[Halld-cal] FCAL CW bases

[Fernando J. Barbosa]

Hi Matt,

I have a few concerns about the plan for the CW bases you suggested at 
the GlueX meeting and regarding the HV sparks observed on the present 
design:

1. HV sparks across components - Due to close proximity of components, 
HV sparks or leakage are observed. The present proposed solution is to 
pot with epoxy. This seems to prevent HV sparks.

My concern is that this is temporary solution but not a cure for the 
inherent problem. I really don't want to rely on potting with epoxy 
(long term issues) and a conformal coating should only be used as a 
preventive measure.

2. HV sparks are observed across one HV capacitor - the capacitor is 
rated for operation at 2,500V but it cannot hold 1,500V. The proposed 
solution is to use a low viscosity epoxy to under fill the capacitor and 
prevent sparking.

A capacitor that is sold and rated at 2,500V should withstand that same 
voltage or the manufacturer has a big problem. Most likely the PCB pads 
to which the cap is soldered to are too close or have sharp edges - this 
limits the maximum operating voltage. Alternatively, you could choose a 
larger capacitor with the pads spaced farther apart but this may not be 
necessary.

I strongly suggest that the base be re-designed to space the components 
and pads farther apart to provide for reliable operation by design, 
without a conformal coating. Once that is verified, a conformal coating 
is highly recommended because of moisture and dust.

Best regards,
Fernando
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