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Method
1.
(Figure 3-20.]
Method 2. (Figure 3-2b.)
The maximum current i s limited to that given in
Table
3-2.
The dc supply may be tied t o ground and
the instrument left floating a s shown, where the ca-
pacitance of the bridge t o ground shunts Rn and causes
a
D
(1/Q) error of -wRnC.
If the dc supply has low
internal capacitive coupling to the power line, the
bridge may be grounded and the dc supply left floating.
The blocking capacitor, Cb, must be of high
enough rating t o take a voltage equal to the maximum
direct current in amperes times
la,
the dc resistance
of the transformer secondary.
The source impedance of the dc supply must be
low compared with that of the unknown, since the
bridge measures both of these impedances in series.
A large capacitor (Cd) shunting the dc supply is some-
times useful.
The maximum current in this method is limited
to that given in Table
3-2.
The d c supply is connected
t o the BIAS jack with the signs reversed in order to
keep the bridge c a s e and dc supply both a t zero volts
dc from ground.
The blocking capacitor Cb must be
able t o take the full dc voltage. The ground connection
may be made t o either the panel or the dc supply.
I H I 3
L t
3-Z
MAXIMUM DC THROUGH INDUCTORS
OR RESISTORS
(METHODS 1 AND 2)
Method
3.
(Figure 3-2c.)
This method is recommended for large inductors,
since the maximum current is the same for any range.
In this method both the bridge and the dc supply are
grounded.
The maximum allowable current for any range is
40
mA. The output impedance of the dc supply should
be high enough to avoid loading the detector (a series
resistor is often useful) and should have low hum.
The blocking capacitor C, must be able to take
the dc IR drop across the unknown inductor, and Cb
must be able t o take the whole dc voltage.
Range
Multiplier
Method
4.
(Figure 3-2d.)
Maximum
Current
100 mA
100 mA
71 mA
22
mA
7.1 mA
2.2 mA
0.5 mA
L
1 0 0 p H
1 m H
10rnH
100rnH
1 H
10 H
100H
This method must be used with very large dc.
The maximum voltage on the unknown i s limited only
by the rating of Cf. The a c source impedance of the
dc supply must be much higher than the impedance of
the unknown since the bridge measures the parallel
combination of these two impedances.
A
large induc-
tor,
La,
may be connected a s shown t o provide a high
source impedance. Often it i s possible to resonate the
feed inductor t o increase the source impedance further.
Ra (Ratio
Arm)
l a
I 0
0
100R
I k R
10 k R
100 k R
1
MR
R
100mR
1R
ion
100R
1 kR
10 k R
100 k R
+DET
DET
Figure
3 -2.
Methods of applying dc to inductors. (Blocking capacitor CB i s not supplied with the bridge.)
S P E C I A L M E A S U R E M E N T S
3-3
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