-4-
RESISTANCE MEASUREMENTS
Figure 5 shows a simplified diagram of the resistance measurement function.
Figure 5 Simplified Resistance Measurement Diagram
External
Resistor
100
900
90k
900k
9M
9k
2k
200
20k
200k
2M
20M
Voltage
Source
Reference
Voltage
Low
Pass
Filter
7106R
DC
Ω
AC CAP
A simple series circuit is formed by the voltage
source, a reference resistor from the voltage divider
(selected by range switches), and the external
unknown resistor. The ratio of the two resistors is
equal to the ratio of their respective voltage drops.
Therefore, since the value of one resistor is known,
the value of the second can be determined by using
the voltage drop across the known resistor as a
reference. This determination is made directly by
the A/D converter.
Overall operation of the A/D converter during a
resistance measurement is basically as described
earlier in this section, with one exception.
The
reference voltage present during a voltage
measurement is replaced by the voltage drop
across the reference resistor.
This allows the
voltage across the unknown resistor to be read
during the read period. As before, the length of the
read period is a direct indication of the value of the
unknown.
-31-
(D) Diode Test
1. Set the selector switch to the “Ohm” position.
2. Connect the red test lead to “V/
Ω
” input jack and
the black one to the “COM” jack.
3. Set the range selector knob to the “ ”
position.
4. If the semiconductor junction being measured is
connected to the circuit, turn off the power to the
circuit being tested and discharge all of the
capacitors.
5. Connect the test leads to the device and read
forward value on the digital display.
6. If the digital reads overrange (1), reverse the
lead connections.
The placement of the test leads when the
forward reading is displayed indicates the
orientation of the diode.
The red lead is positive and the black lead is
negative.
If overrange (1) is displayed with both lead
connections, the junction is open.
(E) Transistor h
FE
Measurement
1. Set the selector switch to “DC”.
2. The transistor must be out of circuit. Set the
rotary/function switch to the h
FE
position.
3. Plug the emitter, base and collector leads of the
transistor into the correct holes in either the NPN
of the PNP transistor test socket, whichever is
appropriate for the transistor you are checking.
4. Read the h
FE
(beta or DC current gain) on the
display.
(F) Capacitance Measurement
1. Set the range selector knob to the “FARAD”
position.
2. Set the range selector knob to the desired
capacitance position.
3. Short the leads of the capacitor to be tested
together to insure that there is no charge on the
capacitor.
4. Insert the capacitor leads into the capacitor test
socket. Note that there are two groups of holes.
One lead must be inserted into one of group one,
and the other lead must be inserted into one of
the holes of group two.
5. Read the capacitance value on the digital
display.
