Model 34SSA
THEORY OF
OPERATION
Section
Vlll
8-64.
Output Buffer Amplifier. An
Operational Amplifier
with unity gain is used to isolate the output of the AC Con-
verter. Full scale output of the AC
Converter
is
+ 6.6667 V
dc for all ranges.
8-65.
OHMS CONVERTER.
8-66.
General.
8-67. The
Ohms
Converter
is a
voltage limited current
source which supplies
a
constant current through the
unknown and reference resistors until the output voltage
reaches approximately
4.7S
volts dc. At this point the con-
verter becomes a constant voltage source. During the cur-
rent mode of operation the converter supplies a constant
current of .7 mA on the 100 ohm through 100 kilohm
ranges
or .7 microamps on the 1 and 10 megohm ranges.
The converter
becomes a
constant voltage source when
measuring resistance greater than
S.8
kilohm on the 10 and
100 kilohm ranges and greater chan 5.8 megohm on the 10
megohm
range.
Since
the same
current flows through both
the unknown resistance and
the
reference resistance, the
voltage drops across them
are
directly proportional. The
unknown resistance value
is the
ratio of the voltage drop
across the unknown resistance times circuit gain to the
voltage
drop across the reference resistance times
circuit
gain multiplied
by
the range constant
;
or
Vref
G2
8-68. Circuit Description.
8-69.
Ohms Converter Power Supply. An inverter circuit
is used to
derive
power for the ohms converter. The
inverter
operates
at a frequency of 30.72 kHz on 60
Hz
operation
or 2S.6 kHz on 50 Hz operation. Transformers AlOTl
and
AI2T1
provide
complete isolation of
the ohms converter.
8-70.
Current Source.
Figure
8-21
shows a simplified sche-
matic of
the current source used in the ohms
converter. The
circuit is designed to provide an
output current of .7 mA or
.7
pA. Output current is
determined
by
resistors R3, RS,
and R6. During the .7 mA
mode
of
operation, (100 ohm
through 100 kilohm ranges) relay
K8 shorts resistor R3.
The output current is then
determined by R6 and is equal
to the reference
voltage
(+ 6.2
V) divided
by
the resistance
of R6, or l<j
=
6.2/8.87
K
=
.7 mA. During the
.7
pA
mode
of operation.
(1
megohm and 10
megohm ranges) both K7
and 1C8
ate
open. Resistors R5 and R6 form a
divider
which
divides the
+
6.2 V reference to + .7 V. The
output current
is now determined
by
the .7 V across R3 or Iq
=
.7
V/1 M
=
.7
pA. Operational Amplifier U1 drives
output transistor
Q4
and provides the gain necessary to maintain
the proper
output current.
Relay K7 is used to select a reference
resistance of
1
kilohm for the . 1 kilohm through 1 0
kilohm
ranges or I megohm for
the
100
kilohm through 10
megohm ranges. Both the reference resistance and
the
unknown resistance
are in
the feedback circuit of the
operational amplifier.
Figure 8-21.
Ohms
Converter Current Source.
8-13
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