406 Electrical Systems Many digital multimeters also have a min/max/aver- age memory. With this feature, a DMM holds the mini- mum value measured, the maximum value measured, and the average value over a period of time. In addition, some advanced DMMs measure frequency and convert the current measurement of a 4-20 mA process control loop to a percentage reading. T E C H T I P DMM Safety Features Low-cost DMMs may not include safety features included in some higher-end models. These extra safety features may someday prevent damage to the device or (more importantly) injury to its user. When working in an industrial environment, the extra safety features are worth the extra cost. Ohmmeter Mode Before measuring the resistance of a device or compo- nent, disconnect electrical power from the device. Th e circuit must be de-energized before resistance can be measured. In the ohmmeter (resistance measurement) mode, the DMM provides voltage to the circuit under test. Most DMMs can measure resistance ranging from fractions of an ohm to several megohms. S A F E T Y N O T E Ohmmeter Mode When measuring resistance with a DMM, never apply the test probes to an energized circuit. If you do, the DMM may be damaged and you could be injured. Diode Test Mode Often, DMMs have a diode test function. Th is function allows you to measure the voltage drop across the junction of the diode. When in the diode test mode, the DMM supplies voltage to the device under test from its internal batteries. Depending on the type of diode, you can expect to measure a voltage drop of 0.4 V to 0.7 V on a good rectifi er diode. LEDs (light-emitting diodes) often have a voltage drop of 1.8 V to more than 3 V. Advanced DMMs have suffi cient voltage to cause LEDs to illumi- nate when tested in the forward direction. If a diode is good, it should show no conductivity (infi nite voltage drop) in the reverse direction. Capacitor Test Mode Many DMMs also have a function for measuring capaci- tance. In the capacitor test mode, the DMM supplies voltage to the capacitor under test. Th e capacitance displayed on the DMM is typically measured in either microfarads (μF), nanofarads (nF), or picofarads (pF). T E C H T I P SI Conversions Capacitors are generally specifi ed in either μF or pF, but the DMM may display the reading in a different unit. Therefore, you may occasionally need to convert from one unit to another. One microfarad is 1000 nanofarads, and one nanofarad is 1000 picofarads. To convert between these units, multiply or divide the number by 1000, which moves the decimal point three places. When converting from a larger unit to a smaller unit, the number increases. When convert- ing from a smaller unit to larger unit, the number decreases. For example, to convert 4700 nF to micro- farads (smaller unit to larger unit), move the decimal point three places to make the number smaller: 4.7 μF. To convert 6.8 nF to picofarads (larger unit to smaller unit), move the decimal point three places to make the number larger: 6800 pF. Before testing a capacitor, de-energize the circuit. Capacitors that have been in an energized circuit may retain their charge for a certain period of time after the circuit is de-energized. Always discharge a capacitor immediately before testing by shorting it across a resis- tor. Th e NEC recommends using a resistor of 20-30 kΩ with a rating of 4 W, but your particular needs may vary depending on the capacitor being discharged. If a capacitor remains in the circuit during testing, the infl uence of other components and capacitors may aff ect the DMM measurement. It is best to remove the capacitor to be tested from the circuit and then discharge it before attempting to test it. S A F E T Y N O T E Capacitor Test Mode When measuring a capacitor, never apply the test probes to an energized circuit. If you do, the DMM may be dam- aged and you could be injured. Also, be sure to discharge the capacitor after the circuit has been de-energized. If you fail to discharge the capacitor, it may still hold a charge that can damage the DMM and cause you injury. Copyright Goodheart-Willcox Co., Inc.