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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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    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? – ThreePhaseEel Oct 07 '16 at 02:04
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    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. – pipe Oct 07 '16 at 02:06
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    Probably pebbles are cheaper than potting compound. – brhans Oct 07 '16 at 02:23
  • @ThreePhaseEel: I just looked at 2 more dead units of the same model (LighTech LET-303 300W) and neither of them have pebbles. –  Oct 07 '16 at 02:26
  • Do they have any UL/ETL/other markings on them? – Daniel Oct 07 '16 at 02:47
  • @Daniel It's UL/cUL listed. –  Oct 07 '16 at 02:50
  • It should have an e-number or something if it's legit. – Daniel Oct 07 '16 at 03:01
  • Yeah, that UL E-number would be good to post :) – ThreePhaseEel Oct 07 '16 at 03:04
  • The case (an 1/8in thick aluminum box) is completely mangled from getting it open, so figuring that out might be hard but I'll try. –  Oct 07 '16 at 03:21
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    I thought of trying this, but I didn't want to be the first to cast a stone. – Spehro Pefhany Oct 07 '16 at 06:07
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    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 :) –  Oct 07 '16 at 06:54
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    @SpehroPefhany I'm impressed ! :-). I'll pass that on to my brother who is a master punster. – Russell McMahon Oct 07 '16 at 07:01
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    @spehro - Apparently you are not the one without sin. – Whiskeyjack Oct 07 '16 at 08:08
  • See also http://electronics.stackexchange.com/questions/240122/best-materials-to-prevent-reverse-engineering-of-pcba-components/240126 –  Oct 07 '16 at 09:59

5 Answers5

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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.

sweber
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I love the innovative thinking

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

Tony Stewart EE75
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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

filo
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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.

Uwe
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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.

Rob
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  • 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. – Dmitry Grigoryev Oct 07 '16 at 14:10
  • @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. – Rob Oct 07 '16 at 14:33
  • 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. –  Oct 07 '16 at 14:41