Re: Measuring self-capacitance directly (Re: flat secondary)

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Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Metlicka Marc by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>

> I use aluminum "gutter guard" which is an expanded aluminum screen ,
> that i cut three lengths of that equal the height of the coil. I place
> these in a star like pattern or three legged cross on the carpet bellow.
> I try to put each coil 3" above because this is the usual height that i
> start my windings, if less then i use spacers.

Ok for a solenoidal coil, but a flat secondary coil would be just 3" 
above the ground?

>   I worried about doing this with the 20" flat coil for just the reason
> you mentioned, If a bigger plate is used i might get higher c readings?
> Oddly enough I only get higher c readings if i raise the coil, size of
> the grid makes very little difference (unless i make it very small).

If the ground below the coil is partially conductive, it works as
the other plate too, and so the size of the metal ground plane is
of little importance (for the measurement).

>  I unhook the dedicated earth ground for selfc readings, my thinking was
> that i only wanted to measure the c of the effective plate of the
> windings, I also thought that even if the grid plate was huge i should
> get accurate readings because the effective c of a two plate capacitor
> is only as much as the area of the smaller plate?

Only if everything is totally insulated, and far, from a partially 
conductive ground, and the distance is much smaller than the size
of the smaller "plate".

>  The twisted pair leads have one wire as the test lead and the other is
> connected to the bridge chassis ground, unconnected at the other end.
> This idea came from reading about some leads used on a fancy l-c-r meter
> and as Antonio mentioned, it may not be optimum, i will try with a set
> of single wire leads tonight. 

You can just discount the capacitance read with only the twisted
wires. 

> I do notice that while position of single
> wire leads does effect readings, the twisted pair doesn't seem to?

The parasitic capacitance that dominates in this case is from one
wire to the other of the pair, and this is insensitive to the
position of the wires.

> I don't think i would take these readings to heart until some others can
> confirm, They are simply something i wished to try, mostly for toploadC
> readings?

Topload capacitances can be measured in this way.

Antonio Carlos M. de Queiroz