76
Service
2-6
OHMS AND
CAPACITANCE
VOLTAGE
SOURCE
A/D
INTEGRATE
REFERENCE
LOW
HIGH
R13
+
–
S1
R1, R8,
R12, RTI
ACTIVE
FILTER
A/D
INTEGRATE
UNKNOWN
R
HIGH
LOW
I
X
VR
X
VR
X
VR
REF
=
I
X•RX
IX•RREF
R
REF
R
X
=
+
–
R2
R
X
UNKNOWN
RESISTOR
VR
REF
3
7
Z1
1.0001k
REFERENCE
RESISTOR
S1
Z1
9.996M
2
1
6
7
9
7
l3f.eps
Figure 2-3. 400 Ohm Range Simplified Schematic
Current Functions 2-9.
Input current through R5+R6 (for mA) or R6 (for amps) develops a voltage that is
proportional to the input. The dc voltage is routed to the active filter and a/d converter
inside U4. Ac voltage is routed to the ac buffer, ac converter (U2), active filter, and a/d
converter. The 4 mA dc and 4A dc ranges use the a/d converter is 40 mV range. The 4
mA ac and 4A ac ranges use the (x10) ac buffer range.
Continuity, Low Ohms and Diode Test Functions 2-10.
Q13 provides the source current for the continuity, low Ohms and diode test functions.
Inputs are sensed through R2. R2 and R62 form a 10:1 divider for measuring voltages in
diode test and the 8 kΩ range of low Ohms. The continuity function is the 400 Ohm
range of low Ohms, and uses a comparator to turn on the beeper when the input is ≤20
Ohms. Low Ohms makes voltage measurements of the input to provide the higher
resolution readings of the 40 Ohm range. Diode test also makes voltage measurements of
the input. A single beep sounds when the input is ≤ 0.75V; a continuous tone sounds for
inputs ≤ 60 mV.
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