A few days ago I made a USB charger for my bike.

Today I made another using the same model charger as the base of my charger. This was a bit different.

While last time I just potted the where the leads attached to the main PCB, this time I potted the whole thing in the kneadable epoxy.

Why? It’s even more stable than the silicon tape I used to hold things together.

Aren’t I afraid that the unit will short out or overheat?

No. Because the specs say so.

I looked up the technical data sheet for the epoxy I was using from Henkel and got the low-down on it.

The resistivity of the epoxy is measured in Tera-Ohms. TΩ. This means it’s effectively not conductive. The oils from your skin lead to larger leakage currents.

The thermal conductivity is also pretty good. At 1.016 W/(m•K) it’s far better than that of free air. Not only that but the charger was designed to sit in the cigarette lighter socket and it’s fully enclosed in plastic itself. By potting it in the epoxy I’m doing it a favor!

Otherwise, the procedure I followed last time is the same here. Strip off the outer case. Solder the pairs of positive and negative terminals together (not to each other!). Solder on the leads that go to the SAE connector. Make a light pipe.

The only thing I had to do was use some card stock as a shield to prevent the epoxy from interfering with the grounding fingers of the USB socket.

Now I have, essentially, a rock that is a USB charger.  :-)