Copyright Goodheart-Willcox Co., Inc. 180 Essential Electrical Skills for HVACR: Theory and Labs Currents below 0.5 A do not induce a strong enough current into the inductive clamp. The ten loop coil method must be used, which was first introduced in Lab 7.1, Induc- tive Reactance and Transformer Evaluation. The typical HVACR clamp measures up to 600 A, although greater capacities are available. When using the inductive clamp, only one wire can be placed inside the jaws. The wires that have opposite current flow cause the magnetic fields to cancel out, resulting in no induced current in the clamp. Even if the current flows in the same direction, the reading can be unreliable. The rule is one wire only. Avoid placing the clamp near strong magnetic fields. The magnetic field generated by transformers, solenoids, relays, and contactors alter the measurement. 10.2.3 Voltage Measurement When the meter is configured for voltage measurement, current flows through a high resistance, Figure 10-14. A high resistance in the millions of ohms limits the current for the delicate electronic components. The series resistor passes current through the fuse to feed the A/D. The digital information is fed to the microprocessor. There, the current and type ac or dc, selected by the user, is processed and sent to the display unit. Digital voltmeters use an insignificant amount of power when connected to a cir- cuit. They are high impedance measuring devices, and thus, they do not become part of the circuit under test. This is different from the low-impedance analog voltmeter. When measuring voltage, the meter probes are placed in parallel across a voltage source, load, or switch. Note: Voltage is measured across, and current is measured through loads, per industry terminology. Current flow through the meter is shown in Figure 10-15. When the meter’s probes are across the source voltage, current flows only through the meter. For the meter testing, voltage across the open switch current flows from the source through the meter and load back to the source to complete the circuit. In Figure 10-15A, the meter’s internal 10 MΩ resistor is in series with the load’s 1 kΩ resistance. In a series circuit, the largest resistance drops the most voltage. In this case, the meter drops (and displays) 119.99 V while the much smaller load resis- tance drops 0.01 V. In Figure 10-15B, most of the current flows through the closed switch. Because there is contact resistance, a minute amount of current flows through the meter and the meter displays 1.2 V. The switch resistance is in series with the load, so the load drops the remaining 118.8 V. Microprocessor 120.0 Vac A/D Fuse L1 N 10 MΩ 3 V Frequency The A/D reads the current through a 10 MΩ resistor and the processor calculates E = I × R While in Volts mode, some meters can also measure frequency and duty cycle % dc = 50% Probes are not polarity sensitive for ac. However, there are some meters that are sensitive with low voltage ac if one of the low voltage legs is tied to chassis ground requiring the red probe placed on the hot leg. For dc, a negative sign will be displayed if the polarities are switched. Voltage Goodheart-Willcox Publisher Figure 10-14. The digital multimeter is configured to measure voltage. Frequency and duty cycle can also be measured while using this function with some meters.