Currentmeasurements; Test; Bear Input/Ratio Input; General - Datron 1061 Calibration And Servicing Handbook

32
RANGE
CONTROL
FIG.3.32 SIMPLIFIED DIAGRAM
OF
CUBRENT ASSEMBLY
To DC
lsolator
or AC Assembly
Lo Out
Hi Out
I+H
I-o---o
RL3
RL3
Analog
lntorlaco
3.5,1
CurrentMeasurements
Precision current
shunts
of 0.1O,
1O,
gfl,
90O
and
900Q connected
in
series
provide
an output
of
100mV
for
a
fulÊrange signal. To eliminate errors
in
measurement
due
to
lead
or
contact
resistance,
all
current shunts
are
4-wire
sensed
i.e. a pair
of
current
leads
and
a pair of
voltage leads
to
the
shunt(s) switched separately. The
voltage developed across
the
shunt(sl
is fed
to
the
DC
lsolator
in
DCI and the AC
assembly
in
ACI
or
DCI
+
ACI. The latter, DC coupled
mode,
computing
the
RMS
value
of the
DC and AC component
of the input
current.
These
circuits
are placed
in the '.1V
range'
amplifying
the
signal
by
3.16. The
output
of
buffer M5
is used
to
guard
leakage
paths on the current board.
Overload
protection up
to 2
amps
is
provided
by
diodes
D13
-
D16.
An input
greater than 2
amps causes
the
current fuse,
located on the
rear
panel, to blow.
3.5.2
Test
Duríng
the self test
routine, the Current
assembly
is
checked
for
correct operation. The
circuitry
is placed
into
the .1mA DC
current
range as described
in
Section
3.2.1 .3
with the
DC
lsolator ín
the
100mV
range.
Filter
is selected
and F.E.T.
09
closed
from
M4
-
10
allowing current
to
flow through R18
to
the
100¡rA
range shunts,
from
the
+15V supply. Thus a
voltage
of
approximately 0.3
volts
is
developed across
the shunts
and
fed
to
the DC
lsolator.
This voltage combined
with
the effect
of
the
voltage
injected
due
to
the
DC lsolator being
in
Test
(section
3.2.2.61 causes
the
output of
the
DC lsolator to
be
approx-
imately
5.75
volts.
After
measurement by the A-D
conver-
ter, the value
is
compared
to
the
stored value.
lf
the
measured
signal
is
within
6%
of
the
stored
value,
the
test
is
complete.
3.6
REAR INPUT/RATIO INPUT (Circuit
Drawing
No.430307).
3.6.1
General
The Rear lnput/Ratio lnput
assembly
contains the
switching
circuitry to
enable one
of
the
three analog signal
sources
to
be
connected
to the
measurement
circuits
of
the
DVM.
When
Rear
lnput
is
selected
either
remotely
or
on
the
rear panel
of
the
instrument and
the RATIO key
is
depressed,
the
sw¡tching
circuitry,
under
microprocessor
control,
selects
the ratio (referenc¡) input then the
rear
(signal) input, taking
one
valid reading at each stage.
3.6.2
Front
Panel/Rear Panel
Input
When
Front lnput
is selected,
either remotely
or
on
the
rear
panel, this
causes
the
base
of
O1
to
be
connected
to 0 volts, turning on the
transistor. Thus relays
RL1
and
RL2
are
energised, causing
the
front
signal
input
terminals
to
be
connected
to
the
measurement
circuits.
Should
Rear
lnput'be
selected,
relays
RLI
and
RL2
are
de-
energised,
connecting
the
rear
input to the
measurement
ci rcu
its.
3.6.3
Ratio
During
the
last
part
of
the
analog
interface
update
sequence'(see
Fig. 3.6)
M1'5
is takqn high
causing
the
flip-
flop
{M1)
to
be clocked
high
(0
volts) on
pin
1.
The
signal
is
applied
to 02
energisíng
the ratio
mode
input
selector
relays,
RL3 and RL4.
Thus
the
inputs
to
the'Ratio lnput'
on the
rear
panel
are connected
to
the
measurement
c¡r'
cuits. Once
a
valid
reading
has taken place, the
'Rear
lnput'
lines
are
connected
to
the measurement
circuits by
leaving
M1-5
low.
This
de-energises
the
relays
as
02
is
turned
off. Another reading is then
taken
and
the ratio
calculated.