Copyright Goodheart-Willcox Co., Inc. 182 Essential Electrical Skills for HVACR: Theory and Labs 10.3. Additional Multimeter Functions The capability to miniaturize electronic components has made way for using special- ized microprocessor integrated circuits that house multiple meter functions in one test instrument. The most common functions found in a test instrument are audible continuity, diode, capacitance, frequency/duty cycle, and noncontact voltage. 10.3.1 Audible Continuity Test The audible continuity test is a quick way to verify continuity, or continuous flow cur- rent, between two points. This could be used for a length of wire or between switch con- tacts. Internally, the meter is configured similar to the resistance function. The meter supplies battery power to the component under test. The returning current is sampled by the A/D and sent to the processor. If the resistance of the component tested is less than a specific value, then the audible buzzer sounds off. If the resistance is greater, there is no sound but the resistance is displayed. After a certain value of resistance, the display will show OL. 10.3.2 Diode Test Recall from Chapter 6, Alternating and Direct Current that diodes allow current to flow in one direction from cathode to anode. Diodes are used to rectify ac to dc power. When the diode test function is selected, the meter configures itself to supply battery power to the diode. The red positive probe is connected to the diode’s anode, while the black negative probe is con- nected to the cathode. See Figure 10-17. Since the diode is forward biased, current flows through a good diode, and the meter dis- plays the voltage drop across the diode. The value is between 0.4 to 0.7 V. OL indicates a faulty diode. The probe connections to the diode are reversed so that the diode is wired in reverse bias. Current shows no flow and is indicated by OL on the display. If current flows and the meter displays a voltage drop, the diode is faulty. Resistance Values The owner’s manual specifications must be checked to find the resistance value when the audible alarming feature will not sound. A typical value is between 40 Ω and 60 Ω. When this resistance value is exceeded, the meter displays the ohmic value but up to a specified amount. A typical value is about 200 Ω and the display will show OL. The technician should verify the condition of the component with the ohmmeter before making a troubleshooting decision. Pro Tip Goodheart-Willcox Publisher Figure 10-17. A—The diode is forward biased, and the meter displays voltage drop across the diode. B—The diode is reverse biased, and the meter displays OL. Forward Bias Voltage drop Diode symbol Positive lead Anode Cathode Negative probe A Reverse Bias Over the limit No current flow Negative lead Anode Cathode Positive probe B