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