Multimeters – Measuring Voltage
Voltage measurements are usually made to determine source voltage, voltage drop, and/or voltage imbalance. Be sure to always connect the multimeter across (in parallel with) the circuit being measured. Know the capabilities and limitations of your multimeter before attempting any measurements. Read and follow the manufacturer’s instructions.
See Figure 3-24. Make voltage measurements as follows:
1. Use the function/range switch to select A C or DC volts. If using an analog meter, always select a range higher than the highest anticipated reading. For example, if you expect to measure 24 volts AC, select the 300-volt A C range on the meter. Once a reading is obtained, switch to a lower range when making the measurement. For accuracy, select a range where the meter pointer reads in the mid to upper half of the selected range scale.
2. Plug the test probes into the meter jacks. Usually the black probe is connected to the common (COM) or minus (—) jack and the red probe to the plus ( + ) or V-Ohm jack.
3. Connect the test probe tips to the circuit in parallel with the load or power source. Measurement is easier and safer if an alligator clip is used on one of the leads. Be sure to shut off power to the equipment before attaching the alligator leads.
If using an analog meter to measure DC voltage, you must observe correct polarity (+/—). Connect the red test probe to the positive side of the circuit and the black test probe to the negative side or circuit ground. If you reverse the connections, the meter movement will go off the scale in the opposite direction and damage to the meter may result. If using a digital meter with auto polarity, the reading will display a minus sign to indicate negative polarity.
4. View the reading on the digital meter readout. Be sure to note the unit of measurement indicated. If using an analog meter, read the voltage value indicated by the pointer on the A C or D C voltage scale. Make sure to use the scale that matches the selector switch voltage setting.
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