[Halld-cal] 2500 volt capacitors

[Paul Smith]

Hi Fernando,

I've tried several of these capacitors, AVX #2225WC103KAT1A, .01 uF @ 2500 volts.  I solder wires to the metallization, then connect to a high voltage power supply.

The ones off of a cut tape I got from DigiKey typically spark at ~1500 volts, then stop.  This is independent of humidity from 40% to 55% RH.  If you gradually raise the voltage, they will spark at higher voltages, but eventually will hold 3 kV without sparking.

I tried one from a reel I got from another distributor; it holds 3 kV at 53% RH without any "conditioning".  If I breath on it, it will start sparking, but then stops.

If you look at the metallization on the ends of these caps under a microscope, it isn't anything close to smooth.  The 2nd batch is better than the first, but still not great.

I'm pretty sure that the sparks burn off the "points" of the metallization.

It's hardly practical to "burn-in" each of nearly 10,000 capacitors.  They will work with just the 1B31 acrylic conformal coating (and no epoxy), but I think it needs to wick under the capacitor as well so that the capacitor is surrounded by the coating.  I've been using some coating in a syringe which I squirt along the sides.  Removing a capacitor verifies that the coating does indeed wick underneath.

I don't see how we can rely on these capacitors without some sort of coating surrounding them on all 4 sides.

Paul