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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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