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

[<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, so if the C changes by 0.1%, it's
not a big deal.  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,
again the wide tolerance range isn't a problem.


> ...............................................................
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