OHMMETERS The resistance of a component or circuit, in ohms, can be determined by using Ohm's law. With the instruments we just discussed, you can find circuit current and voltage. From electrical theory you already know that voltage divided by amperage equals resistance. But the fastest method of determining resistance is by taking a resistance reading directly from an ohmmeter.

The simplest type of ohmmeter consists of a housing that includes a milliammeter, a battery, and a resistor connected in series, as shown in figure 7-21. The ohmmeter is designed so that the resistor R1 limits the current though the milliammeter to a value that results in a full-scale deflection of the meter needle. The scale (fig. 7-22) is calibrated in ohms. By using several resistors, more than one battery, and a selector switch (to select one of the several resistors and batteries), you can make the ohmmeter include more than one resistance range.

Figure 7-21.- A simple series ohmmeter circuit. You may use a variable resistor in the meter circuit (R2 in fig. 7-21) to compensate for variations in battery voltage. Before using an ohmmeter for a precise resistance measurement, short the leads together and set the needle to zero by rotating the "zero ohms" (variable resistor) knob. The result is a full-scale reading at zero ohms.

CAUTION Be certain not to place the ohmmeter leads across an energized circuit or a charged capacitor. Ignoring this rule will likely result in damage to the test equipment. Always turn off the power on a circuit to be tested before making continuity or resistance tests. Before you test with an ohmmeter, bleed any capacitors that are included in the circuits under test. Use extreme care in testing solid-state components and equipment with an ohmmeter. The voltage from the internal batteries of the ohmmeter will severely damage many solid-state components. Always turn an ohmmeter off after you have completed your test to lengthen the life of the batteries.

Figure 7-22.- Typical scale of a series type of ohmmeter.

After you zero the meter, place the leads across the circuit or component under test. The resistance of the unknown resistor between the ohmmeter leads limits the current through the meter, resulting in less than a full-scale deflection of the needle. The resistance reading may then be taken from the point along the scale at which the needle comes to rest Accurate readings become progressively more difficult to take toward the high-resistance end of the scale. When the needle comes to rest at the high end of the scale and the ohmmeter has several resistance ranges, you may simply switch to a higher range for a reading closer to center scale. The resistance is read directly from the scale at the lowest range (for example, the R x 1 range on some ohmmeters). At the higher ranges the reading may be multiplied by 100 or 10,000 (as on the R x 100 or R x 1,000 ranges). The higher resistance ranges in a multirange ohmmeter use a higher voltage battery than do the lower ranges. We will discuss multimeters (meters that perform more than one function) later in this chapter, but since we have already discussed the ammeter as a clamp-on ammeter, we will look at the same instrument as an ohmmeter.

Figure 7-23.- Clamp-on ammeter with ohmmeter battery adapter.

To use the ammeter as an ohmmeter, you should plug a battery adapter into the jack on the side of the case (fig. 7-23). The battery in the adapter powers the ohmmeter function of this instrument. One of two test leads that may be plugged into the instrument (for voltage measurements) is used for the second lead of the ohmmeter. This test lead is plugged into the jack marked "COMMON." The ohmmeter scale is a fixed scale at the right side of the scale window opening. It is not part of the rotating scale mechanism. The rotating mechanism has no effect on the ohmmeter operation. The leads are applied to the circuit or component, and the reading is taken as with any ohmmeter.

The series type of ohmmeter is only one type of instrument used for resistance measurements, but it is common in the design of ohmmeters used by Construction Electricians.