Current Conditioner; Conductance Conditioner; Range Switches; Meter Protection Circuits - Fluke 8010A Instruction Manual

THEORY OF OPERATION
CIRCUIT DESCRIPTIONS
3-33.
CURRENT CONDITIONER
3-34.
The
Current Conditioner changes
current inputs
into representative voltages.
This
is
done by
passing the
unknown
current
through a precision
resistor
and
using
the voltage
developed across the
resistor. If the
proper
resistance
(i.e.,
range)
is
selected,
the voltage across the
resistor (after
passing
through
the
True
RMS
Converter)
will
be
within the
range of the
A/D
Converter. These
resistors are
a
special
type of high
power
precision
resistors
known
as current shunts.
(They
route or shunt
the current
around
the meter.)
Figure
3-3 (part B)
shows
a
simplified schematic of the current input signal
conditioner for current
measurements. The
range
switches determine the
combination of current shunts
used the
measurements.
The
resistances
of the
five
current
shunts are precisely
lOOOO, 1000,
1.000,
and
0.1000.
The
position
of
the
AC/
DC
switch (on the front panel)
sends the voltage
output of the current conditioner to
either the
True
RMS
Converter or the
A/D
Converter.
3-35.
The
10
Amp
function of the
8010A
sends the
measured
current
from
the
lOA
input terminal
through
a
separate current shunt of 0.010.
The
voltage developed
across the shunt
is
processed the
same
as
all
other current
measurements.
3-36.
RESISTANCE CONDITIONER
3-37.
The
resistance
conditioner uses the
same
voltage
divider as for voltage
measurements. For
resistance
measurements
the signal
conditioner
is
used only
as
a
reference resistor.
3-38.
Your Multimeter
uses the ratio
method
to
measure an
unknown
resistance.
Figure
3-3 (part
C)
shows
a simplified
schematic
of the input
signal
conditioner
for resistance
and conductance
measurements.
The
reference
resistor
is
placed
in
series
with
the
unknown
resistance
and a
voltage source (V
SOURCE)
is
applied across
them.
The
ratio
of the
resistances
equals the ratio of the voltage
drops
across the
respective resistances:
n
Unknown
__
V
Unknown
n
Reference
V
Reference
The
A/D
Converter measures
the voltage
ratio
and
the
value of the
unknown
resistance
is
calculated.
3-39.
The
voltage drop across the
unknown
resistance
is
applied to the
HI and
LO
inputs of the
A/D
Converter.
The
voltage
drop across the reference
resistor
is
used
as
the reference voltage for the
A/D
Converter.
The
range
switches select
the reference resistance (from the voltage
divider)
and
V SOURCE
so that the inputs to the
A/D
Converter, stay
within the proper range.
The
reference
resistance
is
selected
so that the output of the
A/D
Converter, displayed
on
the
LCD,
is
the value of the
unknown
resistance.
3-40.
CONDUCTANCE
CONDITIONER
3-4
1
.
The method
of
measuring conductance
is
the
same
as for
measuring
resistance
with
one
exception.
The
controller inside the
custom IC
commands
the
A/D
Converter to
interchange the
unknown and
reference
voltage inputs.
That
is,
the
A/D
Converter
uses the
reference
voltage during the integrate period
and
the
unknown
voltage during the read period. This
inverts the
voltage
ratio
so the reciprocal of resistance
(i.e.,
conductance)
is
displayed
in
the
LCD.
3-42.
RANGE
SWITCHES'
3-43.
Though
the range switches
are
not input
signal
conditioners, they
do
control the input
signal
conditioners so inputs to the Multimeter can be processed
properly.
In addition
to
the functions previously
discussed, the
range switches also position the decimal
point
on
the
LCD
display.
3-44.
Meter
Protection Circuits
3-45.
The
various protection
circuits
in
your
Multimeter protect
its
critical circuits
from
accidental
abuse,
such
as
overvoltage conditions, overcurrent
conditions,
and making measurements
with
the
wrong
function selected.
Even
if
the
Multimeter
is
subjected to
unreasonable abuse
(i.e.,
applying
lOOOV
on
the
2000
resistance
range) the protection
circuits will
absorb the
damage
while
protecting the
more
expensive
(critical)
circuits within.
The
following
paragraphs on
the
protection circuits
of the
Multimeter
are referenced
to
the
Main
PCB
Assembly schematic
in
Section
8.
3-46.
VOLTAGE MEASUREMENT PROTECTION
3-47.
When
the voltage
measurement
function
is
selected,
the
critical circuits
of the
Multimeter
are
protected
from
voltages
slightly
higher
than
the highest
range, regardless
of the selected range.
If
the input voltage
is
higher than the
limit
of the range
selected,
an
overrange
indication
is
displayed.
The
critical circuits
are protected
from input voltages
above 1200V
in
the following
manner:
when
an abnormally high
voltage
is
applied to
the voltage
inputs, the resistance of the three varistors
(RVl,
RV2, and RV3)
drops,
clamping
the Multimeter
side
of
R2
to
1200V. These
varistors
can
compensate
for
transients of
up
to
6000V.
If
the input voltage exceeds
6000V and
generates 'enough heat
to
destroy R2, the
resistor will
destnict (open) without creating a potential
fire
hazard. Resistor
R16
(1
Mfl,
1
W)
is
a
current-limiting
resistor
that protects
U3
from
dc
voltage inputs of
up
to
lOOOV (on
low
ranges).
Resistor
R14
(100
kO,
2W)
is
a
current limiting
resistor
that protects the input buffer of
the
True
RMS
Converter
for inputs of
750V rms
or
lOOOV
peak continuous for the
+200
mV
and
±2V
ranges.
On
these
two
ranges,
if
a high
level
input
is
applied
for
more
than
the 10
second
maximum, R14 may
be
damaged and
no
longer provide
the
proper
protection.
The
input
divider attenuation provides additional protection
on
all
other ranges.
3-6