How to Measure Negative Voltage

By William Hirsch
Use a digital multimeter to determine voltage.
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Learn to measure negative voltage of a electronic device to be sure its functioning properly. Voltage is a measure of a device's ability to pull or produce electrical energy. In the case of a device in a circuit, such as a current limiting resistor, the device draws voltage where a battery creates voltage. Voltage is measured across the positive and negative terminals of a device. Depending on how the wiring for the measurement is made, a negative voltage should be measured.

Turn on the digital multimeter. Switch the multimeter measurement dial to either the alternating current (AC) voltage or direct current (DC) voltage setting denoted by a "V" with wavy lines above it or straight lines, respectively. For example, if measuring the voltage of a standard 9-volt battery select DC voltage. If measuring the voltage of a wall outlet, choose AC voltage.

Connect the red probe of the multimeter to the positive port of the device. Connect the black probe of the multimeter to the negative port of the device. Frequently the negative port is labelled "COM," for common.

Touch and hold the red probe of the multimeter to the negative terminal of the device being tested. Now touch the black probe of the multimeter to the positive terminal of the device being tested. The multimeter should read a negative voltage, or the device being tested may be damaged. For example, a 9-volt battery would produce a reading close to -9 volts.