Electrical Projects The most widely used of all test instruments is the volt-ohm-milliammeter, more commonly known as the VOM. You won't find a repair shop without one. Of the VOM's three measurement functions, the ohmmeter circuit is most often used, since this is the one which can give an indication of continuity. The two selector switches on the face of the meter must be set properly to achieve accurate measurement. The left-hand selector switch designates the function (voltage, current, or resistance measurement), while the right-hand switch is used to select the range appropriate for the quantity being measured. When checking continuity, the function switch must be set in the ohms position, and the range should be set for one of the lower scales (R X 10). Since you are not after a precise resistance reading at this point, it is important only to have the range setting low enough to indicate when there is some internal resistance within motor-powered appliances (caused by the windings on the armature).

Although you are not concerned with an accurate resistance measurement at this point, it is important that you "zero" the meter before making any continuity checks so that a shorted circuit will be apparent. To zero the meter, connect the test leads to their appropriate jacks on the face of the meter—plus is red and minus (common) is black—and while touching the leads together, adjust the zero control for a 0 reading on the ohms scale. If touching the leads together doesn't cause deflection of the blade indicator, or if the indicator won't adjust to a zero position, the meter battery is weak or making poor contact.




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Home > Electrical Projects > Tools And Components > The Volt - Ohm - Milliammeter

October 12, 2008