AC power supply and it is working OK now. The sound quality seems improved and is comparable with the commercial power supply. The Midi->CV circuit is not doing anything at all now but that is a different issue. So I can move forwards now I have adequate power.Regarding the minimum load issue, the LEDs in my design are supposed to fulfil that function. Seehttps://wired.chillibasket.com/2020/06/dual-power-supply/See also the notes about the revised version of the 'wall wart bipolar supply' board here, which includes resistors to create load:http://musicfromouterspace.com/index.php?CATPARTNO=SLMS001PCB&PROJARG=SOUNDLABMINISYNTH%2Fpage1.html&MAINTAB=SYNTHDIY&SONGID=NONE&VPW=1420&VPH=1185Cheers.

author: srb1954
author_rank: Super Contributor
timestamp: February 01, 2021, 01:28:02 am
content: Quote from: mike-k on January 31, 2021, 01:09:27 pmYes, small capacitors in parallel with the larger electrolytics do help filtering out the higher-frequency noise in some cases but if this filtering was required I would definitely use only ceramic or plastic film capacitors on the input side of the regulator due to the failure risk with tantalums.I have seen a case on an expensive piece of test gear where a tantalum on the input side of a regulator failed and shorted out. This caused excessive current through the rectifier diodes which in turn shorted out. The shorted diodes caused excessive current to flow through the power transformer burning up its insulation. The original manufacturer had exited that business so a replacement transformer couldn't be obtained and as a consequence that expensive piece of test gear had to be written off because of a single tantalum capacitor being used where it shouldn't have.