Ac And A/D - Fluke II Series Service Manual

Theory
of
Operation
DETAILED DESCRIPTION
The
pins
of
Z1
are used
as
follows:
Z1-PIN
NUMBER
FUNCTION
1
input
3
3.2V range tap
4
32V range
tap
5 320V
range
tap
6 1000V dc/750V
ac range
tap
Overvoltage
protection
is
provided
by El, Rl, R2,
RV1,
and
RV2.
El
is
a spark
gap
that
fires
between 1200V and 1800V.
Rl
is
a fusible resistor that
opens with excessive
inputs.
RV1
and
RV2
are metal-oxide
varistors that
clamp
around
910V
each.
Current Conditioning 2-5.
Current input conditioning
is
provided
by R6, R7, R5, and R13.
R6
and
R7
develop
input
voltage from
the applied current.
R5
and
R13
act as a
voltage divider for the
300-mA
range.
FI
protects the
mA
circuitry;
F3
protects
the
10A
circuitry.
(Model 70
has
no
current-measuring
capability;
Model
73 measures
current only
on
the
10A
range.)
Ohms
2-6.
When
the
ohms
function
is
selected, the
meter compares
the
unknown
resistance
at J1
with
the reference
resistors in
Zl. (See Figure
2-2,
Ohms
Function.)
Input protection
for the
ohms
ranges
consists
of
Ql, Q2, Q3, R2, R3, R4, and RT1.
Q1
and
Q2
serve
as
back-to-back zener diodes
which
limit
the input
to
between 7 and
9V.
RT1
is
a thermistor
that
normally has about
1
kfl of
resistance
but
increases to
very
high
impedance
as
it
heats
up with an overload voltage. Also,
R2, R3, and
R4
limit
current,
and
Q3
clamps
pin
29 of
U1
to
approximately 2.5V.
Additional
Circuitry
2-7.
In addition
to the
above
circuitry,
the following
circuitry
is
also
used
in
the Series
II
meters:
AC
Converter, Active
Filter,
Rotary
Switch,
and
A/D
Conversion.
AC CONVERTER
2-8.
The
ratio
of
R17
and
R18
determines
ac voltage
and
current
accuracy
(see
Figure
2-3,
AC
and
A/D
Converter), and
Cll
is
the
averaging capacitor for the half-wave
ac
converter portion of Ul.
ACTIVE FILTER
2-9.
An
active
filter
that includes
R9, R10, C5, and
C6
is
located in
Ul. Conditioned
input
signals are passed through the active
filter
in route to the a/d converter section
of Ul.
(See Figure
2-3,
AC
and
A/D
Converter.)
2-4
The
clock frequency
for the
digital
portion of the
circuit
is
a function
of 32.768-kHz
crystal
Yl. Yl, C12, C15, and
amplifiers in
Ul make
up
the
oscillator circuit.