Does a multimeter measure rms or peak

When an AC voltage or current is measured with a multimeter, the meter reading is the "RMS" or "root mean square" reading. Sometimes the RMS value of the AC voltage or current is also called the "root mean square". This means that the RMS value of the AC voltage or current has the same effect as the same value of the DC voltage or current.

Let's take an example. For a 10V DC supply connected to a 10 ohm resistor, the power dissipated by the resistor is.

p = v 2 /r = 100/10 = 10 w

If you replace a 10V DC power supply with a 10V RMS AC power supply, the power calculation will be exactly the same and the resistor will dissipate 10W. However, the AC voltage is constantly changing, so 10 W is the average of the measurements over a period of time. At one time the resistor will dissipate more power and at another time less.

Assuming the AC voltage supplied is a pure sine wave, the maximum AC voltage is 1.414 times the effective voltage, i.e. 14.14V. If we take the inverse of this, we can see that the effective value of the sinusoidal AC voltage is 0.707 times the peak value.

Most inexpensive meters use the averaging technique to find the root mean square value of an AC voltage or current. This method will give you an accurate reading if you are measuring a sinusoidal AC voltage, but if the AC waveform or current you are measuring is not pure sinusoidal, the reading will be inaccurate and generally low. Depending on the type of AC waveform, the reading may be as much as 40% lower than it should be.

If you need to measure the voltage or current of an AC signal that is not a pure sine wave, such as when measuring the output of a variable speed motor controller or an adjustable heating controller, then you will need a "True RMS" meter. A True RMS meter takes the square of the instantaneous value of the input voltage or current, averages this value over a period of time and then displays the square root of this average.

The bottom line is that better results are obtained with a "True RMS" meter. Better readings pay off in the long run, whether these measurements are made in the lab or in the shop.

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