Resistance/Conductance Measurement Protection; Current Measurement Protection; Power Supply; Touch And Hold Circuit - Fluke 8010A Instruction Manual

THEORY OF OPERATION
CIRCUIT DESCRIPTIONS
3-48.
RESISTANCE/CONDUCTANCE MEASURE-
MENT PROTECTION
349.
Selecting either the resistance or conductance
measurement function places
Ql, Q2, and
RT
1
in parallel
with the
meter
across the resistance
input
and
common
lines.
These
three
components
provide protection
from
inputs
up
to
300V.
3-50.
The
LO
RANGE O
function of the
8012A
has a
separate input protection
circuit
rated to
300V. Refer
to
the
schematic of the
Low Ohms PCB
in
Section
8.
The
protection circuit
must
protect
two
separate
parts,
the 10
mA
current source
and
the input to
the
op
amp
circuit.
The
op
amp
input
is
protected
up
to
300V
by R59, CR51,
and CR52. The
10
mA
current source
is
protected by
the
circuit
formed by R51, R52, Q51, and
Q52. This
circuit
can withstand
up
to
700V.
Diode
CR50
protects the
current source
from
negative voltages.
3-51
.
CURRENT MEASUREMENT
PROTECTION
3-52.
The
mA
input
is
protected
two
ways.
Diode
CRl
and IC
U4
limit
the voltage drop across the current
shunts.
Fuses
FI and
F2
protect the current shunts from
overcurrent conditions.
Fuse FI
is
a fast-blowing fuse
rated for
2
A
at
250V.
It is
the type
with a wire thread
in a
glass
envelope.
Under
moderately abusive overloads FI
will
open
to protect the
circuits. If
the
overload voltage
is
high enough, the voltage
can
arc
through
the
metal vapor
caused
by FI
blowing. In
this
case,
F2
opens.
Fuse
F2
is
rated for
3A
at
600V.
3-53.
The lOA
function of
the
8010A
has
no
fuse
protection
circuitry.
However,
it
can handle
overloads of
up
to
12A.
3-54.
Power Supply
WARNING
LINE
VOLTAGE POTENTIAL
IS
ON
THE
PRIMARY
POWER
CIRCUIT
ANY
TIME
THE
INSTRUMENT
IS
PLUGGED
INTO
A
LINE
POWER
OUTLET.
3-55.
The standard instrument can be ordered with
a
power
supply that operates
from
115V, 60 Hz; lOOV, 50
Hz
or 60
Hz
selectable;
and 230V, 50 Hz, This power
supply provides
all
basic operating voltages
for the
instrument.
These
voltages are
rectified
and
filtered
but
are
not regulated. All regulated voltage needs
are
supplied
either
by
U7
or
by
the
custom
IC, U3.
The
POWER
switch opens/closes the secondary
circuit.
The power
supply transformer (Tl)
is
a self-limiting
transformer and
therefore
no
input
line
fuse
is
required.
3-56.
An
optional rechargeable battery
pack
is
available
for
both Multimeters. This option allows the Multimeters
to
operate
from
90V
to
264V,
50
Hz
or 60
Hz
(selectable).
Refer
to
Section 6 for additional
information on
the
Rechargeable
Battery Option.
3-57.
Touch and Hold
Circuit
3-58.
The
touch
and
hold
circuit
allows the use of the
optional
Touch
and
Hold
Probe
with
your
instrument.
Refer to the
Main
PCB
Assembly
schematic
in
Section
8.
The
CL2
line to
the
crystal
Y1
continues
to
the
left,
passing
through
Q4
and Q3
to
ground. This path
allows
the crystal to
oscillate. If
a ground
is
applied to the
mA
terminal,
Q3
is
turned
off,
stopping
the
instrument
clock.
The
last
value displayed
on
the
LCD
remains displayed
for a
few minutes
if
the
mA
terminal
is
kept grounded.
CAUTION
The Touch and
Hold
function
is
meant
to
be
used
for intervals of
no
longer than a
few
minutes.
If
the display
is
stopped
for
longer
periods of
time (about
1
/2 hour) the
outline
of
the
digits
may
remain
for a
few hours
after
the display
is
started again.
Even
longer periods
of
time
may
cause
permanent damage.
3-7/3-8