non-conductive gel

[drHathaway]

I am looking for a non conductive gel or liquid that I can submerge some PCBs loaded with LEDs into?

I am hoping to use a gel for thermal control as well as an optical diffuser.

[Bob Scott]

I am looking for a non conductive gel or liquid that I can submerge some PCBs loaded with LEDs into?

Let me guess.... You are building LED tail lights but they corrode from water leakage and road salt? Use the old trick of filling the tail light sockets with grease. If the grease is already there, water can't get in. Grease does not conduct electricity.

I am hoping to use a gel for thermal control as well as an optical diffuser.

A good solution might be silicon oil or grease. It conducts heat but is an electrical insulator. Silicon oils and greases maintain constant viscosity. ie: Viscosity does not change with temperature. !Silicon oil is not a good lubricant.

I'm thinking of GC Electronics brand Z5 heat sink grease. It is a heavy almost clear grease. It is slightly milky and transluscent. It is also expensive because it is sold in small quantities. Vaseline is cheaper.

[haklesup]

Bob, I think you mean Silicone (a polymer) and not silicon (a solid element) dispite the similar name a very different substance (sorry but the spelling of silicone is a pet peeve of mine)

I would definately research the thermal conductivity of silicone as I never regarded it as being good at conducting heat but right now I am unsure. Silicone can be purchased from a chemical supply company like Baxter and is not a controlled substance but may be considered hazmat for disposal purposes.

Mineral oil is the usual fluid used as a dielectric and coolant but the stuff in the drugstore may be contaminated with perfumes so may be too conductive to be useful. Make sure to get pure fluids. You might also want to learn how to measure the resistivity of your fluid so you can verify its quality. with a rsistivity meter, you can test all manner of liquids. You can get one at a pool supply store (for testing chlorine concentration) or as a water purity meter

Aside from car lamps, such a backfilled assembly would also be useful underwater at considerable depth. A good initial experment might be to fill one of those cheap LED flashlights and try it at the bottom of a pool or bathtub. a plastic cylinder filled with water with a plunger at the top can be used to simulate deep water pressure without leaving the bench. Make sure to test at the voltage and current your lamps will run at.

[Bob Scott]

Bob, I think you mean Silicone (a polymer) and not silicon (a solid element) dispite the similar name a very different substance (sorry but the spelling of silicone is a pet peeve of mine)

Of course. I have pet peeves too, "intermittent" and "preventive" commonly spelled "intermittant" and "preventative.

I would definately research the thermal conductivity of silicone as I never regarded it as being good at conducting heat but right now I am unsure.

I thought that being sold as heat sink grease by GC as being enough research, but they stopped carrying Z5 recently. Sony also used that clear stuff in their early KV1200/KV1201 Trinitron TVs for sinking H Out Xistors.

Mineral oil is the usual fluid used as a dielectric and coolant but the stuff in the drugstore may be contaminated with perfumes so may be too conductive to be useful. Make sure to get pure fluids. You might also want to learn how to measure the resistivity of your fluid so you can verify its quality. with a rsistivity meter, you can test all manner of liquids. You can get one at a pool supply store (for testing chlorine concentration) or as a water purity meter

I've seen submersible sump pump induction motors filled with mineral oil. It works. Baby oil.

Aside from car lamps, such a backfilled assembly would also be useful underwater at considerable depth. A good initial experment might be to fill one of those cheap LED flashlights and try it at the bottom of a pool or bathtub. a plastic cylinder filled with water with a plunger at the top can be used to simulate deep water pressure without leaving the bench. Make sure to test at the voltage and current your lamps will run at.

Good idea!

[sofaspud]

Any chance of expanding on this project a bit, drHathaway? It sounds to me to be more involved than taillights.
If I'm not mistaken, pure water is nonconductive. But perhaps you need more damping ability for a nonstationary
device? More than mineral oil can provide? Why not an epoxy?
Somewhere here at the house I have a big tube of food equipment lubricant. I believe it is called "Thermo-gel." It
maintains its very high viscosity in the presence of heat so as not to contaminate the food. It has a whitish color,
less opaque than Vaseline. I don't know its electrical properties but a mfgr or dstr can surely be found online.

[haklesup]

Indeed DI water is non conductive but increadibly easy to contaminate. Even a small ionic contamination will start the electrolysis process which will spill many metal ions into solution eventually turning into a run away situation. Its best not to use a polar solvent as a dielectric for long.

Silicone is often used as a vehicle to distribute otherwise solid compounds as a colloidal mix, it resists dissolving such ingredients. For exampe Silicone mixed with Zinc Oxide would be an excellent thermal conductor. However, since a substance shows good thermal transmission as a thin film, it dosen't necessarily translate into the same performance as a bulk liquid.

BTW, I am intermittantly guilty

Also, I was in error, Silicone is a synthetic oil not a polymer