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.

Fluke II Series Specifications

General

True RMS	Yes
Temperature	-40 °C to 400 °C
Continuity	Yes
Diode Test	Yes
Safety Rating	CAT III 600V
Voltage DC	600V
Voltage AC	600V
Current DC	10A
Current AC	10A
Resistance	40MΩ
Battery Life	400 hours
Warranty	3 years