Measuring PWM with an Oscilloscope
It is the process of reading the frequency and duty ratio of the pulse train at the driver output from the waveform. The frequency is found by measuring the period and taking its reciprocal, and the duty ratio by dividing the pulse on-time by the period. Because an averaging multimeter only shows the average voltage, it can never give these two pieces of information.
For the measurement the probe tip is connected to the motor output terminal and the ground clip to the driver's power chassis. The time axis is set so that a few pulses are visible; on a 20 kHz signal, 10 microseconds per division is a good start.
- Frequency: Measure one full period and take its reciprocal. A period of 50 microseconds means 20 kHz.
- Duty ratio: Divide the time the pulse stays high by the period; 25 microseconds on with a 50 microsecond period is a duty ratio of 50%.
- Average voltage: It is close to the duty ratio multiplied by the supply voltage; a 50% duty ratio on a 24 V supply produces roughly 12 V average.
The long coiled lead of the ground clip produces large ringing on high-frequency edges; use a short spring ground tip if possible, otherwise part of the oscillation you see is the probe's own doing.
When measuring, make sure the driver chassis is at the same potential as the ground of the measuring instrument.
Context where this term is used: DC motor and driver fault diagnosis matrix