[Halld-cal] FCAL CW bases

[Ashot Gasparian]

  I also share Fernando?s concerns about the HV sparks in the FCAL
  dividers. The epoxy potting is a long term preventive measure for the
  dividers only. We are going to have ~3000 of them in the setup, and
  unless they are very good in the beginning (no sparks before any painting)
  we may end up with a non-effective calorimeter during the operation.

  Ashot


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Ashot Gasparian                    Phone:(336)285-2112 (NC A&T)
Professor of Physics
Physics Department                       (757)-269-7914 JLab
NC A&T State University              Fax:(757)-269-6273 JLab
Greensboro, NC 27411               email: gasparan at jlab.org
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On Fri, 18 Jun 2010, Fernando J. Barbosa wrote:

> 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
>