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

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 12 Nov 2004 09:06:43 -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 8:33 AM, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Thu, 11 Nov 2004 08:03:50 -0800
> From: Jim Lux <jimlux@xxxxxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>
>> ---------- 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.
>
> I don't know about the slope, but a voltage dependent capacitance 
> effect is
> certainly possible.
>
>>
>>   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?
>
> The first two are non-frequency critical,

Provided you stay within the frequency range permitted by the FCC for 
such machinery.

>  so if the C changes by 0.1%, it's not a big deal.

I guess that depends on the L.

>   After all, the tolerance on these caps is typically 20%.
> The last one uses these caps as RF bypass, not as part of a tuned 
> circuit,

Transmitter output filters are very much tuned circuits, I am not 
talking about
power supplies, I'm talking about low pass and bandpass circuits.

> again the wide tolerance range isn't a problem.

Depends on how clean you want your signal to be, I suppose.