Re: Calculatating RC parallel and series impedance networks.

In article <jks_j.6881$_03.2254@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Iain Churches" <IainNG@xxxxxxxxxxx> wrote:

"John Byrns" <byrnsj@xxxxxxxxxxxxx> wrote in message
news:byrnsj-E8A548.16090425052008@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
In article <fde_j.6628$_03.220@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Iain Churches" <IainNG@xxxxxxxxxxx> wrote:

"John Byrns" <byrnsj@xxxxxxxxxxxxx> wrote in message
news:byrnsj-69CE13.07511523052008@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
In article <VogZj.5023$_03.2171@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Iain Churches" <IainNG@xxxxxxxxxxx> wrote:

http://www.kolumbus.fi/iain.churches/ThermionicThoughts/Impedanc
eCa

Either my browser is retrieving an old version of your page, or you
aren't following what I am saying. What I am objecting to is the
mention of a series RC as an example when you are discussing the
parallel RC. In the parallel RC discussion you say "A common example
seeen often in schematics is the series RC combination in parallel with
the anode (plate) resistor on the first stage." This is not an example
of a simple parallel RC network, it is an example of a series RC network
in parallel with the plate resistor, a more complex combination that you
haven't yet addressed.

This is now changed

I thought you would see where I was going without my being too explicit,
the point is that "Theta = arctan (Xc/R) =-90+DEGREES(ATAN(Xc/R))" is
incorrect as it stands.

The second " =" should have been "Or for the parallel case"
This too is changed.

The write-up would have a more consistent look if you dropped the
"DEGREES" from the formula for the parallel case. Also Xc must be
negative for the series case formula "Theta = arctan (Xc/R)" to give the
correct result for Theta, this is inconsistent with the way you use Xc
in the formula for the parallel case where you assume Xc to be a
positive value. I'm not smart enough to know if Xc should be a positive
or negative value, I would take it as negative for a capacitor, however
in any case I would think the usage should be consistent in the two
formulas.

Also what do you intend the "capacitive reactance Xc" to represent in
the parallel case? I would expect Xc to be the capacitive reactance to
represent the equivalent series reactance part of the parallel
combination, but that isn't what you calculate. Your intention here is
both unconventional, and quite obtuse.

What do you mean by "the equivalent series reactance part
of the parallel combination"?

What I mean is that impedance is commonly expressed as the sum of an
imaginary reactance value and a real resistance value, in the case of
the series RC network the reactance is simply the reactance of the
capacitor, however in the case of the parallel RC network the equivalent
or apparent reactance is also a function of the resistance, it is not
simply the reactance of the capacitor as in the series case.

Xc in both cases is simply the reactance of the cap at the frequency
Hence the values E10 and M10 are the same. This is true right down
to E46 and M46.

Since the two columns are redundant, and since the column under the
parallel network is misleading, especially for the more advanced
student, I would delete the duplicate column and retain only one copy,
perhaps placed between the columns for the series and parallel cases.

Actually if I were doing this spread *** I would leave the two Xc
columns in place and add two R columns. The R column for the series
case would simply reflect the entered value of R for all frequencies.
The X and R values for the parallel case would reflect the the imaginary
and real components of the impedance which I called the "equivalent"
reactance and resistance in my previous post. Presenting it in this way
would help provide more insight into what is actually going on with
these two networks.

Further your column headings for the two "dB" columns are misleading and
wrong in some of their implications, I would change the column headings
to more correctly reflect the quantity you are actually calculating.

No-one I have asked can suggest a better heading. Do you have a
suggestion?

I would suggest that "dB change in V for constant I wrt F1" would be a
better and more accurate heading. The "V or I" part in your heading
seems both incomplete and inaccurate to me.


Regards,

John Byrns

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