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Je
Millivoltmeter
MV 40
Hierbei ist zu berticksichtigen, daB der Serienwiderstand
R,
1
gegentiber dem Blindwiderstand X. =
vernachlassigbar klein sein soll.
AuBerdem nimmt man fir diesen Serienwiderstand einen eng-
tolerierten Wert (z.B. + 1°/o), um die MeBgenauigkeit zu er-
hdhen. Die Frequenz ist so zu wahlen, daB der Serienwider-
stand R; gegeniber dem Blindwiderstand X. vernachlassigbar
klein ist (X_ > 100 - Rs).
Es kann dann gesetzt werden:
U
Uj
Io
pe)
R.
Rs;
U
1
d.h.
=
U,;
2u°f:-C,°R;
Uj;
oder C=
U:-2nf-R,
Note that the series resistor R, should be very much smaller
1
o-cCc
than the reactance
X. =
Also the series resistor should
be a close tolerance
com-
ponent (e.g. + 1°/o) to obtain the required accuracy. Select a
frequency where the series resistor R; is very much smaller
than the reactance X< (X_ > 100 R,).
It follows that:
V
V1
Jew
Fae
Re
R;
V
1
1. e.
=>
_—_
<<. ove
V1
2u°f-Cy:R;
V1
or C, = —-—_——-
V:-2af-R,
Example:
V =
10V, f =
1 kHz, R,; =
100 Q, V; across R,; =
10 mV
102V
10°
Cc, =
—
=
=
1.59- 10°F
10V-10°Hz-22-100Q
2m
C, = 1.59 nF
To approximate the value of the capacity, use an RF nomo-
gram where
the reactance
X. of the capacitor must be at
least 10 times greater than the series resistor R, (X_ > 10° Rs).
It can now be said that:
V
X
=
> Rs
Vi
13
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