"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