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.
Up to this point, each of the instruments we have discussed, for the most part, performs only one function. The exception was the clamp-on ammeter/ohmmeter. In a similar way the analog meters and digital meters perform several (or multiple) functions and are therefore referred to as multimeters.
An analog instrument usually makes use of a needle to indicate a measured quantity on a scale. Digital
meters indicate the quantity directly in figures. We will discuss both types here because you will use both types.
Notice that each multimeter in figure 7-24 (A, B, C, and D) consists of a case to enclose the indicating device, one or more functions and/or range switches, and internal circuitry and jacks for external connections.
Before plugging the test leads into the jacks, set the switches for the measurement. Let's look at anexample. You are about to measure the voltage at a standard wall outlet in an office. You already know from experience that the voltage should be in the area of 115 to 125 volts ac. You have one of two types of multimeters-an analog meter or a digital meter. Because you do know the voltage to be tested, you would set the function switch to AC and the voltage to 250V. For the operation of the range and function switches on the particular meter, you should check the manufacture's literature.
What should you do if you have no idea what the voltage is? There are times when you should not getContinue Reading