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I'm making a decade box and I need 100 Ω resistors for the hundreds increments range. I'm quite picky; I want the resistors to have 0.05% tolerance. I've checked online and 100 Ω resistors with that tolerance are pricey.

So I've decided to roll my own. I can take resistance wire, measure precisely (assuming I already have a very accurate ohmmeter), and cut my own resistors from them.

Now, the wires may be very thin and get very hot. Eventually they could degrade or burn up to nothing in open air, so I need to pot them to prevent their exposure to air.

What's the ceramic or cement that you typically see used to pot high-power resistors? I would like the sinter-able type, so I know it'll be permanent, airtight, and waterproof.

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  • \$\begingroup\$ You can buy the stuff off-the-shelf, like Aremco Ceramacast 586 (no affiliation), and others brands. Not cheap, though. Google around. \$\endgroup\$
    – ocrdu
    Commented Mar 5, 2023 at 7:58
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    \$\begingroup\$ 500 ppm of 100 ohm is 50 mohm. Will you consider the reproducibility, aging, wear, tempco of the switches ? otherwise going to such extremes seems like wasted effort. \$\endgroup\$
    – tobalt
    Commented Mar 5, 2023 at 8:24
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    \$\begingroup\$ My firm doubt is that you'll never make a home made resistor that matches the temperature coefficient of a purchased item from a reputable dealer. \$\endgroup\$
    – Andy aka
    Commented Mar 5, 2023 at 11:23

2 Answers 2

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If they get "very hot" they're likely not going to be precise for very long.

My precision Leeds & Northrup etc. resistor boxes use a wax coating which keeps the air away and does not add mechanical stress to the construction.

You could try a vitreous enamel coating (as used for cloisonné) for high temperatures but I'm not all that confident on the outcome.

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You can buy specialised potting compounds for (power) resistors off-the-shelf, like Aremco Ceramacast 586 or Epic Resins Epic 0154 (for both: no affiliation). Not cheap, though. There are several other brands and types of the stuff; Google around.

Some will require a mould and be poured, others can be applied with a brush.

Note that some will cure properly at room temperature over about 24 hours, but others need to be heated once to 200-300°C, which may take your resistance wire out-of-spec permanently.

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