PCB design considerations for boards with line voltage and logic levels - Page 1
Forum section: electronics / projects

author: pstemari
author_rank: Contributor
timestamp: October 08, 2014, 07:35:35 am
content: Anyone have pointers to guidelines for PCB layout when you have line voltage and logic levels on the same board? Right now I have all the 120VAC on one side, with trace separations of at least 50 mils and a fat protective earth trace between the high and low voltages.Is the earth trace a good idea? Any other suggestions?

author: Phoenix
author_rank: Frequent Contributor
timestamp: October 08, 2014, 12:25:01 pm
content: Quote from: pstemari on October 08, 2014, 07:35:35 amNo. Mains voltage getting to earth is a very bad idea.QuoteLook up the terms creepage and clearance with reference to a PCB, it will give you a general idea of the safe distances.Looking at some standardshttp://www.pcbtechguide.com/2009/02/creepage-vs-clearance.htmlI'd say something like minimum 2mm (functional, basic) creepage between tracks that people can't touch (i.e. L and N) and 4mm (reinforced) between mains voltage (L and N) and parts people can touch (logic circuitry, earth, any external buttons).Use appropriate sized terminals for the voltage (meeting creepage distance).Do you care about having an EMI filter? If not you probably don't even need the earth connector on the PCB.

author: pstemari
author_rank: Contributor
timestamp: October 08, 2014, 06:11:03 pm
content: Quote from: Phoenix on October 08, 2014, 12:25:01 pmThat's the entire point of a grounded case. Mains voltage goes to ground and blows fuse, instead of creating a shock hazard. Applicability to PCB's, not so sure.QuoteThose numbers are quite useful. I looked at creeepage/clearance references before I posted, but they didn't address the ground issue and I didn't get a clear sense of what the appropriate numbers to use.4mm (160mil) is easily doable if I drop the earth trace, but a short without the earth trace gives line voltage on the low voltage side. Everything is insulated, but there is a push button and a low-voltage cable going out to a smaller plastic box. The main enclosure is a standard cast aluminum junction box and lid, with rigid conduit going to the main device.Terminals all have appropriate voltage ratings, and not worried about EMI. I just don't like the idea of a short putting line voltage on that low voltage cable.

author: Phoenix
author_rank: Frequent Contributor
timestamp: October 08, 2014, 08:16:45 pm
content: You need Y rated caps between L/N and Earth for the same reason you need proper isolation - to prevent shock hazard in the case of component failure. Having a Earth plane on the PCB increases the chance of the chassis inadvertently becoming live. Fuses take a long time to blow vs someone touching the metal case getting a shock, best case scenario the RCD picks up the fault but not everywhere has an RCD...If you want to go further you can specify route outs in your PCB to provide greater creepage for any traces that are getting too close.If you don't want a short to put line voltage on a low voltage circuit then increase the clearances, consider using optocouplers to get more distance (I know relay's are isolated... provided that they meet the appropriate safety standards). Or even putting the low voltage and high voltage on separate boards, in a segregated parts of the box. Plenty of safer options than putting an earth trace in.

author: pstemari
author_rank: Contributor
timestamp: October 09, 2014, 06:58:56 am
content: RCD? You're in the UK, perhaps? Called GFCI's here, and typically only present on kitchen, bathroom, and outdoor circuits.I've never seen caps between L/N and protective earth in a US device. MOV's for transient protection, yes. I believe some of the EMI entry modules use shunt caps along with chokes, but I've not sliced open to verify.My understanding is that UK wiring is done a bit differently than the US. Here the protective earth runs back the the main panel where it's bonded to neutral. Grounded equipment with metal housings have the housings tied to the PE, and the idea is that any short will cause the breaker to trip immediately, before someone has an opportunity to touch it. Hand-held power tools typically aren't grounded anymore but instead rely on double-insulation. Don't know if UL still permits the grounded style or if the double-insulated stuff is is cheaper to build.The signal ground plane on the low-voltage side of the circuit here is not tied to the PE.At any rate, if the PE trace to separate the HV side isn't a good idea, getting rid of it gives better clearances.

author: Kremmen
author_rank: Super Contributor
timestamp: October 09, 2014, 07:52:59 am
content: Quote from: pstemari on October 09, 2014, 06:58:56 amThe EU apparently directs all installations to include RCDs/GFCIs in all circuits with only some specific exceptions. I think you are allowed to exclude them from a fridge/freezer supply to prevent a thaw in case of an unrelated fault.QuoteThose are commonly used with SMPS converters of any significant power. You could say they are always used. X-rated caps between phases and neutral; Y-rated caps between PE (ground) and the previous.QuoteThat is the TN-S or TN-C-S distribution systemhttp://en.wikipedia.org/wiki/Earthing_systemcommonly used in EU as well, esp. the TN-S.QuoteI included a couple of pics of a measurement/display board i made some time ago. It is 230VAC single phase supplied and lives in an industrial environment. If you look carefully you will note that the circumferential trace is connected to the PE input in the supply input connector block. The PSU is a semi-traditional one with an iron transformer separating grid from the regulator which is a simple buck arrangement. Since the device is an "inherently benign one" or as close as makes no difference, the line filter is simple - just 1 cap. There is an additional MOV to guarantee the fuse separates the device from grid in case of lightning surge or some similar nastiness.This gadget is meant for fixed installations where the PE connection can be relied upon. Therefore the signal ground on the secondary side is equalized to PE using a simple EMI ferrite to keep possible line noise at bay. That of course creates a galvanic connection to the primary side and would be a no-no in a class 2 device.This one has served with zero failures since deployment a couple of years afo.

author: Phoenix
author_rank: Frequent Contributor
timestamp: October 09, 2014, 10:14:25 am
content: Quote from: pstemari on October 09, 2014, 06:58:56 amAustralia, mate. Multiple Earth Neutral system, sounds the same as the UK's where the neutral is tied to the earth at the main switchboard.I guess what's "safe" depends on the class of device and location. If there is no specific board size, I'd rather make the board that little bit bigger and fit proper creepage/clearance in.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 01:44:05 pm
content: Quote from: Kremmen on October 09, 2014, 07:52:59 amAnd the value of the Y rated capacitors is limited by the allowed ground leakage current.QuoteIt is the same here in the US. Ground and neutral are bonded together at the main panel.The double-insulated stuff is just cheaper.