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The various methods of applying dc bias to ca-
pacitors and suggestions for their use are described
in the three methods that follow:
Method
1. CS
Bridge (Figure
3-la).
In this method, up to 600 V may be applied on
any range.
Connect the negative terminal of the un-
known capacitor (if polarized) t o the
LOW UNKNOWN
terminal.
The dc supply used should have a low a c
output impedance. It is usually helpful to ground the
negative side of the dc supply and t o leave the bridge
floating to avoid hum from the power line. If the nega-
tive side of the supply (BIAS jack body) is grounded,
the bridge panel and
LOW UNKNOWN
terminal will be
at low dc potential with low signal voltage on them.
Method
2.
Cp Bridge (Figure
3-lb).
The same precautions mentioned in Method 1 ap-
ply here, and a blocking capacitor should be added
using the EXT DQ jack.
The positive side of the
blocking capacitor should be tied to the tip of the
phone plug. The voltage rating of this capacitor should
be sufficient for the full dc applied. The capacitance
required depends on the D of the unknown and on the
accuracy required. The errors caused by this capacitor
are:
n
L t
C measured = Cx (1
-
D~
)
where Cr
=
0.1 /uf
5
and
C,,
>>
C
D measured
Dx (1
+ _
D I )
Method
3.
C s or Cp Bridge (Figure
3-lc
and e).
This method is recommended for small capaci-
tors.
The maximum voltages that may be applied to
the Cs and Cp bridge are given in Table 3-1, but the
maximum voltage on the bridge are a function of the
CGRL-and-DQdial settings.
The ac impedance of the dc source should be
high (>10kR) to avoid shunting the detector, and the
dc source should have low hum.
The advantages of
this circuit are that the bridge and supply are both
grounded and the dc current can be easily limited by
a
resistor, since the impedance of the source should be
high.
WARNING
Note that the LOW UNKNOWN terminal
has the high voltageon i t in this method.
3.2.2
EXTERNAL AC GENERATOR OPERATION.
When both external a c and dc supplies are used,
hum may be introduced by the capacitance t o the line
in the power transformers of these generators.
The
bridge should be s e t up a s shown in Figure 3-1, with
both the a c and dc supplies grounded and the bridge
not grounded. The ac generator should be shunted by
a resistor if it does not provide
a
path for dc.
Method 3, paragraph 3.2.1, may also be used to
apply dc bias. The bridge and both the a c and dc sup-
plies are grounded (Figure 3-l), and the ac generator
is connected to the EXT GEN jacks.
This method is
particularly useful for high-frequency measurements of
small capacitors (paragraphs 2.4 and 3.2.1).
TABLE
3-1
MAXIMUM DC VOLTAGES APPLl ED
T O CAPACITORS
BY METHOD
3
3.3
A P P L I C A T I O N O F D C TO INDUCTORS.
Range
Multiplier
100 pF
1 nF
10 n F
100 nF
1 PF
10 pF
100,uF
Direct current may be supplied to inductors dur-
ing measurement by any of several different methods
s o that incremental inductance measurements may be
made. The various methods are described below along
with suggestions for their use.
A blocking capacitor
(Cb in Figure 3-2) i s needed only for the Ls bridge
shown. This capacitor (not supplied with the bridge)
should be connected by a phone plug inserted into the
EXT DQ jack. The errors caused by this capacitor are:
To get the corrected results add(%)(A)to
the
Qx
Max Volts
On
Bridge
505 V
242 V
142 V
78 V
72 V
71 V
71 V
measured
Ls
and Q. It will be necessary to solve for
Qx in equation (2) but usually Qmeas
ured
2 :
QX.
Max Volts
On Unknown
500
V
220
V
71
V
7 V
0.7 V
0.07 V
0.007
V
WARNING
Large inductors carrying high currents
are shock hazards.
Reduce the dc to
zero before disconnecting the dc supply
or unknown inductor.
SPECIAL MEASUREMENTS
3-2
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