PCB design review - Page 1
Forum section: electronics / projects

author: PL
author_rank: Contributor
timestamp: October 18, 2022, 07:43:07 pm
content: I'd like to have the design reviewed of schematic and PCB board layout on a couple of interconnected boards. I'm personally developing several prototypes, but nearly nil electronics knowledge. I have contracted out the design work, but I think its good to double-check everything. I can't afford to have them made and assembled to find out that there are critical errors.The boards are small and not exceptionally complicated. Well known microcontroller with WiFi, relays, ADC, IO expander, etc. Have schematics and Gerber files.Any suggestions on finding a well experienced person to provide some reviews? I don't have the budget for the fees of an EE engineering firm, and an hoping find someone who freelances. Preferably someone in Australia, as possible future work would go better if they're here.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: October 18, 2022, 08:54:32 pm
content: What I do is for first batch make a set of 5 pcb's to prove the concept.If that works well only then go on to larger batches.I remember many years ago a manufacturer doing a hundred pcb's for a job I worked on.They came back and none worked.No one had told them the PIC micro had to be programmed.Muggins here got to desolder all the PIC's, program them and solder them back in !PCB design can be a nightmare so best left to an expert.In my early years I designed a USB mixer.PCB came back and I built one up.With inputs shorted I was getting 1VAC on output ! should be zero volts.I looked into it and I hadnt separated out power supply and smoothing caps charging impulses were modulating teh ground line whic was amplified through the op amps.I reworked pcb separating out pwoer supply, used star grounding and that fixed it.I also designed a USB scope. It worked but had 10KHz signal superimposed on sine wave.I had run the scope signal beneath a ICL7660 negative voltage generator and it was picking up the 10KHz oscillator signal.A reroute of the scope signal fixed it.

author: Abdurrahman YAMAN
author_rank: Newbie
timestamp: October 21, 2022, 01:17:56 pm
content: If it is not confidential you can share it in here. There are kind people check it for you.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 21, 2022, 04:09:54 pm
content: This is exactly the kind of thing I do for a living - but let me offer a little advice for free.If you've contracted out the design work, it's their responsibility to produce a design that works. A competent engineer should have performed checks, simulations and reviews, but there's no substitute for testing a real, physical prototype. Anyone who claims otherwise has never designed something non-trivial.Often problems can be solved in a multitude of ways. Something that one engineer is confident will work, might be a source of concern to another. If you ask 'n' engineers how to solve a problem, you'll get at least 'n+1' different answers.I recall one design I was asked to review; I pointed out that although it might function OK, it lacked protection components that I deemed necessary. The response from the original designer was that they weren't needed, it would be OK as-is. We may never know who was "right", if indeed it was that simple.With that in mind, it might be worth considering what you'll do if your reviewer raises concerns. Without the ability to evaluate their responses yourself, do you go back to the original designer and insist they make changes? Or trust that they're right and your reviewer is wrong?Might you just end up paying two people to do the same design work?Who, ultimately, is taking responsibility for the design being right?

author: PL
author_rank: Contributor
timestamp: October 22, 2022, 09:22:26 pm
content: Quote from: AndyC_772 on October 21, 2022, 04:09:54 pmI'm able to evaluate some fundamental aspects of design (I have experience with commercial building controls BAS/BMCS). For example - checking for proper polarity, N.O, N.C. & Com. relay circuiting, etc. Part of my current review, revealed that polarity had been reversed on a component. I advised the EE and he quickly corrected. I have scant comprehension of the many other aspects of electronics; but with sufficient documentation and the basis for a reviewer's opinion, I have been able to draw some useful conclusions.The circuit attached illustrates another similar concern. It appears to be laid out with reversed polarities - nominal 3vdc landed on the negative ("-") pin, and via an NPN transistor, the ground is landed on the positive ("+") terminal. It is possible that I don't fully understand the ways in which it can be successfully circuited. Also attached is a page from the relay datasheet.Page from Relay-V23079G2008B301.pdf(440.68 kB - downloaded 108 times.)If in fact this is an error, it begs the question - are there additional critical issues. If its OK, then I'd glad to hear it.Quote from: AndyC_772 on October 21, 2022, 04:09:54 pmIt really depends. Reversed polarity is a show-stopper - it just won't work. Same applies to landing traces on the wrong pins, etc. I'm a firm believer in QA/QC with another set of eyes. We all make mistakes and I'm glad when a reviewer catches them. However, If the design is littered with these sort of issues, then I'll have to take a hard look at how to proceed. The extent of protection is a judgment call (BTW I did ask for additional protection and he willingly added TVS's: MOV's, Zener diodes, etc.)So to my way of thinking, it depends on the nature and extent of errors, VS a designer's personal preferences that will work but is approached differently than another design that will also work. Example - the EE designed the board using a 12v supply, even though there was no 12v components. Only 5v & 3.3/3.0v. He wanted it just in case something was added in the future (.e.g. 0-10vdc outputs) It added cost and complexity unnecessarily, so I had it changed to 5v, as its just the first PCB prototype.Any further thoughts are appreciated.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 22, 2022, 10:54:36 pm
content: First off, listen to Andy. I do this for a living (both the design side and the review side), and the question of how to handle review feedback is real and serious. If it's two coworkers who you've worked with for years and know well (maybe Alice is super conservative about everything and Bob is generally carefree, except gets really bothered about regulator feedback nodes for whatever reason), then it's not too hard to handle. If it's a hired gun versus the new guy... who knows, right? And if it's hired gun versus hired gun, well, good luck deciding who's right, who's wrong, who's panicking about a waste of time, and who's panicking about saving $1M.Your designer's schematic is, frankly, terrible (even in such a small snippet), and that combined with your description of his behavior gives me low confidence. It is not easy to read, mixes American and European symbols, and leaves off important part numbers and component descriptions. Correctness comes first on a schematic, clarity comes second, and everything else comes later. Butcorrectness and clarity go together, because if you can't understand it, you can't check it(or repair it). Forcing designers to match personal preferences after the fact is a waste of time, yes... but sloppy schematic capture is usually sloppy circuit design is usually sloppy final product. So pay attention!The relay datasheet (and the one for Panasonic TX relays, a drop-in replacement for these TE Axicom guys) suggests you shouldn't drive the coil this way. I've tested the smaller versions of these relays on the bench and found that they don't appear to care too much, so this will probably work. But it's not what the datasheet says to do, it's drawnreally badly, and it's not something I would do in production (though I also wouldn't stop the presses for this). So... your designer is sloppy.

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 01:38:03 am
content: Good lord, that snippet was done by a "professional" EE? A relay circuit is very simple to draw, you usually need the transistor to provide the current as the driver (usually a micro) cannot provide enough current itself, the diode is a back emf diode when the relay is turned off the energy in the coil can still dissipate through the diode, some circuits have extra components or are driven using opto couplers etc, depends on spec and requirements.Some people want belt and braces some want MVP, but in either case that schematic looks like a beginner has drawn it.

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 01:56:58 am
content: Quote from: Wilksey on October 23, 2022, 01:38:03 amFor my education, could you give an example circuit?

author: AndyC_772
author_rank: Super Contributor
timestamp: October 23, 2022, 12:05:47 pm
content: The circuit you already have is - aside from the relay coil being back to front - functionally correct, as far as we can see.What's ringing alarm bells here is the way it's drawn. An experienced, professional engineer simply wouldn't produce a drawing that looks like that, because there are some simple conventions that are normally followed that make the schematic clearer and easier to read. These conventions are so deeply ingrained that a professional engineer wouldn't even think about them.For example, things that leap out at me are:Relay coil is connected with positive to the pin marked '-' and negative to the pin marked '+'. Since this looks so obviously like an error, I'd always add a note alongside to explain why it's done here (eg. "note: reversed polarity allows easier PCB routing and is confirmed by experiment to work on this relay type"). Or just fix the error, if that's what it is!T4 has no part number. Why? It looks like a package with integrated resistors, why aren't their values marked either? Is there really not room? Does missing them off make it easier, or harder, to check and understand how the circuit works?'T' is an unusual prefix to use for a transistor - 'Q' is more usualWhile I'm nit-picking T4... the symbol should really show circular blobs where the wires are connected, and personally I'd put a box round the entire symbol to make it clearer that the resistors are integrated. It does suggest that the designer has picked a symbol that's already in a supplied library and decided it's 'good enough'.Supply rails are normally drawn with the most positive toward the top of the page, so having a positive rail "Cr5_3V3" at the bottom is odd.The part type for D15 has just been randomly chucked somewhere near the part, with a wire cutting through which makes it harder to read. This is just sloppy.The name "Cr5_3V3" implies it's not a rail that's used elsewhere, so why is a rail symbol used at all? Why not just show FB4 connected directly between VCC_3V3 and the relay?That kink in the wire by relay pin 12 is ugly, though it's something we all have to do from time to timeFinally - and I almost know I'm going to regret saying this because someone will undoubtedly complain about my clearly unjustifiable CAD-ist prejudice here - it looks like the schematic has been drawn using Proteus, which is an inexpensive CAD tool thattendsto be bought by beginners, hobbyists and the smallest of small businesses.I've never used it myself, I've no reason to say the tool is "bad" in any objective way, but it does just hint that maybe the designer has yet to outgrow it and buy what I might call 'real' CAD software... Cadence, Altium or PADS.OrCAD PCB Designer Standard is really not that expensive for a professional engineer, and it has an upgrade path all the way up to Allegro while still being able to read and edit existing schematics and PCBs.

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 01:00:14 pm
content: I thought it was Altium

author: AndyC_772
author_rank: Super Contributor
timestamp: October 23, 2022, 01:21:30 pm
content: Possibly, in which case I'm wrong. It does happen!

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 05:51:07 pm
content: Quote from: PL on October 23, 2022, 01:56:58 amAs Andy has said the circuit is there it is just very badly drawn, god knows what the layout is like!

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 10:18:13 pm
content: Quote from: AndyC_772 on October 23, 2022, 12:05:47 pmI've spent untold hours and days trying to understand the drawings. Had the EE followed proper conventions and drawing practice it would have removed a lot of mysteries that I had to sort out.It really looks like back to the drawing board on this. Rather get it right at this point. Lot harder to remedy things when its on a PCB.

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 10:21:23 pm
content: Quote from: Wilksey on October 23, 2022, 01:00:14 pmCorrect

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 10:28:08 pm
content: Can you share the entire schematic? I can only imagine what other horrors lie within.Can you say what the board is for? Driving relays I presume, but what is the purpose?I expect we could decode it for you or to what it is attempting to do at least.

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 10:33:08 pm
content: Can share portions. Some other parts involve IP.Will send soon. Thanks.

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 10:44:40 pm
content: Fair enough, have you already paid this EE for work? I'd request some kind of refund if so.

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 10:50:51 pm
content: Haven't paid for most recent work.

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 11:00:37 pm
content: Good, where did you find him?

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 11:10:34 pm
content: Online search for freelancing (chartered) EE's.

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 11:12:48 pm
content: Ah, ok, well, I guess somebody was lying.

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 11:20:58 pm
content: I checked his EE registration. Baffling.

author: Wilksey
author_rank: Super Contributor
timestamp: October 23, 2022, 11:28:38 pm
content: Is that an Australian thing? You don't have to be registered here in the UK for Electronics, just Electrical work (house mains etc).

author: PL
author_rank: Contributor
timestamp: October 23, 2022, 11:56:57 pm
content: Not sure if you have to here either.Perhaps a requirement of some employers?Certainly in commercial building services design its a standard thing for engineers.

author: julian1
author_rank: Frequent Contributor
timestamp: October 24, 2022, 12:18:05 am
content: For more than one relay, consider a bjt relay driver. Takes care of the resistor biasing, and incorporates flyback diodes. One part, generic, common, inexpensive etc. eg. uln2003https://www.mouser.com/datasheet/2/308/ULN2003A_D-1814978.pdf

author: PL
author_rank: Contributor
timestamp: October 24, 2022, 12:28:29 am
content: I need completely isolated DPDT relays. For 1 or 2 of them, they need rated for mains voltage ~240vac.

author: Psi
author_rank: Super Contributor
timestamp: October 24, 2022, 12:32:09 am
content: Quote from: PL on October 18, 2022, 07:43:07 pmAlways test the PCB with a small sample before going to low volume production. Don't bother getting PCBA done for a sampleJust get a panel or a few PCBs made using your final gerbers and then hand assemble them according to your final BOM.Test and confirm everything is ok.Skipping this step is something people do do, but only when they have many PCBs under their belt and have put it significant time checking over the PCB. Usually by more than 1 person. And even then, problems still slip through from time to time.However, if you are contracting the development that maybe something they should do. I dunno, depends on your agreement with them

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 24, 2022, 12:33:02 am
content: Quote from: julian1 on October 24, 2022, 12:18:05 amYes, I was looking for an example of mine to redact and share and then realized that I didn't have any that weren't done with ULN200x or NUD3xxx drivers! But the good old '2003 is harder to use than it looks. Most people follow most of the datasheets and connect its COM pin to the relay supply. This isn't great; as AoE chapter 1x reminds us, we actually like the voltage to spike up a bit when you stop driving the coil. So you want to stick a Zener diode inline with the supply going to COM, and allow this. I think there's one figure in one datasheet that shows this. The others all show the more basic arrangement.

author: Psi
author_rank: Super Contributor
timestamp: October 24, 2022, 12:40:46 am
content: I wouldn't criticize a schematic too badly when the criticisms are things that don't actually matter.If they are errors, absolutely that is bad. And a badly layed out schematic or a schematic that is not very tidy is a bad sign.but. There could be reasons for this. eg, the contractor might be giving you a very good price and taking shortcuts on things that wont affect the final product. etc.Not saying that's a good thing, just that I wouldn't focus on things that don't matter.The schematic doesn't really matter as long as it's functionally correct, the PCB design does matter.

author: julian1
author_rank: Frequent Contributor
timestamp: October 24, 2022, 12:46:52 am
content: I'm sorry, if I pull out my copy of AoE an entire stack of books will collapse. Super briefly... what's the reason for desiring a voltage spike on the rail? When I used uln2003, I just put 1uF decoupling cap right on the COM pin.

author: PL
author_rank: Contributor
timestamp: October 24, 2022, 12:51:40 am
content: Quote from: Psi on October 24, 2022, 12:40:46 amThe errors have also showed up on the PCB traces, when EE said all was good to go. Gulp.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 24, 2022, 02:05:02 am
content: Quote from: julian1 on October 24, 2022, 12:46:52 amI'm, uh, not digging anything out from my own stack(s) right now, but from memory, allowing higher voltage here speeds up field collapse in the coil (by a surprising amount), making the whole thing open faster (by a surprising amount), which quenches any arc on the contacts faster and helps keep the contact material intact. This was very important to us to get right on a project with Very Special Relays whose deaths tended to be the main cause of service in previous versions of the unit. So we stuck that zener in there!

author: Wilksey
author_rank: Super Contributor
timestamp: October 24, 2022, 01:32:22 pm
content: Quote from: PL on October 24, 2022, 12:51:40 amHe's done a PCB too??Do share some snippets of PCB layout, I could do with a good laugh today.Seriously, get someone else to do it, if his PCB is anything like his schematic and he takes just as much care(!) then it will be riddled with issues.