Many advanced digital multimeters measure the duty cycle -- the on and off period of a square wave -- and display the results as a percentage. A duty cycle of 60 percent may indicate the voltage is on for 60 percent of the time and off for 40 percent. Control systems found in cars, industrial processes and computer power supplies all rely on duty cycle pulses.
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Read the frequency of the circuit under test before performing the duty cycle measurement. With the Fluke 87, for example, connect the test leads and select AC volts on the mode selector switch. Press the Hertz push button and the meter will display the frequency up to 200KHz. If it doesn't trigger reliably, manually select a lower voltage range.
Turn the mode selector switch to the duty cycle setting. On some advanced multimeters, you'll find this function under one of the settings or options menus. With the Fluke 87, pressing the audible signal push button changes the triggering slope of the meter. This means the displayed percentage is selectable for on time or off time.
Measure the duty cycle of a sine wave to reveal distortion. Set up the meter to measure duty cycle, then look for any deviation from a 50 percent reading. Ideally, any sine wave is at 50 percent, so a change indicates distortion. This checks AC voltage inputs to a power supply or audio from an audio generator. Multimeters measure frequency accurately up to 200KHz, but audio and most control systems are well below that limit.