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Capacitance
Bridge
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circuit of a simple capacitor tester based on bridge principle
is described here. The use of a capacitance bridge is
illustrated with the help of Fig.1. Fixed capacitor C1
and unknown capacitor Cx form the two arms of the bridge.
The other two arms of the bridge are formed by potentiometers
RA and RB. Whenever the bridge is not balanced, an AF
voltage appears across points V1 and V2, and hence a tone
is heard in the ear-piece. By adjusting the potentiometers
RA and RB the potential at points V1 and V2 can be made
equal, in which case no tone is available in the ear-piece.
Under such conditions the bridge is said to be balanced.
The value of the unknown capacitor Cx can be calculated
using the formula: Cx/C1=RA/RB, or Cx = C1 x (RA/RB).
Please note, here RA and RB denote the in-circuit resistances
of respective potmeters. The actual circuit utilising
the above-mentioned principle is shown in Fig.2. A multivibrator
using transistors T1 and T2 forms an AF oscillator circuit
which provides an AC voltage to the bridge. Here the unknown
capacitor is connected across point B and one end of fixed
capacitor using crocodile clips. The points A and B shown
here are the same as those shown in Fig.1. Potentiometer
VR1 gets divided into two variable resistors RA and RB
which form two arms of the bridge. When the supply is
connected to the circuit, a tone is heard in the ear-piece
if the bridge is unbalanced. The arm of potentiometer
VR1 is adjusted in such a way that a null voltage is obtained
across the ear-piece and it stops producing the tone.
For accurate calibration of the circuit the following
procedure should be adopted: 1. Use a fixed capacitor
of, say, 100 pF for the known capacitor (C1) and use capacitors
of known different values such as 10 pF, 22 pF, 33 pF,
47 pF, 82 pF, etc for the unknown capacitor (one at a
time). 2. For each capacitor connected in place of Cx,
vary VR1 to obtain nil tone in the ear-piece and mark
VR1 position in terms of the capacitor used for Cx. Thus
different positions of VR1 will signify different values
of the unknown capacitor. 3. For changing the range use
a capacitor which is, say, ten times the value of fixed
capacitor used in step 1. The values of unknown capacitors
will also increase by ten times for the same position
of VR1. The bridge can measure values up to about 10 nF
in different ranges. |
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