In view A of figure 3-11, a source of 150 volts is applied to a series circuit consisting of two 10-kilohm resistors. View A shows the voltage drop across each resistor to be 75 volts. In the 150-volt range, the voltmeter to be used has a total internal resistance of 10 kilohms. View B shows the voltmeter connected across the circuit. The parallel combination of R2 and the meter now present a total resistance of 5 kilohms. Because of the addition of the voltmeter, the voltage drops change to 100 volts across R1 and 50 volts across R2. Notice that this is not the normal voltage drop across R2. Actual circuit conditions have been altered because of the voltmeter.

Voltmeter Sensitivity

The sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of the resistance of the meter (R_m), plus the series resistance (R_s), by the full-scale reading in volts. In equation form, sensitivity is expressed as follows:

This is the same as saying the sensitivity is equal to the reciprocal of the full-scale deflection current. In equation form, this is expressed as follows:

Therefore, the sensitivity of a 100-microampere movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.

Q.19 What term is used to express the sensitivity of a voltmeter?

METERS USED FOR MEASURING RESISTANCE

The two instruments you will use most often to check continuity, or to measure the resistance of a circuit or circuit component, are the OHMMETER and the MEGGER (MEGOHMMETER). The ohmmeter is widely used to measure resistance and to check the continuity of electrical circuits and devices. Its range usually extends to only a few megohms. The megger is widely used for measuring insulation resistance, such as that between a wire and the outer surface of its insulation, and the insulation resistance of cables and insulators. The range of a megger can be extended to more than 1,000 megohms.

Q.20 What instrument is used for measuring the insulation resistance of cables?

The Ohmmeter

A simple ohmmeter circuit is shown in figure 3-12. The ohmmeter consists of the dc milliammeter, discussed earlier in this chapter, and the added features shown below:

Figure 3-12. - Simple ohmmeter circuit.

Q.21 What added features enable a dc milliammeter to function as an ohmmeter?

The deflection of the pointer of an ohmmeter is controlled by the amount of battery current passing through the moving coil. Before you can measure the resistance of an unknown resistor or electrical circuit, you must calibrate the ohmmeter to be used. If the value of resistance to be measured can be estimated within reasonable limits, select a range on the ohmmeter that will give approximately half-scale deflection when the resistance is inserted between the probes. If you cannot estimate the resistance to be measured, then set the range switch on the highest scale. Whatever range you select, the meter must be calibrated to read zero before the unknown resistance is measured.

To calibrate the meter, you first short the test leads together, as shown in figure 3-12. With the test leads shorted, a complete series circuit exists. The complete series circuit consists of the 3-volt source, the resistance of the meter coil (R _m), the resistance of the zero-adjust rheostat, and the series multiplying resistor (R_s). The shorted test leads cause current to flow and the meter pointer to deflect.

Notice that the zero point on the ohmmeter scale (as opposed to the zero points for voltage and current) is located at the extreme right side of the scale. With the test leads shorted, the zero-adjust potentiometer is set so that the pointer rests on the zero mark. Therefore, a full-scale deflection indicates zero resistance between the leads.

Q.22 A full-scale deflection on an ohmmeter scale indicates what resistance between the leads?

If you change the range on the meter, you must "zero" (calibrate) the meter again to obtain an accurate reading. When you separate the test leads, the pointer of the meter will return to the left side of the scale. This action, as explained earlier, is caused by the restoring force of the spring tension acting on the movable coil assembly. The reading at the left side of the scale indicates an infinite resistance.

After you have adjusted the ohmmeter for zero reading, it is ready to be connected to a circuit to measure resistance. A typical circuit and ohmmeter arrangement is shown in figure 3-13. You must ensure that the power switch of the circuit to be measured is in the de-energized (OFF) position. This prevents the source voltage of the circuit from being applied to the meter, a condition that could cause severe damage to the meter movement.

Figure 3-13. - Measuring circuit resistance with an ohmmeter.

Remember that the ohmmeter is an open circuit when the test leads are separated. To take a resistance reading with a meter, you must provide a path for current flow produced by the meter's battery. In view A of figure 3-13, the meter is connected at points A and B to produce this path. Connecting these test leads places resistors R1 and R2 in series with the resistance of the meter coil, the zero-adjust potentiometer, and the series multiplying resistor. Since you previously calibrated the meter, the amount of coil movement now depends only on the resistances of R1 and R2.

The addition of R1 and R2 into the meter circuit raises the total series resistance and decreases the current. This decreases the amount of pointer deflection. The pointer comes to rest at a scale reading that indicates the combined resistance of R1 and R2. If you were to replace either R1 or R2, or both, with a resistor having a larger ohmic value, the current flow in the moving coil of the meter would be decreased even more. This would further decrease the pointer deflection, and the scale indication would read a still higher circuit resistance. View B is a simplified version of the circuitry in view A.

From our ohmmeter discussion, two facts should be apparent: (1) Movement of the moving coil is proportional to the amount of current flow, and (2) the scale reading of the ohmmeter is inversely proportional to current flow in the moving coil.

The amount of circuit resistance to be measured may vary over a wide range. In some cases, it may only be a few ohms; in other cases, it may be as great as 1 megohm. Scale multiplication features are built into most ohmmeters so that they will indicate any ohmic value being measured and offer the least amount of error. Most ohmmeters are equipped with a selector switch for selecting the multiplication scale desired. For example, view A of figure 3-14 shows a typical meter that has a six-position switch. The positions are marked on the meter in multiples of 10, from R X 1 through R X 100K.

Figure 3-14. - Ohmmeter with multiplication switch.

The range used to measure any particular unknown resistance (R _x in view A of figure 3-14) depends on the approximate ohmic value of the unknown resistance. For instance, the ohmmeter scale of the figure is calibrated in divisions from 0 to infinity. Note that the divisions are easier to read on the right-hand portion of the scale than on the left. For this reason, if R_x is greater than 1,000 ohms and if you are using the R X 1 range, you will be unable to accurately read the indicated resistance. This happens because the combined series resistance of resistors R_x is too large for range R X 1 to allow enough battery current to flow to deflect the pointer away from infinity. You need to turn the range switch to the R X 10 position to obtain the 1,000-ohm reading.

Let's assume that you have changed the range switch to the R X 10 position and the pointer now deflects to a reading of 375 ohms, as shown in view B of figure 3-14. This would indicate to you that unknown resistance R_x has 3,750 (375 times 10) ohms of resistance. The change of range caused the deflection because resistor R X 10 has only 1/10 the resistance of resistor R X 1. Therefore, selecting the smaller series resistance allowed a battery current of sufficient value to cause a readable pointer deflection. If the R X 100 range were used to measure the same 3,750 ohm resistor, the pointer would deflect still further to the 37.5-ohm position, as shown in view C. This increased deflection would occur because resistor R X 100 has only 1/10 the resistance of resistor R X 10.

Q.23 The R X 100 resistance selection on an ohmmeter has what amount of resistance compared to the R X 10 selection?