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SECTION
3
SPECIAL
MEASUREMENTS
3.1 APPLICATION OF DC BIAS TO UNKNOWN.
3.1.1
APPLICATION OF DC BIAS TO CAPACITORS
(OPERATION WITH INTERNAL OSCILLATOR). Up to
600 volts ofdc bias may be applied to the unknown capa-
citor by any of several methods.
The simplest method
can be used only for measuring series capacitance; for-
tunately, this is how most capacitors are specified.
WARNING
Charged capacitors form a shock hazard, and
care should be taken to ensure personal safety
during measurement and to be sure that the capa-
(:itors are discharged after measurement.
The
external
dc supply should also be carefully
handled and connecting leads insulated wherever
possible.
It is' advisable to limit the power that can be
drawn from the external dc supply to 1/2 watt (by a re-
sistor, fuse, or circuit breaker) in order to protect the
bridge components in case the unknown is short-cir-
cuited.
The various methods of applying dc bias to capaci-
tors are described below, along with suggestions for
their use:
Method 1. C s Bridge (see Figure 3-1a).
With this method up
to
600 volts can be applied on
any range. Connect the negative terminal of the unknown
capacitor (if polarized) to the LOW UNKNOWN terminal.
The dc supply should have a low ac output impedance.
For this method of bias the bridge and the dc supply do
not have a common ground and one must be left floating.
This problem is discussed in paragraph 3.1. 5.
Method 2. C p Bridge (see Figure 3-1b).
This method is the same as Method 1 above, except
that a large blocking capacitor is placed in the standard
bridge arm to prevent direct current from flowing through
the D adjustment rheostat. Connect this capacitor, C y of
Figure 3-1, between the EXT DQ terminals, with the
positive terminal connected to the upper terminal.
Since this capacitance is not infinite, there will
be an error in the measured value of C x and
Dx .
The
true values can be calculated from the following formulas:
C
- C
(1
+~D
2)
x -
measured
C
x
y
22
D x
=
Dmeasured
(1-
~t
D x
2)
y
Method 3. C s or C p Bridge. Small Capacitors (see
Figure 3-1c).
This method is recommended for small capacitors.
The maximum voltage that can be applied depends on
the bridge range as given in Table 3-1.
The "Max DC
Current" column is correct only for the C s bridge unless
a blocking capacitor, C y (see Figure 3-1b), is used with
the C p bridge.
If no blocking capacitor is used, the
maximum direct current will depend on the DQ rheostat
setting, but the full current indicated can be applied on
the three lowest capacitance ranges.
The advantage of this method is that both the dc
source and the bridge are grounded and that the dc can
easily be limited by a series resistor since the .imped-
Figure 3-1. Methods of applying dc
bias to capacitors.
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