Please for your input on my first mains pcb - Page 1
Forum section: electronics / projects

author: dimkasta
author_rank: Regular Contributor
timestamp: April 23, 2017, 12:11:38 am
content: I have been doing low voltage pcbs for some time now. I always kept my mains wiring off-boardNow I have to use a specific transformer that comes on a pcb mount footprint.I have faston connectors on the mains side, added slots, put the traces at the bottom layer, and I will use protected parts. The transformer will be wired with series primaries (230V) and 2x24V AC (at 36VA) on the secondaries.Could you please have a look at my pcb?Thanks in advance for your input

author: Ian.M
author_rank: Super Contributor
timestamp: April 23, 2017, 12:26:28 am
content: Actual dimensions so we can check creepage and clearance?I assume this is for 220V AC operation?A link to the transformer would be useful so we can check its mass and mounting requirements.IMHO you've weakened the board far too much with excessively large cutouts, coming far too close to each other and to the board edge.

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2017, 12:30:33 am
content: IMO you need focus less on phase/neutral isolation as the fuse looks after that. I'd say the creepage distance between mains and LV was plenty without slots but it does little harm for them to be there BUT should the PCB with the transformer mounted be subject to any shock it can tear the pins from those potted PCB mount transformers and I strongly advise to have some holes for zip/cable ties to hold it firmly against the PCB.Be aware excessive slotting can make the PCB more flexible and rigid componentry like transformers won't/don't flex like TH passives so some thought need be taken to only addenoughslots to do the job.

author: digsys
author_rank: Unknown
timestamp: April 23, 2017, 12:38:46 am
content: As above +The transformer will already have correct (+) clearances, so you don't need anything under it. Even the slots on the Inputs are superfluous, especially for the lowpower your talking about. The ONE thing I'd add, is a MOV, after the fuse, across the primary. ~270VAC for a 230VAC IP. That will take care of any fly-back / voltage spikesthat would normally affect clearance requirements.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 23, 2017, 12:43:46 am
content: You haven't given any dimensions (that I can see). You just need to be sure you satisfy creepage and clearance requirements for the environment the board is going into.7mm distance should be both easy to achieve and ok, but if you look up the requirements you probably get away with less.One thing I will say is the size of the cutout near F1 (left centre) will make the board unnecessarily weak. That area near the centre tap on the primaries looks very weak.Not even sure if you need that cut out anyway?You need to maintain high isolation ~4000V between primary and secondary but not between active and neutral.IMO You probably only need the big slot in the centre about 3-4mm with rounded ends. Slots with rounded ends will be stronger.Maybe bring the corner posts in a smidge too.What are you using for terminals? I would use properly rated ones on the primary at least.Silk screen the Active, and Neutral and what about earth? Might be nice to have one there in case you have a three wire lead.Sorry 3 replies beat me to it.

author: dimkasta
author_rank: Regular Contributor
timestamp: April 23, 2017, 02:00:02 am
content: Thanks a lot for the quick responses guys.I added a todo note to look for an appropriate mov and add it to the boardThe board is 10x10cmThe slots are more or less 10mm wide and the corners should be rounded by the fab's milling machine. I will make them round anyway just to be sureThe mains connectors will be properly insulated and crimped faston connectorshttp://www.mouser.com/ds/2/418/NG_CD_62409_S_baseFilename-669300.pdfEarth and switch connections will remain off-boardThe transformer is this onehttp://www.mouser.com/ds/2/418/NG_CD_4900-RF65_D-680958.pdfThanks again

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2017, 02:36:27 am
content: Quote from: dimkasta on April 23, 2017, 02:00:02 amLooking at some mains powered PCB's in my junk pile all the slots that provide mains creepage isolation are 1 mm wide.As HFM mentioned without slots ~7mm+ board separation is used.

author: Ian.M
author_rank: Super Contributor
timestamp: April 23, 2017, 02:53:32 am
content: With 46.8mm from primary pins to secondary pins, you don't need any slots under the transformer.You don't have nearly enough copper around the faston connector footprints to give enough strength for the usual stresses of mating and disconnecting that sort of connector.Why not flip the transformer over, reduce the board size, and mount the transformer and board with long screws passing through both the board and transformer core mounting holes, with spacers between the board and the core so the compression doesn't stress the board excessively. Move F1 to be parallel to the line of primary pins, with the tracks leading diagonally to it to reduce the overhang and provide a good place to add the suggested MOV across the primary inline with the fuse.

author: dimkasta
author_rank: Regular Contributor
timestamp: April 23, 2017, 01:07:11 pm
content: I added two more screw holes for extra rigidity and made the slots 2mm wide. I already had screw holes on the footprint of the trafo, so I will fasten it with some screws to further reduce vibration stressI also added a 275VAC (20mm) mov after the fuse.https://gr.mouser.com/Search/ProductDetail.aspx?R=V20H275Pvirtualkey57610000virtualkey576-V20H275PI also beefed up the faston pads and added vias to strengthen them even more. Not that I intend to do any frequent connecting/disconnecting. Just to be sure we won't have any issues there.Once again, thanks a lot for your time guys

author: Ian.M
author_rank: Super Contributor
timestamp: April 23, 2017, 01:26:04 pm
content: Put a spur on the N trace to move the MOV closer to the fuse, to get the L after the fuse further away from the N faston. Move the L and N fastons closer together so you can get rid of the slots between them and the mounting holes, and get rid of the long slot under the transformer - you don't need it for creepage with the transformer's generous pri-sec pin spacing.