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mended.
The available power from such a supply is
0.1 W, which i s a low enough dissipation for almost
a l l resistors, and the maximum current i s 4 mA. Such
a supply permits measurements up t o 1
MO
with 1% ac-
curacy. For r e s i s t a n c e over 1
MR,
a higher voltage i s
desirable for good sensitivity, but i t should be noted
that the maximum EIA t e s t voltage is 100 V, and that
various types of r e s i s t o r s have different voltage ratings.
T h e maximum voltage and current that may be
applied t o the bridge for each range are given in Table
2-1.
Careful observation of both of t h e s e limits will
prevent damage t o the bridge.
Because the full voltage may be applied t o the
unknown, it is advisable t o limit the available power
to a value l e s s than the power rating of the unknown
component.
CURRENT-LIMITINO
RESISTOR
Figure
2
-
2.
Circuit for standard
t e s t voltage measurements.
Various
EIA
standards
for testing different
types of r e s i s t o r s are summarized ii T a b l e s 2-2 and
2-3.
Various military standards are listed in T a b l e s
2-4 through 2-7.
A suggested s e t u p for t e s t s a t t h e s e
voltages i s shown in Figure 2-2.
The voltmeter here
indicates the bridge voltage and should be s e t a s l i s t e d
in T a b l e s 2-2 and 2-3.
An alternate scheme i s t o put
the voltmeter directly a c r o s s the unknown resistor,
assuming that the input r e s i s t a n c e of the voltmeter is
large enough t o c a u s e no error.
2.4 CONNECTION OF EXTERNAL GENERATOR.
In most c a s e s when a n external generator i s used
it should be connected t o the EXT GEN jack on the
s i d e of the bridge.
In t h i s connection, the external
generator i s connected directly t o the internal bridge
transformer when the function switch is in the AC EX-
TERNAL position, and the low generator terminal i s
connected t o the bridge c h a s s i s (which should be
grounded; paragraph 4.6).
A second ground connection
to the generator should be avoided.
If the external generator c a n be overdriven when
connected to a low-impedance load, it i s generally de-
sirable t o p l a c e a resistor in s e r i e s with the ungrounded
generator connection t o the bridge.
T h i s resistor
should be large enough to prevent distortion even when
the bridge input i s short-circuited.
T h e bridge input
impedance a t the EXT GEN jack i s a minimum of 30
fl
(resistive) a t 1 kHz when the bridge i s s e t t o measure
a short circuit on the UNKNOWN terminals.
T h i s i s
shunted by the inductance of the primary of the bridge
transformer, which i s approximately 0.25 13.
In some c a s e s where more input power is re-
quired, particularly in measurements of low impedance,
a matching transformer between generator and bridge i s
useful.
T h i s transformer need not be shielded.
The
GR Type 1311 Audio Oscillator is recommended for
t h i s application at frequencies of 50,
60,
100, 120,
400, 500, 1000, 2000, 5000, and 10,000 Hz because i t s
output will not be distorted by over-loading and it h a s
a matching transformer t o drive low-impedance loads.
When the desired bridge voltage i s higher than
c a n be applied by the internal bridge transformer, the
generator c a n be connected directly in the bridge cir-
cuit by connection t o the BIAS jack (Figure 2-3a). In
t h i s connection, the generator i s ungrounded and ca-
pacitance from i t s terminals t o ground must be con-
sidered. Capacitance from the negative BIAS terminal
to ground c a n c a u s e a large error a t high frequencies
when low impedances are measured. Therefore, use a
shielded c a b l e and u s e the outer conductor t o connect
the low generator terminal to the positive BIAS termi-
nal.
Capacitance of over 100 p F from the positive
BIAS terminal to ground can c a u s e appreciable error
(paragraph 4.6).
A bridge transformer c a n be used t o
connect a generator t o the BIAS jack, but t h i s h a s no
advantage over the u s e of the internal bridge transform-
er u n l e s s the external transformer h a s a higher voltage
rating, a s d o the GR Type 578 Transformers (Figure
2-3b).
SHIELDED
BRIDGE-TF~NSFORMER
OSC
Figure
2
-
3.
Methods of applying external ac.
2,s MAXIMUM APPLIED AC VOLTAGE.
T h e maximum a c voltage that may be applied to
the 1650-B Impedance Bridge depends on:
a. the voltage and power ratings of e a c h compon-
ent (including the unknown),
b. t h e bridge circuit used,
c.. the range used,
d. the position of the variable components,
e. the method of applying the voltage.
Exact limits for any specific measurement c a n be cal-
culated from the circuit diagrams of Figure 2-1, and by
insuring that the power dissipation i n the ratio-arm
BASIC M E A S U R E M E N T S
2-6
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