Test; Current Assembly - Datron 1061 Calibration And Servicing Handbook

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¡