i can't tell the difference of true RMS and RMS. Is there a false RMS ??

## what is the difference between true RMS and RMS?

Depends on the exact context we're talking about. In terms of a multimeter, the usual cheaper style of meter 'fakes' the RMS reading by just scaling the peak measured value ( times 0.707) - this is fine if the measured value is a pure sinewave. Things get inaccurate if the measured value isn't a sinewave. Thus we have 'true RMS' that actually calculates the 'true' rms value regardless of the waveform. Since the RMS value is taken to be the AC voltage that creates the same heating effect in a resistor as compared with a DC voltage. Therefore 240VAC (RMS) is actually around 320V peak but heats a resistor the same as 240V DC, so true RMS meters can sometimes use a cell to measure the heating effect in order to measure 'true RMS'. Analog Devices have a number of chips that can derive true RMS. Another method is to use an ADC to sample the waveform and to perform the RMS calculation to obtain true RMS. RMS is simply the square root of the mean sum of squares. Say we have 10 samples, square each value, add them together, divide by 10 then find the square root of the value.

There is RMS that is only approximated... IIRC, RMS on most voltmeters only approximates unless the signal is a true sinewave.

And the different meters (methods) have

different ranges where they are more or

less accurate (true) :

frequency range,

crest-factor,

......

There are true rms analog chips.... they square the input (multiply it by itself, not clip it to a square wave), average this 'power' signal, then square root the avg. If you measure an AC dimmer waveform that looks like a chopped off half a sine wave, this gives you the equivalent DC level to the wierd waveform.

There were several threads relating to this exact same subject about a year ago. Do a forum search on "RMS, " "True RMS " or Google on the same key works and read all about it.

Extra! Extra! Read all about it!!!