[ARC5] Diode effect in Caps.

[Sean Barton]

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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