To explain the relationship between the meter readings and the range switch setting, let's use an example. Suppose you have a 2,400-ohm resistor, which you have identified by the resistor color code. With the range switch in the R X 1 position, you connect the meter across the resistor. The meter point then deflects between 200 and the point labeled with the infinity symbol on the extreme left side of the scale. Because the R X 1 range is selected, you multiply the reading by 1. Obviously, the scale reading is not accurate enough. Therefore, you move the range selector switch to the next higher scale position (R X 100) to obtain a more easily read value.

In the R X 100 position, you again zero the meter. This time, the pointer moves to the 24 mark on the scale. Because the R X 100 scale is selected, the reading is multiplied by 100. This gives a more accurate reading of 2,400 ohms (24 times 100).

If you position the range switch to the R X 10,000 scale, accuracy decreases. The most accurate readings are obtained at or near midscale. Other VOM instruments have ranges with other settings, such as R X 10, R X 100, or R X 1,000, to make it easier to make such readings.

Another thing to remember when you are measuring resistance is the tolerance of the resistor. If the tolerance of the resistor in the preceding example is 10 percent, we would expect a reading between approximately 2,160 and 2,640 ohms. If the reading is not within these limits, the resistor has probably changed value and should be discarded.

An open resistor will indicate no deflection on the meter. A shorted resistor causes full-scale deflection to the right on the lowest range scale, such as if the leads were shorted together.

Measuring dc Voltages

You set the multimeter to operate as a dc voltmeter by placing the function switch in either of two positions: +DC or -DC. The meter leads, as in the case of the ohmmeter function, must be connected to the proper meter jacks. When you measure dc voltages, be sure the red lead is the positive lead and the black lead is the negative, or common, lead. View A of figure 4-4 is a functional block diagram of dc voltage circuits in a multimeter. View B shows the jacks and switch positions for measuring dc voltages.

Figure 4-4. - Functional block diagram of dc voltage circuits.

When the meter is connected in a circuit, it becomes a circuit component. Because all meters have some resistance, they alter the circuit by changing the current. The resistance presented by the voltmeter depends on the amount of voltage being measured and the position of the function switch.

Some multimeters use a 20,000 ohms-per-volt meter sensitivity for measuring dc voltage and a 5,000 ohms-per-volt sensitivity for measuring ac voltage. The higher the meter resistance, the less it will load the circuit. The idea is to keep circuit loading to an absolute minimum so that the circuit under test is unaffected by the meter. In this way, you can get a clearer picture of what the circuit malfunction is, not the effect of the meter on the circuit.

Again, refer to figure 4-4. With the function switch set to either +DC or -DC, let's consider the effect of the range switch on the meter scale to be used. When measuring dc voltages, you have eight voltage ranges available: .25V, 2.5V, 10V, 50V, 250V, and 500V (1- and 1,000-volt special application plug-ins are also available). The setting of the range switch determines the maximum value represented on the meter. When measuring dc voltages, use the scale marked DC (figure 4-3). The last number at the extreme right side of the DC scale indicates the maximum value of the range being used. When the range switch is in the 2.5V position, the scale represents a maximum of 2.5 volts.

To simplify the relationship between the digits on the meter scale and the setting of the range switch, always use the multiple of the full-scale-deflection digits on the meter face that correspond to the numbers of the range switch. For example, use the 250 scale for the 250MV jack, 2.5V, and 250V ranges; the 50 on the scale for 50V and 500V ranges; and the 10 on the scale for the 10V and 1,000V ranges.

For explanation purposes, let's assume you wish to measure 30 volts dc. In this case, select the next higher range position, 50V. When you place the range switch to the 50V position (as shown in view B of figure 4-4) , the meter pointer should rise from a little more than midscale to 30, which represents 30 volts dc.

When measuring a known dc voltage, position the range switch to a setting that will cause approximately midscale deflection. Readings taken near the center of the scale are the most accurate. When measuring an unknown dc voltage, always begin on the highest voltage range. Using the range switch, work down to an appropriate range. If the meter pointer moves to the left, you should reverse the polarity of the function switch.

Always check the polarity before connecting the meter.

Q.4 Besides setting up the meter for expected voltage ranges, what must be strictly observed when taking dc voltage readings?

Now let's discuss how you take a voltage measurement on a component within a circuit. As an example, let's measure the voltage drop across the resistor shown in figure 4-5.

Figure 4-5. - Measuring the voltage drop of a resistor.

When measuring a dc voltage drop across a component in a circuit, you must connect the voltmeter in parallel with the component. As you can see in figure 4-5, the positive (red) lead is connected to the positive side of the resistor, and the negative (black) lead is connected to the negative side. A voltage reading is obtained on the meter when current flows through the resistor.

Some voltmeter readings will require the use of a ground as a reference point. Under these conditions, one voltmeter lead is connected to the equipment ground, and the other lead is connected to the test point where voltage is to be measured. Be sure to observe polarity.