Learning Event 4:
DETERMINE VOLTS AND OHMS READING ON THE PSM-6
1.
Multimeter PSM-6.
As an aid to our discussion and for simplification, we
will use the ME-70 PSM-6 multimeter illustrated in Figure 1-18.
2.
The sensitivity of the PSM-6 voltmeter is the ohms-per-volt rating of the
meter circuit, either 1000 ohms per volt or 20,000 ohms per volt.
Ordinary
voltmeters are not extra-sensitive, since the energy they use is only a very small
percent of the energy produced by the current of the circuit being tested.
For
accurate readings of delicate network circuits where normal current is small, the
current which energizes the meter becomes such a large percentage of the total
current that erroneous readings and circuit malfunctions occur when you use a
common voltmeter.
3.
Reading the multimeter. Look at the dotted line shown on the meter face in
the illustration in Figure 1-18. It shows an imaginary line where the pointer of
the meter comes to reset.
Suppose the function switch is turned to the direct-
current voltage position, 20,000 ohms per volt (20K).
This indicates that the
middle scale (black) is to be read. Now suppose that the range switch is on the 50
volts position.
This indicates that the maximum deflection of the meter needle
represents 50 volts. Therefore, make your reading on the 5 scale, since there is
no 50 scale, and 50 is a multiple of 5. The multiple selected is always 1/10th,
10, 100, etc.
However, instead of the indicated numbers 1, 2, 3, 4, and 5,
visualize this scale as reading 10, 20, 30, 40, and 50 volts respectively.
Each
numbered segment of the arc has a value of 10 volts.
Therefore, each small
division of the scale has a value of 1 volt.
Thus the reading is one increment
past the number 2 (visualized 20). It, therefore, represents a value of 21 volts
a. Suppose that we turn the range switch to 250 and, checking a circuit, we
notice that the needle again comes to rest as shown. There is no maximum reading
of 250 on the direct-current scale, but we can use the 2.5 scale.
This time we
visualize the indicated numbers 0.5, 1, 1.5, 2, and 2.5 as readings of 50, 100,
150, 200, and 250 volts respectively. Each number segment of the scale has a value
of 50 volts; therefore, each small increment has a value of one-tenth of 50, or 5
volts.
b. Turn the function switch to the 1000 ohms per volt, alternating-current
voltage position and make the reading on the AC scale, which is the lower one and
is red.
Take the reading with the range switch on the 500 position (fig. 1-18)
making it on the 5 scale.
The numbered positions of the scale are visualized as
representing 100, 200, 300, 400, and 500 volts.
Then with each increment of the
scale representing 10 volts, you are reading 240 volts.
4.
The dual external shunt shown in Figure 1-18 extends the direct current range
of the instrument from its normal range (0.5 to 1000 milliamperes----1000
milliamperes equal 1 ampere) to provide either a 0-to 2.5-ampere range or a 0-to
10-ampere range.
Three terminals provided at each end of the molded plastic are
standard pin jacks which accommodate the test lead
20
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