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CONTEXT

The other day I was checking the heat coming off an aluminium heatsink on the power PCB of an eizo EV line led display monitor. It was okay, not too hot, just warm and tad bit hot. When I touched this it was okay. The display for my custom project was on and still works perfectly. Then I touched the aluminium bezel of the monitor and that was okay too.

BUT... when I touched with both handes BOTH the lcd panel's aluminium back (to check if the led backlighting was warm) with one hand AND the aluminium heatsink on the power board of the monitor with my other hand, I got a huuuuge electric continuous shock for a full second. Boy it hurt! I've had a few times in my life touched live wire accedentally while fixing lightbulbs etc, but this time it was long and Im lucky to still be here telling the tale. This seemed or felt more dangerous / higher voltage! How can that be I thought LED was cute and safe!!

QUESTION

How can I insulate the tiny aluminium heatsink on the power pcb of the monitor?

I am planning on covering these PCBs with an aluminium cover as a ground encasing for protectingmyself and children, so first I must insulate this PCB and make sure no heatsinks touch the aluminium plate covering it. HOW can one insulate a Power PCB?

Can I pot the power PCB and the monitor PCB completey submerge it with hot glue or with epoxy resin for example? Or could I just cover the pcb with a rubber layer? All suggestions will count as answers as not only I appreciate your input, it (your answer) might in some cases even save lives! So, thanks in advance!

enter image description here

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  • \$\begingroup\$ Are you operating 230V electronics without a case? \$\endgroup\$ Mar 27 '18 at 10:59
  • \$\begingroup\$ This was temporary layout. These two PCBs will be placed on the backside of the montor (laying on top of rubber insulation sheets). Then the entire rear will be covered with an aluminium metal sheet (connected to ground) that will function as a case. BUT... the ground/monitor rear cannot touch that one aluminium heatsink as there is a huge potential difference, apparently(!), which I did NOT expect! \$\endgroup\$
    – Sam
    Mar 27 '18 at 11:05
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    \$\begingroup\$ +1 great drawing, you have an unusually long right thumb, you should get it checked out :) \$\endgroup\$ Mar 27 '18 at 11:40
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    \$\begingroup\$ Live heat sinks are common, as the insulating washer between power semiconductor and heat sink reduces thermal performance. There is no reason to add one if the product is enclosed with "no user serviceable parts inside"... watch out! \$\endgroup\$
    – bobflux
    Mar 27 '18 at 13:10
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    \$\begingroup\$ Did you see the big sticker "warning lethal electricity inside, do not open of you have no clue"? \$\endgroup\$
    – PlasmaHH
    Mar 27 '18 at 15:01
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You see that thick white line on the power supply board? Everything to the right of it is to be assumed to be at mains potential, that line marks the safety isolation barrier.

You cannot pot a HEATSINK! Think about it, the whole point of the thing is to have surface for air to flow over to remove the heat.

I would note that each of those board has a perfectly good set of mounting holes, get some ally, some standoffs and make up a case (With vent holes too small to admit a finger), job done. You will want to make sure the metal case is connected to the power supply earth connection (The green/yellow wire bottom right of the power board).

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Before you continue messing with that for your personal safety get a GFCI to protect the outlet from which you are powering your test setup. Plug-in inline devices exist. That way if some current leaks to ground (potentially through you) it will kill the power.

To help the GFCI detect that you should connect the aluminum chassis to the ground wire (green and yellow green) coming from the power connection.

Everything on the right side of the white line of the bottom PCB should be treated as live at mains voltage. with the only exception being the ground wire.

The insulation can be handled using airgap of 1/4 inch (5 mm) using standoffs or a plastic sheet lining the inside of the enclosure where you cannot have that gap. For airflow you want slots in the encasing and matching ones in the plastic sheet. You can skip the plastic sheet if the enclosure is plastic.

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Obviously, the electronics need to be in an enclosure that allows plenty of airflow while preventing physical access. A metal or plastic box, with plenty of holes or slots, but with the individual openings too small to admit a finger.

Potting the PCBs, or anything else that would reduce airflow across the heatsinks, would be a very bad idea.

This is precisely why the monitor originally had a case!

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