Re: cheap way to test "doorknob" capacitors? (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 12 Nov 2004 09:10:43 -0700
From: Gomez Addams <gomez@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: cheap way to test "doorknob" capacitors? (fwd)


On Nov 12, 2004, at 8:38 AM, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Thu, 11 Nov 2004 15:19:10 -0600
> From: Bert Hickman <bert.hickman@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>
> High Voltage list wrote:
>> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>> ---------- Forwarded message ----------
>> Date: Thu, 11 Nov 2004 07:12:17 -0700
>> From: Gomez Addams <gomez@xxxxxxxxxxxx>
>> To: High Voltage list <hvlist@xxxxxxxxxx>
>> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>>
>>
>> On Nov 10, 2004, at 7:14 PM, High Voltage list wrote:
>>
>>
>>> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>>>
>>> ---------- Forwarded message ----------
>>> Date: Wed, 10 Nov 2004 19:31:13 -0600
>>> From: Shaun Epp <scepp@xxxxxxx>
>>> To: High Voltage list <hvlist@xxxxxxxxxx>
>>> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>>>
>>> I read that the capacitance of door knob caps are a function of
>>> voltage.
>>> The capacitance is higher at high voltages.
>>
>>   Also: I have seen standard barium titanate doorknobs used as tank
>> circuit caps in RF induction furnaces, diathermy machines, and in 
>> tuned
>> filter stages of radio transmitters.  How could that be if their
>> capacitance varies with applied voltage?
>>
> <SNIP>
>
> Shaun is partially correct, and so is Gomez! HV ceramic capacitors are
> considerably more complex than they seem... and all doorknob caps are
> not the same.  :^)
>
> There are two major "classes" of ceramic dielectric materials used in
> HV ceramic capacitors.
[snip]
Thanks for the detailed info, Bert!  Before this, I had always thought 
"a doorknob is a doorknob is a doorknob", and while I knew some of 
their limitations and advantages, I never really looked at tempco, etc. 
  I'll be filing this away for future reference.

  - Bill "Gomez" Lemieux,
	Denver, Colorado, USA

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