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Radial electrolytics on PCB

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  • Radial electrolytics on PCB

    I was curious if it was acceptable practice to put positive traces on top of a PC board for a high votlage radial electrolytic. Is there any danger of the aluminum case touching the trace, or is it insulated?

    I checked with a meter and the can does not have continuity with the negative terminal (I thought it did), and was worried the conformal coating on the PCB and electrolytic sleeve/casing was not adequate insulation. That said, I have seen example where the route the positive terminal trace on top. Sorry if that's a stupid question, I can't seem to find the answer!

  • #2
    There may not be a direct connection, ie that would indicate continuity, but my understanding is that an ecap’s can should be assumed to be at the same potential as its negative terminal. eg there’s no insulation per se between them.
    A trace under the can probably wouldn’t cause a problem but it doesn’t seem like it would be good practice.
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    • #3
      I've had the odd board where its arced through the insulating shrink sleeve from a positive track to the can. I think it's mainly been when an old cap has leaked a little and initially caused a bridge around the insulation. Maybe in all the years I've been doing this I've only seen this a couple of times.

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      • #4
        Originally posted by Mick Bailey View Post
        I've had the odd board where its arced through the insulating shrink sleeve from a positive track to the can. I think it's mainly been when an old cap has leaked a little
        I had a newish AC15 a little while ago, under the main HV filter a trace runs very close to the + lead. I think it was a heater trace (or maybe ground). Anyway, a bit of cap leakage and it arced. Because it was under the cap it was not easy to find.

        As far as the can shorting to trace question, another good reason for a bead of sealant holding the cap solid (adds insulation).
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #5
          Thanks for the thoughts so far. I was thinking about adding insulation or a silicone pad of some kind to insulate better.

          This struck me when I noticed some electrolytics I ordered for a board I made didn’t have the normal rubber spacer (pictured below). This would be mean that the only thing between the aluminum casing and positive trace (450vdc) would be the can’s plastic insulator sleeve and black mask. I’m not sure if all board have any other conformal coating.

          Here’s a pic of both styles of cap and the board in question.

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          • #6
            I'd want something insulating between the cap and the board - maybe a disk of bare FR4 or some other material drilled to take the leads.

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            • #7
              Ok, so I came up with a tidy solution so I can still use the board. Hopefully this can help someone else that needs to isolate a radial cap for any reason.

              I found this 3M 1mm thick silicone rubber strip intended for use under doors. Comes in different sizes, and if you cut of the adhesive backed half, it's conveniently about the width of most radial caps. While enticing, I'm not going to use the part with adhesive because I don't know the chemical makeup of the glue. Last thing I'd want is accidentally short my caps with some conductive adhesive!

              The silicone is thin enough that I can simply cut it, lay it over the board and pop holes with the leads. A lot less messy that liquid silicone (which I will still use a dab of to hold the caps), and provides a certain gap between cap and board.

              Here's a link:

              https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

              Next time I'll just put the traces on the bottom, but glad I found a solution, and a cheap one to boot!

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              • #8
                Originally posted by Gaz View Post
                liquid silicone (which I will still use a dab of to hold the caps), and provides a certain gap between cap and board.
                Avoid any silicone that smells like vinegar.

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