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Figure 3·2. Methods of applying de
to inductors.
3.1.3 DC BIAS FOR AC RESISTANCE MEASUREMENTS
(OPERATION WITH INTERNAL OSCILLATOR).
A dc
bias voltage and current may be applied to various types
of nonlinear resistive elements such as diodes, varis-
tors, and thermistors in order to measure the incremen-
tal resistance.
For voltage-sensitive devices, the ac
Method 3. (see Figure 3-3c).
This method is very similar to Method 2 but here
all the current flows through the unknown and a very
low-impedance dc supply is required.
If the dc supply
has high ac output impedance, it should be shunted with
resistance is the slope of the dc voltage-current charac-
teristic.
For thermally sensitive devices the ac resist-
ance is equal to the dc value if the same total power is
applied in both cases (as long as the thermal time con-
stant is much longer than the period of the signal).
Method 1. (see Figure 3- 3a).
Also, for this method the bridge and dc supply do
not have a common ground and one must be left floating.
This problem is discussed in paragraph 3.1.
5.
There is
a dc potential difference between the chassis and the
negative terminal of the dc supply that varies with the
adjustment of the CGRL control up to a maximum of 37
volts.
This method is preferred because the bridge and
dc source are both grounded and all the applied current
flows through the unknown. A maximum current of 30 ma
may be applied to the unknown resistors. The total volt-
age applied to the bridge should not exceed 400 volts.
The
impedance of the blocking capacitor, Cd,
should be small compared with that of the unknown re-
sistor (this may be difficult when R x is small), and the
voltage rating of Cd must be greater than the IR drop of
the unknown resistor.
The voltage rating of capacitor,
C e , connected to the BIAS terminals should be greater
than Idc x 7.6 k.o and the capacitance should be over
50 !-Lt.
A resistor should be placed in series with the dc
supply to avoid shunting the detector with a low ac im-
pedance.
A variation in this method is to short-circuit the
two blocking capacitors, Cd and Ceo
Then the current
through the unknown will be Iinput (R
Tha R ), where
a
x
R a is given in Table 3-2, and the voltage and current
limits of Table 2-7 apply.
Method 2. (see Figure 3-3b).
This method can be used to get higher currents
through small unknown resistors, and the current limit
for each range is given in Table 3-2.
The maximum
voltage is limited to 71 volts. Also, this method avoids
the use of a capacitor in series with the unknown or
ratio arm.
In this method the current through the unknown is
the total current multiplied by (
R~
), where R t is
R a
R
t
6667 ohms and R a is given in Table 3-2.
On the lower
ranges this ratio is near unity.
CHASSIS
(e)
DEl
supply do not have a common ground and one must be
left floating. This problem is further discussed in para-
graph 3.1. 5.
Method 3. Large Currents (Figure 3-2c).
This method must be used for very large currents
and the bridge does not limit the amount of current ap-
plied, since none of the current flows in the bridge. The
ac source impedance of the dc supply must be very high,
since it is in parallel with the unknown. An inductor, La'
very large compared with the unknown, may be used.
Often it is possible to resonate this shunt inductor to
increase the source impedance still further. The imped-
ance of the blocking capacitor, Cf, must be low compared
with the unknown since it is in series with it. If the dc
supply is grounded there will be a dc voltage between
the bridge chassis and ground, equal to Idc (dc resist-
ance of Lx).
The same grounding difficulties are present for
this method as are present for Method 2 above.
24
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