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ance of the dc source should be high (above 10 kO) to
avoid shunting the detector. The dc source should have
low hum on its output because it is tied to the detector
input.
External filtering on the dc source may be re-
quired, but is relatively easy to obtain when the required
current is small.
WARNING
Note that the LOW UNKNOWN terminal has high
voltage applied to it in this method of biasing
capacitors.
TABLE 3-1
MAXIMUM VOL T AGE APPLI ED TO
CAPACITORS BY METHOD 3*
RANGE
MAX VOL TS
MAX DC
ON UNKNOWN
CURRENT
1100 pf
393v
0.39ma
11 nf
nov
2.2ma
110nf
71v
7.1ma
1100 nf
22v
22ma
11 flJ
3v
30ma
110,uf
0.3v
30ma
1100,uf
0.03 v
30ma
*Methods 1 and 2 allow 600 v to be ap-
plied to all capacitors
3.1.2
APPLICATION OF DIRECT CURRENT TO IN-
DUCTORS (OPERATION WITH INTERNAL OSCILLA-
TOR). Direct current can be applied to inductors during
measurement by several different methods to permit in-
cremental
inductance
measurements.
The 'various
methods are described below aiong with suggestions
fo~
their use. An external blocking capacitor, C y in Figure
3-2a, is needed only for measurements on the L s bridge.
It should be connected between the EXT DQ terminals
with its positive terminal connected to the upper of the
two bridge terminals. There is a slight error due to the
finite size of this capacitor, and the true value of Lx and
Q x can be calculated from the measured values by the
following formulas:
Lx
=
Lmeasured (1
+
C t
2.
2 )
C
y
Qx
Qx
=
Qmeasured (I
+
~t Ql~)
Y
x
WARNING
Large inductors carrying high current are shock
hazards because of the high voltage induced if.
SPECIAL MEASUREMENTS
the connections are broken.
Reduce the dc to
zero before disconnecting the dc supply of the
unknown inductor.
Method 1. (see Figure 3-2a) 30 ma max.
This method is preferred because both the dc sup-
ply and bridge are grounded and· up to 30 rna may be ap-
plied to large inductors. At the 30-ma level there is an
added 0.03% error in inductance and there may be a
1
D
EQ'
error as large as 0.001.
The resistor in series with the supply should be
large enough to avoid shunting the detector, and to keep
the dc constant as the bridge adjustment is made. Con-
nect the capacitor C e between the BIAS terminals, with
its positive terminal connected to the black BIAS termin-
al. The voltage rating of this capacitor should be greater
than the IR drop in the inductor.
The voltage rating of
the capacitor C y (L s bridge only) should be greater than
Idc x 7.6 kO. (7.6 k is the maximum value of the adjust-
able bridge arm). If the dc supply has high hum, external
filtering may be necessary.
Method 2.
(see Figure 3-2b)
High Current in
Small Inductors.
This method permits higher currents in small in-
ductors because the current is fed through the ratio arm
resistor R a , which is small on the lower inductance
range.
The maximum current is limited to that given in
Table 3-2.
The dc supply is connected between the BIAS ter-
minals with the positive supply terminal connected to the
black BIAS terminal in order to keep the bridge case and
dc supply at zero volts dc from ground.
The blocking
capacitor C y (necessary only on the L s bridge) must take
the full dc voltage applied.
With this method of bias, the bridge and the dc
TABLE 3-2
MAXIMUM CURRENT THROUGH INDUCTORS
(METHOD 2) AND RESISTORS (METHODS 2
&
3)
RANGE
MAXIMUM
RATIO
L BRIDGE
R BRIDGE
CURRENT
ARM
(Ra)
1100 f-Lh
1100mO
lOOma
10
11 mh
110
100ma
100
110mh
1100
71ma
1000
1100 mh
11000
22ma
nO
11h
11kO
7.1ma
10kO
110h
llOkO
2.2ma
100kO
1100h
1100kO
OAma
IMO
23
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