That might happen using a capacitance meter than does
not measure the series and parallel resistance. The GR
bridge does, which is not to say that you might not be
right.

Dick,

what I meant to say is the following:

At 1kHz, a 500 pF capacitor with a 500,000 ohm resistance in parallel is
equivalent to the series of of 702 pF capacitor and 144,200 ohm resistior
(using the well known parallel-to-seriel translation formulas).

So, if your meter has the ability to separately measure the series resistance
and capacitance, it should correctly indicate 702 pF, i.e. a value higher than
that marked on the capacitor.

73

Tony I0JX

Re my previous message, I have downloaded a GR bridge manual from BAMA. It
reports that:

- the bridge measures the series capacitance
- if D is low, the series capacitance almost coincides with the parallel
capacitance
- but if D is high, they differ significantly. A chart is provided to convert
series capacitance into parallel capacitance.

This confirms that, if D is high and if the loss is caused by a parallel
resistance (as it actually is), one must convert the measured capacitance value
using the chart.

73

Tony I0JX

"Antonio Vernucci" wrote in message
. ..
Re my previous message, I have downloaded a GR bridge
manual from BAMA. It reports that:

- the bridge measures the series capacitance
- if D is low, the series capacitance almost coincides
with the parallel capacitance
- but if D is high, they differ significantly. A chart is
provided to convert series capacitance into parallel
capacitance.

This confirms that, if D is high and if the loss is caused
by a parallel resistance (as it actually is), one must
convert the measured capacitance value using the chart.

73

Tony I0JX

I think you are looking at a manual for a later model
bridge. My 650A manual has the formulas but not charts.
I remeasured a bad cap and calculated the parallel
capacitance, series resistance, and parallel resistance.
This is a paper cap rated at 0.02 uf. The values I got a
Cs = 4.8 uf
D = 0.3
Cp = 4.3 uf
Rs = 994 ohms
Rp = 12 kohms

Not a very good cap.

New plastic film caps measure very close to the marked value
and have a D which is below the residual of the bridge
(essentially zero)

While there is an error from the rather high D it is not
significant in terms of this measurement, that is, the value
of the cap measures nearly three times its marked value. I
have not dissected one of these but suspect the winding is
distorted. That would also affect the voltage rating. What I
mean is that the plates of the capacitor are closer together
than originally, probably because of loss of the wax
impregnant. I found other caps in this RX which had high
values so this one is not unique.

I have not measured the caps at RF but I seems like an
interesting project and a practical use for my Boonton
Q-Meter:-)

BTW, I think my math is OK but maybe not.


--

--
Richard Knoppow
Los Angeles
WB6KBL

I think you are looking at a manual for a later model bridge. My 650A
manual has the formulas but not charts.

Yes. However, in the 650A manual there is a similar statement. At page 3, item
9, they say that the bridge measures the series capacitance, and they also give
the formula for calculating the parallel capacitance (that is what we need, as
the leaky capacitors have a resistence in parallel).

I remeasured a bad cap and calculated the parallel capacitance, series
resistance, and parallel resistance. This is a paper cap rated at 0.02 uf. The
values I got a
Cs = 4.8 uf
D = 0.3
Cp = 4.3 uf
Rs = 994 ohms
Rp = 12 kohms

Not a very good cap.

Your Cp/Cs ratio corresponds to that calculated using the formula at page 3.
However the other figures do not tie up with what my spreadsheet gives at 1kHz,
that is:

- for measured Cs= 4.8uF and D=0.3 (that is a reactance / resistance ratio =
3.33), then Rs should be about 10 ohm, rather than 994 ohm

Moreover:
- the series of 4.8uF and 994 ohm would corresponds to Cp= 5,335 pF and Rp= 995
ohm
- the parallel of 4.3uF of 12 kohm would corresponds to Cs= 4.3uF and Rs= 0.1
ohm
I get values close enough to yours only if I set a frequency close to 10 Hz, not
1 kHz (unless I did something wrong).

Anyway, you may measure the parallel resistance of your capacitor with an
ohmeter, and check that you really read a value as low a 12 kohm.

New plastic film caps measure very close to the marked value and have a D
which is below the residual of the bridge (essentially zero)

While there is an error from the rather high D it is not significant in
terms of this measurement, that is, the value of the cap measures nearly three
times its marked value.

why just three times? I would say that the ratio between 4.3uF and 0.02uF is
more than 200

I have not dissected one of these but suspect the winding is
distorted. That would also affect the voltage rating. What I mean is that the
plates of the capacitor are closer together than originally, probably because
of loss of the wax impregnant. I found other caps in this RX which had high
values so this one is not unique.

I have not measured the caps at RF but I seems like an interesting project and
a practical use for my Boonton Q-Meter:-)

BTW, I think my math is OK but maybe not.

73

Tony I0JX - Rome, Italy

"Antonio Vernucci" wrote in message
. ..
I think you are looking at a manual for a later model
bridge. My 650A manual has the formulas but not charts.

Yes. However, in the 650A manual there is a similar
statement. At page 3, item 9, they say that the bridge
measures the series capacitance, and they also give the
formula for calculating the parallel capacitance (that is
what we need, as the leaky capacitors have a resistence in
parallel).

I remeasured a bad cap and calculated the parallel
capacitance, series resistance, and parallel resistance.
This is a paper cap rated at 0.02 uf. The values I got
a
Cs = 4.8 uf
D = 0.3
Cp = 4.3 uf
Rs = 994 ohms
Rp = 12 kohms

Not a very good cap.

Your Cp/Cs ratio corresponds to that calculated using the
formula at page 3. However the other figures do not tie up
with what my spreadsheet gives at 1kHz, that is:

- for measured Cs= 4.8uF and D=0.3 (that is a reactance /
resistance ratio = 3.33), then Rs should be about 10 ohm,
rather than 994 ohm

Moreover:
- the series of 4.8uF and 994 ohm would corresponds to
Cp= 5,335 pF and Rp= 995 ohm
- the parallel of 4.3uF of 12 kohm would corresponds to
Cs= 4.3uF and Rs= 0.1 ohm
I get values close enough to yours only if I set a
frequency close to 10 Hz, not 1 kHz (unless I did
something wrong).

Anyway, you may measure the parallel resistance of your
capacitor with an ohmeter, and check that you really read
a value as low a 12 kohm.

New plastic film caps measure very close to the marked
value and have a D which is below the residual of the
bridge (essentially zero)

While there is an error from the rather high D it is
not significant in terms of this measurement, that is,
the value of the cap measures nearly three times its
marked value.

why just three times? I would say that the ratio between
4.3uF and 0.02uF is more than 200

I have not dissected one of these but suspect the
winding is
distorted. That would also affect the voltage rating.
What I mean is that the plates of the capacitor are
closer together than originally, probably because of loss
of the wax impregnant. I found other caps in this RX
which had high values so this one is not unique.

I have not measured the caps at RF but I seems like an
interesting project and a practical use for my Boonton
Q-Meter:-)

BTW, I think my math is OK but maybe not.

73

Tony I0JX - Rome, Italy
I will recalculate, I may have misplaced a decimal
point.




--
Richard Knoppow
Los Angeles
WB6KBL

"Antonio Vernucci" wrote in message
. ..
I think you are looking at a manual for a later model
bridge. My 650A manual has the formulas but not charts.

Yes. However, in the 650A manual there is a similar
statement. At page 3, item 9, they say that the bridge
measures the series capacitance, and they also give the
formula for calculating the parallel capacitance (that is
what we need, as the leaky capacitors have a resistence in
parallel).

I remeasured a bad cap and calculated the parallel
capacitance, series resistance, and parallel resistance.
This is a paper cap rated at 0.02 uf. The values I got
a
Cs = 4.8 uf
D = 0.3
Cp = 4.3 uf
Rs = 994 ohms
Rp = 12 kohms

Not a very good cap.

Your Cp/Cs ratio corresponds to that calculated using the
formula at page 3. However the other figures do not tie up
with what my spreadsheet gives at 1kHz, that is:

- for measured Cs= 4.8uF and D=0.3 (that is a reactance /
resistance ratio = 3.33), then Rs should be about 10 ohm,
rather than 994 ohm

Moreover:
- the series of 4.8uF and 994 ohm would corresponds to
Cp= 5,335 pF and Rp= 995 ohm
- the parallel of 4.3uF of 12 kohm would corresponds to
Cs= 4.3uF and Rs= 0.1 ohm
I get values close enough to yours only if I set a
frequency close to 10 Hz, not 1 kHz (unless I did
something wrong).

Anyway, you may measure the parallel resistance of your
capacitor with an ohmeter, and check that you really read
a value as low a 12 kohm.

New plastic film caps measure very close to the marked
value and have a D which is below the residual of the
bridge (essentially zero)

While there is an error from the rather high D it is
not significant in terms of this measurement, that is,
the value of the cap measures nearly three times its
marked value.

why just three times? I would say that the ratio between
4.3uF and 0.02uF is more than 200

I have not dissected one of these but suspect the
winding is
distorted. That would also affect the voltage rating.
What I mean is that the plates of the capacitor are
closer together than originally, probably because of loss
of the wax impregnant. I found other caps in this RX
which had high values so this one is not unique.

I have not measured the caps at RF but I seems like an
interesting project and a practical use for my Boonton
Q-Meter:-)

BTW, I think my math is OK but maybe not.

73

Tony I0JX - Rome, Italy
Turns out to be a couple of misplaced decimal points.
First of all I mis-typed, the measured value is 0.048uf, not
4.8uf. Recalculating I get:

C parallel = 0.044 uf
R series = 99.5 ohms
R parallel = 1205 ohms


--
Richard Knoppow
Los Angeles
WB6KBL

"Richard Knoppow" wrote in message
m...

"Antonio Vernucci" wrote in message
. ..
I think you are looking at a manual for a later
model bridge. My 650A manual has the formulas but not
charts.

Yes. However, in the 650A manual there is a similar
statement. At page 3, item 9, they say that the bridge
measures the series capacitance, and they also give the
formula for calculating the parallel capacitance (that is
what we need, as the leaky capacitors have a resistence
in parallel).

I remeasured a bad cap and calculated the parallel
capacitance, series resistance, and parallel resistance.
This is a paper cap rated at 0.02 uf. The values I got
a
Cs = 4.8 uf
D = 0.3
Cp = 4.3 uf
Rs = 994 ohms
Rp = 12 kohms

Not a very good cap.

Your Cp/Cs ratio corresponds to that calculated using the
formula at page 3. However the other figures do not tie
up with what my spreadsheet gives at 1kHz, that is:

- for measured Cs= 4.8uF and D=0.3 (that is a reactance /
resistance ratio = 3.33), then Rs should be about 10 ohm,
rather than 994 ohm

Moreover:
- the series of 4.8uF and 994 ohm would corresponds to
Cp= 5,335 pF and Rp= 995 ohm
- the parallel of 4.3uF of 12 kohm would corresponds to
Cs= 4.3uF and Rs= 0.1 ohm
I get values close enough to yours only if I set a
frequency close to 10 Hz, not 1 kHz (unless I did
something wrong).

Anyway, you may measure the parallel resistance of your
capacitor with an ohmeter, and check that you really read
a value as low a 12 kohm.

New plastic film caps measure very close to the marked
value and have a D which is below the residual of the
bridge (essentially zero)

While there is an error from the rather high D it is
not significant in terms of this measurement, that is,
the value of the cap measures nearly three times its
marked value.

why just three times? I would say that the ratio between
4.3uF and 0.02uF is more than 200

I have not dissected one of these but suspect the
winding is
distorted. That would also affect the voltage rating.
What I mean is that the plates of the capacitor are
closer together than originally, probably because of
loss of the wax impregnant. I found other caps in this
RX which had high values so this one is not unique.

I have not measured the caps at RF but I seems like an
interesting project and a practical use for my Boonton
Q-Meter:-)

BTW, I think my math is OK but maybe not.

73

Tony I0JX - Rome, Italy
Turns out to be a couple of misplaced decimal points.
First of all I mis-typed, the measured value is 0.048uf,
not 4.8uf. Recalculating I get:

C parallel = 0.044 uf
R series = 99.5 ohms
R parallel = 1205 ohms


--
Richard Knoppow
Los Angeles
WB6KBL

Oh, Yikes! I did it again. The correct measured value
of the capacitor is 0.048 uf, D = 0.3

I calculate:

C parallel = 0.044 uf
R (AC) series = 995 ohms
R (AC) parallel = 12050 ohms
Xc, at 1000 hz = 3315 ohms

Someone please check this.

Formulas a

Cp = Cs / 1+D^2

Rs = D/wC where w = 2*pi*f

Rp = (1+D^2)/D^2)*Rs

All measurements and calculations for f = 1000 hz

--
Richard Knoppow
Los Angeles
WB6KBL

Turns out to be a couple of misplaced decimal points. First of all I
mis-typed, the measured value is 0.048uf, not 4.8uf. Recalculating I get:

As a matter of fact a value of 4.8uF seemed real odd to me.

Oh, Yikes! I did it again. The correct measured value of the capacitor is
0.048 uf, D = 0.3

I calculate:

C parallel = 0.044 uf
R (AC) series = 995 ohms
R (AC) parallel = 12050 ohms
Xc, at 1000 hz = 3315 ohms

Someone please check this.

Your calculations seem correct to me (assuming that by Xc you mean the reactance
of Cs and not that of Cp, which is 3,617 ohm).

At this point, one would still have to explain how a capacitor marked 0,02 uF
can grow up to 0,044 uF, that is more than twice its value.

Before formulating hypotheses (e.g. that the plates of the capacitor are closer
together than originally because of loss of the wax impregnan) I would rather
try to reconfirm the measurement results.

Measuring the resistance of the capacitor by means of a plain digital ammeter,
do you obtain a value close enough to 12 kohm? Repeating the measurement on a
different scale, do you obtain similar results? My experience with lossy
capacitors is that the apparent Rp varies quite a lot with the scale.

Also it would be useful to repeat the test with the GR set at a diffierent
frequency (should this be possible).

73

Tony I0JX