probe of the tester is on the negative side of the circuit; likewise, the opposite electrode glows when the red probe of the tester is on the negative side of the circuit.
The neon lamp is not the only method used on line voltage indicators to indicate dc polarity; for example, the Wigginton voltage tester, manufactured by the Square D Company, uses a permanent magnet mounted on a rotating shaft The ends of the magnet are colored red and black. The magnet is viewed from a transparent cap located on top of the tester. When the red portion of the magnet is up, the red test prod is positive. When the black portion of the magnet is up, the black prod is positive. Neither type of line voltage indicator vibrates when measuring dc.
Be certain to read and understand the instructions for the particular instrument you use. As you can see from this example about polarity indicators, because of variations in similar instruments, you could easily misunderstand an indication from one instrument when thinking of the instructions for another.
The line voltage indicator is not used to determine the exact amount of circuit voltage. That presents no problem for most of the work done by Construction Electricians. As you become proficient in the use of the solenoid type of voltage indicator, you can tell approximately what the voltage is by the location of the indicator within a voltage range on the scale.
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 R
1 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.Continue Reading