Hand machined mechanical pencils

Film capacitor dissection (12/05/12)

When I was messing around with induction heating, I looked around for a reasonably cheap source of capacitors suitable for that application. These typically are a few µF and handle at least several hundred amps continuously. The typical sort of capacitors used are conduction-cooled and have heavy copper plates as electrodes which can be attached to water-cooled heatsinks (for example, check out Celem www.celem.com). I eventually bought a proper Celem capacitor, and it worked great, but I wondered if there might be a way of more effectively cooling a "normal" film capacitor, since the Celem caps still use a polypropylene film roll, just with better cooling.

Film capacitors are made by rolling up several long strips of polypropylene film (metallised) and/or aluminium foil. Once rolled, the ends of the bundle are sprayed with metal (usually tin or a solder alloy) to form a contact, lead wires are attached and the whole lot potted in a plastic casing. I reckoned if the metal spray could be exposed then it could be soldered directly to a larger metal plate with low-melting solder, thus providing a better cooling path.

Here are some photos of what I experimented with. The first six are just odd capacitors to see what they're like inside. The rest are of a 1µF 630VDC Epcos B32654 capacitor. I removed the plastic and potting just by carefully working it away with a Dremel burr and some sandpaper. There is a fair amount of metal spray present (around 0.5mm thick) so it's reasonably robust. You can see the remnants of the copper lead wire embedded in the metal spray. Once I had got it down to a single "brick" I then tried soldering one end to a copper plate with some Wood's alloy (melts below 100°C) and normal liquid flux (by heating the copper plate with a small gas torch). This worked quite well and the capacitor looks to be securely attached.

I never pursued this further, but I would think that it would be possible to make up something like a Celem capacitor with heavy metal end plates and the roll from a normal capacitor in the center.