Copyright Goodheart-Willcox Co., Inc. 174 Essential Electrical Skills for HVACR: Theory and Labs One percent flows through the coil and deflects the needle to the 10 mA on the display. Shunt 2 is used for a 100 mA range. A 100 mA measured current causes 1%, or 0.001 A, to flow through the coil. The remaining 99% flows through shunt 2. Current measurement requires an in-line connection. This means the meter must be connected in series with the load. The inductive clamp is the safest and most preferred method to check large currents since a direct connection is not required. See Figure 10-5. The conductor being mea- sured acts as a primary winding in a transformer. The ris- ing and collapsing magnetic field induces a current into the clamp. The clamp acts as a transformer secondary and passes this current into the meter movement. An add-on inductive clamp can be connected to many VOM meters, or combination units are also available. 10.1.2 Voltage Measurement Voltage can be measured by placing the VOM in parallel with the power source. A resistor is placed in series with the measured voltage and the meter coil. Like current measure- ments, ac voltage is rectified to dc voltage. See Figure 10-6. In this example, the coil has a dc resistance of 1000 Ω, so a series resistor of 49,000 Ω is required to limit the coil current to 0.001 A when 50 V is measured in the 50 V range. The 50 V is the maximum scale value and is the maximum voltage that can be measured. Higher voltages cause movement damage. To use the 250 V range, a 249,000 Ω resistor is required to limit current to 0.001 A. If the meter is placed across 250 V, then 249 V is dropped across the series resistor, and 1 V is dropped across the coil. The pointer is at full deflection when 0.001 A flows through and a corresponding voltage drop of 1 V. Regardless of the number of ranges, the coil current and voltage drop across a coil does not exceed its limits. The current does not exceed 0.001 A, and the voltage drop does not exceed 1 V for the movement coil in this example. Goodheart-Willcox Publisher Figure 10-5. An inductive clamp is used to measure current without having to place the meter in series with load. Transformer Primary Secondary Clamp jaws secondary Wire primary Goodheart-Willcox Publisher Figure 10-6. A voltage measurement using two ranges. The meter is configured to measure voltage. Note only two ranges are shown for simplicity in this figure, but actual meters have multiple range selections. Coil Range selector 49kΩ 249kΩ Common probe + Probe Power source 1kΩ Meter in parellel with source or load to measure voltage