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Author Topic: conformal coatings for air variable caps  (Read 3838 times)
KN4GE
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Posts: 13




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« on: March 03, 2013, 07:31:00 PM »

Has any one tried using high dielectric conformal coatings (maybe aerosol) for air variable tuning caps to improve the voltage rating / stop arcing? if so wondering what impact the different dielectric constant has on capacitance and if it can actually help stop arcing.
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K1KP
Member

Posts: 14




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« Reply #1 on: March 04, 2013, 03:21:22 AM »

I used Kapton (tm) tape to go over the burned edges. See my note in QST H&K:

OCT 2012 - QST (PG. 65)
Cure for Amplifier Arcing   
(Hints and Kinks)

Absolutely cured arcing in my LK-500ZB.
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WB6BYU
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Posts: 13020




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« Reply #2 on: March 04, 2013, 11:20:22 AM »

You can slip Teflon sheets between the plates - that will increase the
capacitance by some amount that depends on the relative thickness
of the Teflon and air in the gap.
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KD0REQ
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Posts: 854




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« Reply #3 on: March 05, 2013, 10:55:58 AM »

if you spray something on your capacitor, and it gunks the rotor connection that is usually grounded, there is a reasonable chance something more expensive will go BANG!

I like the idea of slipping in some Teflon sheets
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G3RZP
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Posts: 4366




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« Reply #4 on: March 06, 2013, 01:53:18 AM »

Some 40+ years ago, Jackson Bros here in the UK did that with some of their air variables to special order. You didn't win quite as much as you would think: instead of flashing over from plate to plate, it then became a flash from stator to rotor shaft. But it gained about 50% in working voltage. From memory, they used something like 10 mil thick Teflon.
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KN4GE
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Posts: 13




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« Reply #5 on: March 06, 2013, 05:50:06 PM »

Thanks for the input. The teflon sounds appealing but that is a lot of little pieces.
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WB6BYU
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Posts: 13020




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« Reply #6 on: March 07, 2013, 08:33:47 AM »

Quote from: G3RZP
...From memory, they used something like 10 mil thick Teflon.


On your side of the pond they probably used PTFE instead.
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G3RZP
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Posts: 4366




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« Reply #7 on: March 07, 2013, 10:12:52 AM »

But often helps to translate.....
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AB2YC
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Posts: 53




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« Reply #8 on: April 03, 2013, 08:12:03 AM »

What if you only coated the rotor ?

It would seem you could disassemble the cap coat the rotor, then re-assemble
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KD0REQ
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Posts: 854




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« Reply #9 on: April 03, 2013, 11:42:10 AM »

depends on what it takes to flash over from the stator to the cap base, then.  but if you can plate polyfluorimers onto the rotor without compromising the bearing/friction connections that make the electrical contact to the base, it buy you up to that stator/base flashover point.

part of which is dependent on the same factors that affect rotor/stator insulation... air humidity, dust coating, composition of the dust, etc.

if you're looking to turn a triple-365mmf capacitor into a 6 KV rated unit, won't work.  if you're trying to fix the low-end tuning flashover, it might.
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