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I was recently taking a look at a failed power supply for 12V halogen lighting and noticed an unusual texture in the potting compound. I tried chipping some of it away with a screwdriver and got a nice surprise:

enter image description here

Yes, pebbles. But why? Are they special pebbles or just normal ones?

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    \$\begingroup\$ Boy, that sure is a surprise alright! Good find! BTW: do you have nameplate (make, model, etal) data on the failed unit, and if so, can you post it here? \$\endgroup\$
    – ThreePhaseEel
    Commented Oct 7, 2016 at 2:04
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    \$\begingroup\$ I think it's audiophile-grade crystals, leave them in. They improve the sound of your hi-fi equipment connected to the same power outlet as the lamp. \$\endgroup\$
    – pipe
    Commented Oct 7, 2016 at 2:06
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    \$\begingroup\$ Probably pebbles are cheaper than potting compound. \$\endgroup\$
    – brhans
    Commented Oct 7, 2016 at 2:23
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    \$\begingroup\$ I thought of trying this, but I didn't want to be the first to cast a stone. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Oct 7, 2016 at 6:07
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    \$\begingroup\$ Silicaceous materials are also highly resistant to many types of drills. Not that it applies in this instance, but I have seen safes (e.g. for putting money in) that have a thick wall of gravel / flint aggregate between two walls of steel. This will reliably blunt practically any non-specialist drills in seconds, thus preventing easy entry to the safe without the key. Some kinds of electronics might be worth protecting in such a manner, although a halogen PSU probably isn't one of them :) \$\endgroup\$
    – user98663
    Commented Oct 7, 2016 at 6:54

5 Answers 5

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Thermal conductivity is indeed better for stones than for epoxy. Wikipedia gives 2.8W/(K m²) for granite vs. 0.2W/(K m²) for epoxy. Thermal conductivity can be increased by adding some material, but a potting compound should not be electrically conductive, which prohibits metals, graphite etc. Most insulating casting compound of this kind have a value around 0.8W/(K m²), but I also found a silicone based compound with 5.0W/(K m²).

However, I think the price plays a big role, too. Even normal epoxy costs some money, the heat conductive stuff more, and that 5.0W/(K m²) is for sure very expensive. In contrast, I guess the pebbles in that picture cost less than a cent.

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    \$\begingroup\$ I'd wager that the cost saving is the real reason. \$\endgroup\$
    – Li-aung Yip
    Commented Oct 7, 2016 at 7:41
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I love the innovative thinking

stones conduct heat well, epoxy or potting compound conducts heat POORLY

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  • \$\begingroup\$ ...unless it's thermally conductive epoxy... :) \$\endgroup\$
    – Daniel
    Commented Oct 7, 2016 at 3:01
  • \$\begingroup\$ which is expensive or poor in BDV for line Vac \$\endgroup\$
    – D.A.S.
    Commented Oct 7, 2016 at 3:04
  • \$\begingroup\$ At least it's a UL-rated material hah \$\endgroup\$
    – Daniel
    Commented Oct 7, 2016 at 3:10
  • \$\begingroup\$ Not necessarily PB-free though. Could be some Galena in there. \$\endgroup\$
    – user98663
    Commented Oct 7, 2016 at 8:40
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Hypothesis 1 : someone made a good product and wanted to save epoxy by filling as much as possible with junk

Hypothesis 2 : someone made a bad/counterfeit product using less/smaller/lighter components, but wanted it to weigh more

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I have heard of using very finely ground silica mixed with epoxy for potting power transformers or power inductors for very strong magnetic fields. Thermal conductivity of this mix should be better than a mixture with pebbles.

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The pebbles are filler that also add strength. Potting compound, on its own, isn't really that strong. Just like concrete driveways and sidewalks, you use gravel to fill and build strength while using concrete to fill in the gaps and glue things together. Combined, they make a solid block.

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  • \$\begingroup\$ If anything, epoxy with pebbles is weaker than a solid epoxy block. I can guarantee you epoxy filled with stones with crack on stone surface rather than in between. \$\endgroup\$
    – Dmitry Grigoryev
    Commented Oct 7, 2016 at 14:10
  • \$\begingroup\$ @DmitryGrigoryev Take a screwdriver and tap it with a hammer on a solid epoxy block and see how easily you can break it in half. Now look at the supplied image in the question and notice the difference. I said epoxy but the question was about potting compound, so I edited my answer. I don't recall if they are always the same thing. It also doesn't take away from the fact that the pebbles are also filler that are cheaper than epoxy. \$\endgroup\$
    – Rob
    Commented Oct 7, 2016 at 14:33
  • \$\begingroup\$ I think sand would be a better bulk filler because it's less likely to damage underlying electronics than dumping a load of gravel on it. \$\endgroup\$
    – user98663
    Commented Oct 7, 2016 at 14:41

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