Ohms Function - Fluke 78 Service Manual

R1+RT1
Ω
V/
COM
2-8.

Ohms Function

When the 400-ohm range is selected, internal switches connect the resistor Z1-2 (9.996
MJ) to resistor Z1-6 (1.0001 kJ). (See Figure 2-3, 400-Ohm Range Simplified
Schematic.) Then through switch contacts S1 5, 6, and 9, these resistors form a reference
resistor of 1 kJ.
The source voltage is connected internally at both V0 and V4 of U1. The current is
routed through two parallel resistors Z1-6 and Z1-2 (1.0001 kJ and 9.996 MJ) into S1
at contacts 5 and 9. The signal then travels out of S1 at contact 6, through RT1 and R1
and to the z input. The signal then goes through the unknown resistance and back
to the COM input. The same current flows through the unknown resistance and the
reference resistor. The voltage dropped across the unknown resistance is sensed from the
z input jack through R2 and S1 (contacts 2 and 3) to SENS of U1.
The a/d converter senses the voltage drop across the 1 kJ reference resistor through the
low (RRS of U1 through R8) and high (V0 and V4) points. These two voltages are used
by the a/d converter to perform a ratiometric measurement. Since the same current flows
through the reference and unknown resistors, the ratio of the resistance values is the
same as the ratio of the voltage drops across them.
For the 4-kJ range, the 10.010-kJ resistor (Z1-5) used in parallel with the 9.996-MJ
resistor (Z1-2) forms a 10-kJ reference resistor. For the 40-kJ range, 101.01 kJ(Z1-4)
and 9.996 MJform a 100-kJ reference resistor. And for the 400-kJ range, 1.1111 MJ
(Z1-3) and 9.996 MJ provide a 1-MJ reference resistor. The 4-MJ and 40-MJ ranges
use the 9.996-MJ resistor alone.
C1
S1
6 5
1
Z1
C2
9.996M
2
4 VAC
4V
3
Z1
1.1111M
7
12
S1
11
Figure 2-2. 4 V Range Simplified Schematic
DC
ACTIVE
FILTER
AC
AC
CONVERTER
Theory of Operation
Circuit Descriptions
HIGH
A/D
LOW
zr02f.eps
2
2-5