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Eric is right on, I'll elaborate a little. Generally only higher-end meters have the functions in question, of which there are several possibilities that are related but different.
1) A pulse-width modulated signal is on for a period then off the rest of the time and may be displayed as a percentage (25% means on 1/4 of the time) or in degrees (90deg = 25%, 1/4 of a full circle) as is common for setting point dwell if you have an old-fashioned car.
2) The same signal may be displayed in ms (milliseconds) of on time, which tells you how long it is on but not how long it is off, so is not directly equivalent to a percentage or degree reading. If you know the on time and the frequency, you can calculate the percentage of on time, for instance 250 ms (1/4 second) of on time and a frequency of 1 Hz (Hertz, the modern way to denote 1 cycle per second) = 25% on time; 250 ms and 2 Hz = 50% on time.
The above measurements are accurate for square waves or sine waves. They are not so accurately displayed on most multimeters for irregular waves such as fuel injector pulsewidth. An oscilloscope is necessary for accurate display of pulswidth for that type of signal.
3) The third allied function is frequency. This is displayed in Hz, ie 60 Hz if you stick the probes into a wall plug, or alternately on my automotive meter (Fluke 88) as RPM, which displays house current as 60 x 60 = 3600 RPM, but is of course really intended to measure engine speed.
As Eric says, you can make a pretty good estimate of percentage of on-time for a square wave (many automotive signals, such as outputs from the ECU to operate PWM devices such as solenoids) by reading the signal as a DC voltage. If engine-on voltage is 14.4V and you read 7.2V at a controlled device, on-time is 50%. This will not be accurate for irregular waveforms such as fuel injectors.
What do you want to measure?
posted by 198.69.250...
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