PCB Layout AC Question - Page 1
Forum section: electronics / beginners

author: admiralk
author_rank: Regular Contributor
timestamp: June 30, 2019, 11:51:04 pm
content: My current project uses relays and I an not sure how to handle the AC part of the circuit. I am using 10A relays, which will change to 15A if I make a next version.It was suggested that I cut my pours down to ~ 60 mil traces and have them on both the bottom and top layer. I am also concerned about the gap between traces, and if I should include the cutouts or not worry about it. The DC part still has some changes to be made to it. It is the AC part, on the right, that I an concerned with for now. I would really like to hear some opinions on how I have it and I could better handle it.

author: mdszy
author_rank: Unknown
timestamp: July 01, 2019, 12:24:55 am
content: Looks very similar to a board I just did that involved 10A relays and AC stuff. See attached for the PCB I ended up using and that worked just fine.I previously had a board that didn't have sufficient AC isolation, so the AC was coupling back into the circuit and causing my microcontroller to freak out, there was 500mVpp 60Hz AC all over the microcontroller's pins.This board worked just fine, and the main concerns are thick enough traces for current, and excluding your ground plane from the AC area.I don't think the slots are necessary, relays are already quite isolated internally so there don't need to be slots under them, in my opinion.

author: admiralk
author_rank: Regular Contributor
timestamp: July 01, 2019, 01:22:42 am
content: My ground plane is on the bottom layer, so it is farther away from the AC, so I should be good there.It looks like a 60 mil trace would be fine, from your design, especially if I use both sides. That would also give me much more clearance.I do need to double check the pin size on he relays. They are probably the smallest diameter of all the components, yet I have them as one of the biggest.

author: floobydust
author_rank: Super Contributor
timestamp: July 01, 2019, 01:33:15 am
content: I would say the slots are not needed 1.9 mm is decent spacing.The annular ring is looks small on all your pads. IPC for big ones is twice the hole size, so 1mm hole 2mm dia. pad. The relays need more, especially if 1oz copper.The J1 connector pad holes look unequal and the relay coil too but maybe an illusion or rendering is doing that.I would consider moving all the low voltage stuff over so you can have four mounting holes. Right now the one corner can bend down, especially when the connectors are screwed or wire torque, so the PCB could touch the chassis. The connector would be in front of a relay.ULN2003(5V) maybe get rid of 20 some discretes/resistors/diodes driving the relays, even though some outputs wasted.Mains relay contacts arc a little and generate a burst of EMI, which is coupled to your logic from the relay coils. Usually a 0.01uF cap from GND to chassis to cover that, assuming a metal box and spacers.

author: MarkF
author_rank: Super Contributor
timestamp: July 01, 2019, 02:49:47 am
content: Quote from: mdszy on July 01, 2019, 12:24:55 amI would propose a different routing with larger clearances.

author: mdszy
author_rank: Unknown
timestamp: July 01, 2019, 02:59:11 am
content: Quote from: MarkF on July 01, 2019, 02:49:47 amYeah that's a nice layout there, if the project weren't already complete I'd definitely to it that way. Looks nicer too.

author: MarkF
author_rank: Super Contributor
timestamp: July 01, 2019, 04:41:57 pm
content: Quote from: floobydust on July 01, 2019, 01:33:15 amHuh? He already has four mounting holes.

author: free_electron
author_rank: Super Contributor
timestamp: July 01, 2019, 05:08:42 pm
content: you have a double sidedboarduse top AND bottom layer to double up current handling- round edges on all HV lines. you don't want corona effects- no need to extend 'beyond'. current wont go there. current follows shortest path.

author: admiralk
author_rank: Regular Contributor
timestamp: July 01, 2019, 09:46:56 pm
content: Thanks for all the suggestions so far. I had to fix a couple things on the top also and was able to get the mounting holes a little better positioned in the process.free_electron, that is basically what I ended up doing.floobydust, I do not understand the suggestion for a driver? As far as I can tell at best it would replace a resistor, a diode, and a transistor. Also, while my annular rings could be bigger, they do meet the IPC-2221 standard for minimum size. If you can point me to a better calculation I would appreciate it.Anyway, I still need to do some pushing and shoving, but I think it is looking better now.

author: free_electron
author_rank: Super Contributor
timestamp: July 02, 2019, 07:09:55 am
content: there we go. now we're talking.

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2019, 03:43:18 am
content: For heavy or high current parts, I would not use the IPC minimum annular ring.The appliance fires (dishwashers, clothes dryers), class action lawsuits were the PCB relay pins becoming hot spots due to crappy PCB layout or vibration fatigue and cracked joints.I don't know your application or PCB specs. Is it base 0.5oz copper plated up to 1oz?I have to try a simulation but I'm sure there is actual current flow to the backside of the pads and cutting that off can result in localized high current densities on the copper.The ULN driver can drive all four relay coils, it's just a suggestion for ~20 fewer parts.The mounting holes look much better.

author: admiralk
author_rank: Regular Contributor
timestamp: July 06, 2019, 03:09:41 pm
content: ThanksThere is a good chance I will make several of the pads bigger, if for no reason other than to make it easier to solder. However, without thermal reliefs would it really make much of a difference in the case of those? I mean are the pads separated from the traces somehow? I could be missing something, but it seems to me that even if I tripled the size of the annular rings, the only difference would be that they would just add a bulge to the end of the traces.I will look closer at the driver, I was thinking I would need one for each of the relays. Having just the one chip for all of them might be a better idea. The D5_x are connectors for off board LEDs and R1_x are the resistors for them, they would still be there no matter what.