Table of Contents
Advertisement
SPECIAL MEASUREMENTS
a large capacitor since it is in serIes with the unknown
resIstor.
With 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
will be a dc potential between the chassis and negative
terminal of the dc supply, equal to
approximatel~
Idc R a .
Method 4. (see Figure 3-3d).
With this method any amount of dc may be sup-
plied
to
the unknown resistor because none of the cur-
rent flows through the bridge and the applied voltage
is
limited only by the voltage rating of the blocking ca-
pacitor.
Here the dc supply shunts the unknown, and it is
necessary
to
use a series resistor or inductor with an im-
pedance much larger than that of the unknown. There-
fore, this method is limited to relatively small resistors.
Also, for this method there is a grounding problem since
the bridge and the dc supply do not have a common
ground. See paragraph 3.1. 5. There will be a dc poten-
tial between the chassis and negative terminal of the dc
supply, equal
to
approximately Idc R x .
OET
3.1.4
APPLICATION OF DC BIAS WITH EXTE RNAL
AC GENERATOR. When an external generator is used,
the grounding problem (see paragraph 3.1.5) becomes
even more serious since the internal detector is not se-
lective in the EXT AC position and the hum pickup is
unattenuated. In many cases it will be necessary to use
an external selective detector, such as the Type 1232-A
Tuned Amplifier and Null Detector.
In some cases the
induced hum may overload the internal detector, causing
erroneous readings, in which case the external detector
should be connected between the LOW UNKNOWN ter-
minal and the bridge panel rather than
to
the DET OUT
terminals. In extreme cases, the bridge may be discon-
nected from the power line, thus removing all internal
source of hum. This has the disadvantage of turning off
all the indicator lights.
For those biasing methods where the dc supply
and the bridge have a common ground, the external ac
supply should be connected to the EXT GEN terminals
which have the same common ground. With those methods
that do not have a common ground between the bridge
and dc supply, it
is
generally best to ground the external
dc and ac supplies at the same point, as shown in Fig-
ures 3-4a and 3-4b, and ungrou1'1d the bridge. A resistor
should be put in parallel with the ac generator to provide
a dc path.
When the bridge is floating and an external
detector is used, this detector
is
also floating and
should be battery operated (as
is
the Type 1232-A) to
avoid additional hum pickup and capacitance to ground.
3.1.5 GROUNDING PROBLEMS WITH DC BIAS. Forthose
biasing methods described above that do not have a
ground in common with the bridge chassis,
it is
neces-
sary to float (unground) either the dc supply or the
bridge. This results in two difficulties. First, there
is
CHASSIS
INDUCTANCE BRIDGE
(METHOD 2)
CHASSIS
CAPACITANCE BRIDGE
(METHOD 2)
OET
(d)
Figure
3-3.
Methods of applying dc
to resistors for ac resistance
measurements.
Figure
3-4.
Connections of external
ac an d dc supplies.
25
Advertisement
Table of Contents
