Copyright Goodheart-Willcox Co., Inc. Chapter 10 How Electric Meters Work 175 10.1.3 Resistance Measurement The same meter movement used for voltage and current is used for the resistance measurement. This means there are the same coil current limits. The meter must be connected in parallel to the component. The resistance function configures the meter to supply power to the component being tested. The meter’s internal battery supplies the power for current to flow, Figure 10-7. The typical battery (dc) voltage is between 1.5 V and 9 V. When using the resistance function, the resistor under test must form a series circuit with the meter movement coil and shunt. Test probes are then connected together. At this point, the pointer must fully deflect to show 0 Ω. The total coil circuit resistance made up of the coil, variable resistor, and fixed resistor limits coil current to 0.001 A. The variable resistor, named the zero ohms adjustment, compensates for battery voltage loss. If 0 Ω is not displayed when test probes are connected together, the zero adjustment is turned until the pointer is over the zero mark. A weak meter battery does not allow full pointer deflection. When the test leads are connected to a component, current flows from the bat- tery through the parallel branches formed by the coil circuit and shunt resistor. Then it flows through the component under test and continues back to battery positive battery terminal. See Figure 10-7. A 1000 Ω resistor is measured by using the R × 1 and R × 100 range selections on the meter to show how the range setting affects pointer deflection. Maximum Voltage Ratings Always check the manufacturer’s maximum voltage ratings and all safety requirements for the specific meter used. Safety Note Goodheart-Willcox Publisher Figure 10-7. A resistance measurement using two ranges. The meter is configured to measure resistance. Range selector Common probe + Probe + – Coil S1 S2 Under test Zero ohms Variable resistor S1–R × 1 range shunt resistor S2–R × 100 range shunt resistor