Potting just some parts of a pcb?  High voltage (2KV) insulation? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2015, 05:54:48 pm
content: I have a possible pcb design that measures a 1000Vdc source. This signal arrives onto the pcb via a through hole shrouded "HV" 4mm banana socket like this:It then passes down a multi-resistor ladder to be suitably attenuated.So far so good. However, one of the application installations of this pcb is into a (safety grounded/earthed) metal enclosure, and there will be just 4mm of clearance between the enclosure and the pins on that connector. Whilst there is little chance that that 4mm gap could be reduced, except my massive mechanical damage, it's a possibility. So, i'd like to pot over the top of the pins on that connector or something similar?A bit of googling suggests epoxy potting has a dielectric strength of something like 10kV/mm so it should be fine both in terms of mechanical support and electrical isolation.The question, is how to achieve potting just this bit of the pcb, locally? I guess i could make some sort of silicon former / dam that is clamped around the area in question, the potting poured in, and then the silcone former peeled off once set?Or, i though perhaps i could machine some sort of acrylic/plastic plate and glue that onto the pcb somehow?Any ideas anyone? (this is a low volume device, maybe 10 to 50 being made, so the solution doesn't have to be mass producible etc?Anyone done anything similar ?

author: ajb
author_rank: Super Contributor
timestamp: October 14, 2015, 06:17:02 pm
content: There are plastics with even higher dielectric strength. Would applying a plastic film/sheet to the inside of the nearest part of the enclosure solve the problem? You can find ready-made peel-and-stick plastic sheeting that would just need to cut to size and stuck on. Or you could fasten the sheet to the PCB via a couple of standoffs.Alternatively, could you put the input connectors and resistor dividers on a separate board and pot that with leads back to the main PCB?

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2015, 06:26:39 pm
content: I thought about some sort of plastic insulator, but with just a 4mm gap, how can i hold it in place? Perhaps an acrylic sheet, 4mm thick, with some cutouts to clear the protruding pins of the banana socket, and threaded holes to screw it onto the bottom side of the pcb?That's why i though the localised potting idea was useful, as if i can work out how to keep the potting from escaping it would be really easy to do

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2015, 06:54:35 pm
content: Quote from: max_torque on October 14, 2015, 06:26:39 pmJust cut plastic rings from an empty toothpaste tube or something. Crease them into squares. Grease them so they don't stick. Use them to contain the potting compound until it hardens.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2015, 07:42:29 pm
content: If you insist, conformal coat may be an option. I don't know how well it's rated for voltage, in whatever of the many formulations available besides. You'd want to ask around. Assemblers usually do potting and conformal, they might know about material ratings/properties too.Also it might be more help just to add some "corona dope" to the points, perhaps not the literal stuff, but just dabs of silicone on the ends of the soldered pins, perhaps.For clearance, absolutely, just use a strip of plastic glued to the enclosure or PCB. This is done for power supplies all the time (isolation 2.5kV RMS and up). Typical choices: tape (mylar or kapton), fishpaper (rubber-vulcanized fiber paper, tough and insulating), mylar sheet (or other plastic), or I suppose you could make the argument for paint (especially tough varieties like powder coat, epoxy, or rubberization), or anodizing if applicable (hard coat anodize is quite heavy and tough, and can withstand hundreds of volts; though I suppose not thousands, and can still be chipped relatively easily).For the paper or film choice, I'd suggest something in the 1-10 mil (0.025-0.25 mm) thickness range, preferably self-adhesive just so it's easier to use.Tim

author: AMS
author_rank: Contributor
timestamp: October 14, 2015, 08:03:26 pm
content: If you have access to a 3D printer you can make a very thin walled container (boundary box) fill it with the epoxy and break it of or even leave it at the end.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2015, 08:07:09 pm
content: Heck, you could probably 3D print directly onto the board, give or take some prep...

author: plesa
author_rank: Frequent Contributor
timestamp: October 14, 2015, 08:17:38 pm
content: Potting for 2kV device seems to be overkill. By potting you needs to select compatible components and also it will make the device almost non repairable.I suppose that plastic sheet for insulation will be enough ( PTFE sheets are cheap) or two enclosures ( plastic inside and metal outside).

author: ajb
author_rank: Super Contributor
timestamp: October 14, 2015, 08:50:48 pm
content: www.mcmaster.com/#standard-adhesive-back-plastic-film/The 5mil kapton, to pick one example, would give you a nominal 10kV dielectric strength.

author: DTJ
author_rank: Super Contributor
timestamp: October 15, 2015, 01:01:10 am
content: Maybe check out the insulating properties of that "Bergquist" rubbery thermal sheeting.Cut to size, spray adhesive and push into place.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 15, 2015, 01:27:12 am
content: Corona dope.15kV per 0.01 inch thickness, according to the bottle label.

author: max_torque
author_rank: Super Contributor
timestamp: October 15, 2015, 08:54:18 am
content: I think i need more than just a thin film / dope layer, due to the (remote)possibility for mechanical damage! OK, people shouldn't be dropping / kicking this device around, but you never know! An idea option might be a thin layer of potting / enough to just cover the protruding pins / solder on the back of the board, that also "glues down" some form of say 3mm thick plastic sheet that acts as a mechanically robust top cover to prevent the pins ever getting into close proximity with the case etc?

author: daqq
author_rank: Super Contributor
timestamp: October 15, 2015, 10:06:07 am
content: There are companies which do selective conformal coating with something like a printer:If you need to do it en bulk. Or look for "dam and fill".

author: max_torque
author_rank: Super Contributor
timestamp: October 15, 2015, 11:20:57 am
content: Considering soldering one of these to the pcb:then back filling with a "thick" potting material (too thin and the potting will escape through the small gaps, although i guess i could solder it to the pcb etc?So, then the question becomes, what are minimum clearances when we are talking about a potted pcb/components.Normally, you'd leave significant gaps between components to protect against arc-over and creepage, both of which can be degraded by local environental conditions or contaminants on the surface of the pcb. But when potted, assuming the pcb is clean first, and then potted without large voids, do the physical seperation requirements fall significantly??

author: max_torque
author_rank: Super Contributor
timestamp: October 15, 2015, 12:00:53 pm
content: Something like this:Where the 4mm pcb mount socket is mounted on the topside of the pcb, the resistor / capacitor ladders are on the back side, enclosed by that ^^^^ pcb mount tin box that is then filled with potting. The output of the ladders is via a Via () back to the top side of the pcb, which, when everything is potted should mean all HV contacts are completely hermetically sealed and mechanically supported etc!Comments anyone??