Datron 1061 - Test; Current Assembly

M4
+
642rl
M2{A}/o11
R3o/R31
lOmA
r-
10n
e
loo
n
6.42V
RANG
ES
M1(Al
R2s/R3o/R31
1mA
I-
1KN
RANGE
6.42V
M1(Bl
Rz4l*2sl*3ol*31
loopA
I-
lOKN
RANGE
6.42V
o4
R24ln2s/R29/R3o¡h31
I-
lOpA
1OOKN RANGE
6.42V
I-
M1(D}
R2s/R3o/R31
1PA
R3
4Mn
o1
R1
4kn
1OOOKN
RANGE
I-
10OnA
M1(cl
R24lR29/R3o/R3r
R3
4Mn
o1
R1
4kn
1OMO
RANGE
FIG.3.3O
SIMPLIFIED
Q
CURRENT
SWITCHING
M4
+
6.42ko
M4
64 2ko
642kO
6.42kO
6.42V
64.2kO
6.42V
31
The
voltage
developed
across
C8 causes
M4 to
sink
current
through
resistor
chain
R24.
R25,
R29, R30, R3l
until
the
voltage
developed
across
the
chain
balances
that
across
C8.
Thus the
current
required
for
a
particular
range
is selected
by the value
of the
resistor
chain
switched byM1,
M2
and
04.
Simplified
diagram
Fig.
3.30
shows
the
resistor
chain
and switching
for
each
range.
On
the high resistance
ranges
leakage
paths
are
províded
by
03, M2(B)
and
M2(C).
To
produce
good
cornmon
mode
rejection,
M4
supplies are bootstrapped,
the
supply
span
being
defíned
by
a 12 vôlt
zener,
Dl7.
The filtered
bootstrap
supplies
(+OBS
and
-S)BS)
power
the
astable
(M6)
and
bilateral
switch.
The use
of ohms
guard permits
in-circuit measurement
of
resistors,
provided
shunt
paths
are
greater
than
250O
and a suitable tapping
point
is
available. Consider
Fig.
3.31.
Guard
is reference
0, Lo is
actively maintained
within
microvolts
of reference
0
(as
previously
explained).
Thus
there is no voltage across
Rz and consequently
no
current
in Rz. Voltage
follower
O10/M3
will
simply
pass
more
current into Ry
from
the I+ terminal until the
selected
current for the
particular
range flows through Rx.
3.4.3
Test
During
the
self-test
routine
(actuated
from
the front
panel
or
remotely
programmed),
the
Ohms Converter is
checked
for correct
operation'
The
circuitry
is
placed
into
the
1OkO
range
as described
in
Section
3'2.1
'3'
Filter
is
selected
and
F.E.T.
05
'closed'
from M9-1 causing
R8
(9.76kO)
to
be
placed
between
l+
and
I-.
fhus
with
I+
and
Hi,
I-
and
Lo
connected
(2-wire
if front
panel
input
selected),
the DC
lsolator
(which
is
also
in
the
TEST mode)
measures
the
voltage developed
across
the
resistor
(approx
1 volt).
The
resulting
voltage
output
from the DC
lsolator
is applied
to the
A-D
converter,
measured
and compared
to
the
stored
value.
lf
the
measured signal
is within
+
6%
of the
stored
value,
the
test
is complete.
3.5
CURRENT
ASSEMBLY
(Drawins
No. 430304)
The
Current
assembly
contains a set of selectable
precision current shunts,
the
voltage
developed
across the
shunt(s)
being sampled
by the DC or
AC voltage
measure'
ment
circuits.
F¡G.
3.31
USE
OF OHMS
GUABD
OHMS
GUARD
Ry
Rz
Lo
I-
I+
H¡