[ARC5] Diode effect in Caps.

[J. Forster]

Hi Sean,

I'm certain your analysis is correct. In the case of the horizontal sweep, the
rate is too fast for the capacitor to deform and change capacitance. Not so for
the vertical.
'-John

Sean Barton wrote:

> John,
>
> This appears to be the basis of a problem that, as George may allude to, has
> been noted in late 1940s television sets employing electrostatic deflection.
>   These sets originally use ~0.005uF 6000V paper capacitors to couple the
> waveform from the horizontal and vertical output stages to the respective
> deflection plates in the CRT.  In restoration of these sets, many people
> have replaced these capacitors with ceramic capacitors due to low cost and
> ease of obtaining them.  It has been found that while ceramic capacitors
> seem to work fine in the horizontal circuit (15.75kHz), these units
> introduce non-linearity in the vertical sweep (60Hz).  It was thought that
> some sort of piezoelectric properties of the ceramic dielectric were causing
> the distortion.  It has been recommended to replace all of the capacitors
> with appropriately valued film capacitors which work well.  Perhaps the
> frequencies of the two sweep axis determines how much distortion is
> introduced by the ceramic capacitors in each axis and how noticeable it will
> be in the image on the screen of the CRT.
>
> Sean
>
> >From: "J. Forster" <jfor at quik.com>
> >Reply-To: jfor at quik.com
> >To: ARC-5 <arc5 at mailman.qth.net>,"Wireless-Set-No19 @
> yahoogroups.com" <Wireless-Set-No19 at yahoogroups.com>,Milsurplus
> <milsurplus at mailman.qth.net>
> >Subject: [ARC5] Diode effect in Caps.
> >Date: Wed, 06 Oct 2004 22:10:15 -0400
> >
> >The effect George describes is called 'dielectric relaxation' and is a
> result of
> >something akin to a piezoelectric effect.
> >
> >If you consider a capacitor as a pair of plates on each side of a
> dielectric
> >slab, there is a force between the plates when a voltage is applied that
> >squishes the plates together. Just like when you squish a piece of
> cheese, when
> >the force is relaxed the dielectric slab does not fully return to its
> original
> >dimensions right away, but will recover over some period of time,
> perhaps hours,
> >perhaps days. This relaxation causes a slight redistribution of charge
> in the
> >dielectric block, yielding a terminal voltage. High voltage capacitors,
> such as
> >those from 'Plastic Capacitor Corp.' can develop quite a charge due to
> >dielectric relaxation effects. Note that this is a linear effect...
> reverse the
> >polarity of the HV and the 'relaxed' voltage will reverse also.
> >
> >A similar effect can occur in ceramic capacitors. Their incremental
> capacitance
> >can increase markedly when a DC bias is applied. Some years ago, I built
> a
> >triangle wave form generator which produced a slightly curved wave form.
> It
> >turned out to be caused by the voltage coefficient of capacitance.
> Moral: Use
> >high quality caps when making integrators. Again, this is linear...
> that is the
> >capacitance v. applied voltage curve is symmetrical about zero applied
> bias.
> >
> >
> >The effect Keith originally described is non-linear..  the leakage is in
> only
> >high for one applied bias polarity.
> >
> >-John
> >
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