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

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Greenfield by way of Terry Fritz <twftesla-at-qwest-dot-net>" <alienrelics-at-yahoo-dot-com>

I must admit I'm confused.

This will measure capacitance to ground, but what
about turn-to-turn capacitance? Or is this just meant
as sort of a rule-of-thumb sort of measurement? Isn't
it likely that a flat coil will have a very different
self-capacitance than a solenoid coil?

I wouldn't expect connecting your cap meter across the
leads of a coil to work, regardless of AC or DC output
of the meter.

Steve Greenfield

--- Tesla list <tesla-at-pupman-dot-com> wrote:
> 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>
> 
> > Tonight i will sweep for fo, double check selfC
> with my general radio
> > capacitance bridge and do some playing around,
> also try to get Paula to
> > count turns for accuracy (after counting the 3k
> coils turns twice for
> > me, this may be difficult?), I will post results.
> 
> How are you measuring the self-capacitance?
> 
> I was verifying if it's possible to measure directly
> something 
> related to the self-capacitance of a coil. I used a
> secondary
> coil that I know that has a self-capacitance of 5.6
> pF (obtained
> by measuring the main resonance frequency of the
> coil, and
> consistent with Medhurst's formula).
> 
> First I verified if my capacitance meter can measure
> capacitances
> accurately with a high resistance in series with the
> capacitor.
> I simply picked a 180 pF capacitor (a Leyden jar),
> connected one 
> terminal to the meter, and held with one hand one
> terminal of the 
> meter and with the other hand the free terminal of
> the capacitor.
> I obtained correct measurements when measuring far
> from sources
> of interference, as computers or TVs.
> 
> Then I put the coil over an insulating support with
> height similar
> to the height of the coil above the wooden floor,
> connected one
> terminal of the meter to one terminal of the coil,
> and held the
> other meter terminal in my hand, while sitting on
> the floor (so
> I was "grounded"). 
> 
> Before the measurement, I connected the meter
> terminal that would go 
> to the coil to an insulated area of the coil form,
> and set the meter 
> to zero with the wires at that position.
> 
> With the connection to the coil made, the meter read
> 10 pF, with the
> coil in any position. At half the distance from the
> ground, the meter
> reads 11 pF. The readings are stable and consistent.
> If I approach
> my free hand to the coil, the capacitance increases,
> as expected.
> The same if I mount the coil closer to the ground.
> 
> *****
> Conclusion: It is really possible to measure the
> self-capacitance
> of a coil with a direct measurement of the body
> capacitance of
> the insulated coil. The body capacitance is about
> twice the "self-
> capacitance" of the coil, as it should be (the
> correct lumped model
> for a floating coil has two "self-capacitances",
> going to ground, 
> one at each side of the coil).
> *****
> 
> This was discussed in the list some time ago, but I
> had not verified
> yet.
> 
> By the way, a direct reading across the coil leads
> to overrange in
> all scales in my meter (expected, as these meters
> typically try to 
> charge the capacitor with a DC current and measure
> the time taken to 
> reach a certain voltage).
> 
> Also, to use the power line ground as "ground" in
> the measurement
> is useless. Too much interference, and the meter
> reads noise.
> 
> Antonio Carlos M. de Queiroz
> 
> 
> 
> 


__________________________________________________
Do You Yahoo!?
Yahoo! Sports - Coverage of the 2002 Olympic Games
http://sports.yahoo-dot-com