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Also, the impedance of the blocking capacitor, C f ,
should be low compared with the impedance of the un-
known since it i s directly in series with the unknown.
The blocking capacitor, Cb, i s not needed for this
method and can be shorted out or removed.
Method
5.
(See Figure
3-2e.)
This method permits large or small dc currents
by connecting a current source in parallel with the un-
known inductor.
The dc voltage is isolated from the
bridge by capacitor C f . The impedance of Cf should
be low compared to the unknown since they are in
series.
The current source impedance must be high
relative t o the unknown at the measuring frequency.
A current source with the proper impedance must
be constructed because: 1) most regulated supplies
that have current limiting have a large capacitor across
the output terminals causing a low a c impedance, and
2) even the high slewing rate operational supplies
VOLTAGE
OUTPUT
Figure 3
-
3.
Dc-current supply for inductor measurements.
usually have a network across the output terminals that
reduces their impedance t o a few thousand ohms a t
1 kHz. To construct a high impedance supply use any
common ungrounded voltage supply (Kepco ABC series
units) and feed the output through the circuit in Fig-
ure 3-3.
Connect the output of this circuit t o the un-
known inductor (Figure 3-2e).
C A U T I O N
Short out the current source before dis-
connecting the inductor to prevent large
transient voltages.
3.4
D C BIAS FOR AC RESISTANCE MEASUREMENTS.
A dc bias voltage and current may be applied to
various types of nonlinear resistive elements such a s
diodes, varistors, and thermistors in order t o measure
small ac signal resistance. For voltage-sensitive de-
vices, diodes, and varistors, the a c resistance is the
slope of the dc voltage-current curve.
For thermally
sensitive devices, the ac resistance is equal to the dc
resistance a s long a s the time constant is much longer
than the period of the ac signal.
Several methods of
applying dc are shown in Figure 3-4.
Method
1.
(Figure
3-40.)
In this method a l l of the current supplied flows
through the unknown.
The current is limited t o the
amount given in Table 3-2. The dc source impedance
should be low compared with that of the unknown, or
the source should be shunted by a lnrge capacitor a s
shown. I f the dc supply is grounded, the bridge chas-
s i s may be a t a potential of up t o
6
V.
Method
2.
(Figure
3-4b.)
This method removes the dc supply from the
bridge arm s o that its impedance i s not s o important.
The current in the unknown i s equal to the current
supplied multiplied by
Rb
,
and should be limited
Ra
+
R b
t o that given in Table 3-2. The voltage applied should
be limited to 71 V. If the dc supply is grounded, the
bridge chassis may be a t a potential of up to 37 V.
Method
3.
(Figure
3-4c.)
This method permits grounding of both the bridge
chassis and the dc supply.
The current through the
unknown is equal to the current supplied multiplied by
a
,.
The dc current and voltage limits are' given
Ra
+.
R x
in Table 2-1.
Method
4.
(Figure
3-4d.)
This method permits large currents through low
resistors, since no current flows in the bridge.
The
resistor Rf should be large compared with the unknown,
and the blocking capacitor, Cf, should be able to take
the dc voltage Id,R,.
The impedance of the blocking
capacitor should be low compared with that of the
unknown.
Figure
3
-
4.
Methods of applying dc for ac resistance
measurements.
S P E C I A L M E A S U R E M E N T S
3-4
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