About to send my first ever PCB to be manufactured - but will it work!? - Page 1
Forum section: electronics / beginners

author: Lomax
author_rank: Frequent Contributor
timestamp: May 13, 2025, 02:35:45 pm
content: Hello,I've designed a simple Teensy based gadget that listens for data on two RS422 ports, performs some calculations, and transmits the result via CAN bus. This is for an off-grid application that runs on batteries. I've made a prototype on a breadboard which works perfectly, though it uses different bus transceivers to the actual design (I used what I had to hand) and omits the DC/DC converter. I'm confident-ish that this is a viable design, but nervous about pulling the trigger!Primarily:I'm worried about the DC/DC converter layout, since I've never designed a board with one before.While I'm a strong believier in KISS, perhaps I have gone too far in component reduction, for example:An SM712 bidirectional TVS diode is the only input protection.I make no attempt at rejecting noise or common mode voltages.I have not put any pull-up/down resistors on the internal data lines, relying on the Teensy's pull-up/downs.I will be ordering the boards with the SMD components mounted by the manufacturer (on the back; the front of the PCB only has throug-hole compoments, which I will solder myself), and would hate to waste the time, effort, money and parts if it turns out the design is not viable. The problem is I have no-one I can ask for a sanity check - except the EEVBlog forum members of courseIt would be amazing if I could get your help to push this project across the finish line - I've certainly been dithering long enough with it!Other than the Teensy the main components used are:AP63205 2A Buck ConverterBourns SRN4018-4R7M 4.7uH InductorAP2112-3.3 600mA LDO RegulatorSM712 Bidirection TVS DiodeTCAN334 CAN TransceiverTHVD1400 RS-485 TransceiverWhat do you guys think?

author: PGPG
author_rank: Frequent Contributor
timestamp: May 13, 2025, 02:52:51 pm
content: About schematic:- don't use GND label but use GND symbol,- don't use +12V, +5V, +3.3V labels but use supply symbols,- it is surprising when supply comes from right and goes to left,Don't be afraid of assembling SMD elements. Down to 0603 size it is even simpler than assembling THT elements (you don't need to continuously revert PCB.C3,C4 - I would go with track directly through their pads. For DCDC as short connections as possibly counts.I always prefer to have at least one side (or if not possible one internal layer) with continuous GND fill like here:https://forum.kicad.info/t/approaching-pcb-track-routing-for-a-newbie/36302/8

author: Lomax
author_rank: Frequent Contributor
timestamp: May 13, 2025, 03:15:58 pm
content: Quote from: PGPG on May 13, 2025, 02:52:51 pmOk, I'm not sure how to do this; when I click to add a label there are no predefined symbols?Quote from: PGPG on May 13, 2025, 02:52:51 pmYeah, I know, sorry. Might fix that some time. Edit: done!Quote from: PGPG on May 13, 2025, 02:52:51 pmJust hoping to save time; one or two boards might be ok, but I'm having ten made...Quote from: PGPG on May 13, 2025, 02:52:51 pmDone!Quote from: PGPG on May 13, 2025, 02:52:51 pmWould have liked this too but I had to use both layers for some tracks, and don't want the extra cost of a multi-layer board. Hopefully there's enough of a ground plane still there? Gonna have another look around the board and add some more vias where it looks like they're needed...

author: PGPG
author_rank: Frequent Contributor
timestamp: May 13, 2025, 05:02:22 pm
content: Quote from: Lomax on May 13, 2025, 03:15:58 pmIf you are using KiCad than at schematic:hotkey 'a' = add symbolhotkey 'p' = add power symbol

author: SteveThackery
author_rank: Super Contributor
timestamp: May 13, 2025, 05:08:38 pm
content: D1 (top right on the schematic) the wrong way round?

author: Lomax
author_rank: Frequent Contributor
timestamp: May 13, 2025, 05:22:44 pm
content: Quote from: SteveThackery on May 13, 2025, 05:08:38 pmI don't think so - it's the schematic that's the wrong way aroundBut I just realised it won't fully protect against reverse polarity since there are other external ground connections; if you swap the wires on the CANBus you'll have a short to ground through the RS422 portsBut you know what, it's not even needed, since the CANBus connector is pre-wired and therefore rather difficult to get wrong! Imma delete it.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 13, 2025, 06:14:33 pm
content: Oh, sorry! It caught me out.

author: Lomax
author_rank: Frequent Contributor
timestamp: May 13, 2025, 06:56:40 pm
content: Can I assume that there are no obvious issues with my design then?I've added another 20-ish vias to improve the grounding, and removed D1 since it wasn't needed.

author: Lomax
author_rank: Frequent Contributor
timestamp: May 14, 2025, 07:05:49 am
content: I'm having second thoughts about transceiver VCC; would I be better off powering them from the 5V rail instead, using a level shifter (e.g.TXB0108) to interface with the 3.3V Teensy? I'm thinking the communication would be more resilient/reliable at the higher voltage? RS422 spec has 5V signalling. Although the datasheet for the THVD1400 doesn't say, other similar transceiver chips can drive the data lines to (max) VCC. There should be room on the PCB for a level shifter, and it wouldn't add a huge amount to the BOM cost ($0.25 at JLCPCB).Edit:If so, am I right in thinking that I would only need to level-shift the RxD (R) lines going to the MCU, and that the transceivers will be happy with TxD (D) and DD (DE & /RE) signals at 3.3V?

author: AndyC_772
author_rank: Super Contributor
timestamp: May 14, 2025, 08:17:24 am
content: Hi, not a bug as such, but FYI you will need a hot air station to properly solder some of those components. The GND plane will draw heat out of the joints - thinking especially about the SMD caps you have which have one end grounded, also the SMPS controller and inductor.

author: Lomax
author_rank: Frequent Contributor
timestamp: May 14, 2025, 08:50:23 am
content: Quote from: AndyC_772 on May 14, 2025, 08:17:24 amThanks - I presently only have a hot-air "pencil", courtesy of aWeller WMD-3soldering station, which I find isn't powerful enough for a lot of SMD work. It should be ok for smaller components with few legs, but for larger ICs I think I either need to add a hotplate, or get a more powerful hot-air station. This is not essential for this project though, since I intend to have the SMD components fitted by the board manufacturer (JLCPCB). I have already purchased most of the SMD parts and they're waiting for me at the manufacturer. Just need to finalise the design!

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: May 14, 2025, 10:39:37 am
content: I have had a lot of boards made by JLCPCB, they are excellent and very low cost.A couple of points to note.We all make mistakes with layouts, that is pretty normal. Even large manufacturers have v1, v2, boards, etc. Given the low cost of Chinese boards, don’t worry if the first set isn’t quite correct.For those who fit their own parts, it helps to have the parts in front of you and to print the board on paper to make certain they really fit. Datasheets don’t always include the overall outline (Alps mini pots). Also PCB pads are generally designed for commercial assembly and may not necessarily be wide enough to hand solder with an ordinary soldering iron.Home assembly with a soldering iron is straightforward, you don’t have to use hot air, except for those parts with pads underneath. This link shows you how:https://youtu.be/eZtPR8_x0nc?si=CjycyO78_qZVr3fISJ

author: PGPG
author_rank: Frequent Contributor
timestamp: May 14, 2025, 10:42:20 am
content: Quote from: AndyC_772 on May 14, 2025, 08:17:24 amOP will order PCBs with SMD assembled so it is not his problemI'm not sure of it but I think that in reflow soldering solid pad connections to planes are not a problem as whole PCB get enough high temperature.Quote from: Lomax on May 13, 2025, 06:56:40 pmI don't think anyone deeply analysed your project (me not, and have no time for it)

author: AndyC_772
author_rank: Super Contributor
timestamp: May 15, 2025, 01:09:57 pm
content: Quote from: PGPG on May 14, 2025, 10:42:20 amCorrect, if the board is heated sufficiently during reflow (note: "if"), then it's not a problem during assembly. It's an issue to consider when making any changes, though, and if these are prototypes, it's a valid consideration.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: May 24, 2025, 02:18:05 pm
content: Question: Why are you using multiple different sizes of capacitors? Is it to simply differentiate between the two different capacitance on the board?Minor suggestion: If this were my board, I'd personally keep the caps all the same size (unless there's a rational for different sizes, like board space limitations, desired ESR/frequency), and use a larger diode for D1 (because it's quite small and will be annoying to hand-solder).Major Suggestion: For U4: Your switching loop area looks to be huge and is likely to cause problems. Can you decrease the switching loop area?Here's a video on designing switching regulators: