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Re: Method of measuring self C of secondary
Hi Atonio,
I agree with your circuit analysis assuming a three element equivalent
circuit.
Thanks for the explanation. I have now read Medhurst's paper (the one
that Terry gave a link to) and I agree with your definition of Medhurst C.
Medhurst makes the distinction between the one end grounded self C of a
coil and the ungrounded self C by calling the one end grounded C
apparent self C. However he does not use this terminology in the rest of
his paper. My vote is for this term as it will help avoid confusion with
the
real self C and better explains why the lumped equation is valid.
Incidentally he referees to the self C as being external and the turn to
turn as internal.
Medhurst apparently gives no indication of where the ground plain was. I
would assume this means that compared to the dimensions of the coil he
considers it at infinity. So presumable his "apparent" self
C is for a an isolated, one end grounded coil (including C to leads which
will be small).
I believe he assumes the equivalent circuit is valid when resonating with a
comparatively large external C (<1/10Fr). I would expect this to be a valid
equivalent circuit for such conditions. I did not find any justification
for such an
equivalent circuit when the external C has a similar value to the apparent
C. But I will read the paper again.
I also note that as one end is grounded apparent C includes the turn to turn
C.
I wonder if there is a fixed relationship between the two values i.e. 2
(assuming validity for reasonable frequencies and an isolated coil say
hanging from the high ceiling).
Regards bob
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: 29 April 2000 21:53
Subject: Re: Method of measuring self C of secondary
>Original Poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.b with
the poits your>
>
>Tesla List wrote:
>
>> Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
>
>> The self C (true) or better the equation is required for simulation and
>> maths models.
>>
>> The use of a voltage weighted C in a lumped resonance calculation or any
>> calculation is totally new to me. As is the concept of a current weighted
L.
>
>You may be finding strange that the self-capacitance is not the
>capacitance seen from the base of the coil at low frequency (C(0)), as
>should happen if this model is valid:
>
> o-----L-----+
> |
> base -> Cself
> |
> o-----------+
>
>The inconsistency may be solved by adding another capacitor:
>
> o-------+------L-----+
> | |
> base -> Cbase Cself
> | |
> o-------+------------+
>
>Where Cbase = C(0)-Cself
>
>In normal operation the base of the coil is grounded, and Cbase is
>short-circuited. This is the condition assumed by Medhurst. It also
>doesn't affect the frequency the impedance zeros (just one in this
>model) seen from the base. It must be considered, however, in a
>magnifier configuration, where the base of the coil is not grounded.
>Note that Cbase causes an impedance pole (a maximum) at the frequency
>where L resonates with the series combination of Cself and Cbase:
>
>fpole=1/(2*pi*sqrt(L*Cbase*Cself/(Cbase+Cself))) Hz
>
>This can be observed if you measure the impedance seen from the
>base of the coil.
>
>Antonio Carlos M. de Queiroz
>
>
>