what is the working principle of an r.m.s.voltmeter?

Question

about the features and working principle of an r.m.s. voltmeter

Answer 1

A voltmeter is a device that measures the voltage passing between any two points, typically used to check electric circuits for power. It works because it is a high-resistance ammeter that is actually measuring the amount of current that goes through it despite resistance.

One example of a voltmeter, is a moving-coil galvanometer, which works by measuring the rotation of a coil of fine wire suspended in a magnetic field; the angular rotation is directly proportional to the current running through the coil. Series resistance is added to the coil to make the rotation proportional to the voltage.

Another type is the potentiometer, a length of uniform-resistance wire, film, or other substance combined with a "wiper" that can create a short circuit on any part of the substance; this changes the effective resistance. The wiper's position can be changed by the user until the measured resistance is balanced, with no detectable current; the uniform-resistance wire is pre-calibrated so that the user knows what the voltage must be at that position.

There are several other types of voltmeters in use today, the most common being digital voltmeters that use the natural resistance of electric circuits.

Inexpensive multimeters measure the average positive voltage of a waveform and scale this value using the square root of two to produce a display value. Because this method only works for cosines, special purpose "TRUE RMS" voltmeters must be used for nonsinusoidal waveforms. One method of measuring true RMS voltage is to measure the heat generated when the signal passes through a resistor. Regardless of the shape of the input signal, the RMS voltage can be computed as the square root of the power dissipated in the test resistor.

"..only a TRUE RMS Voltmeter gives accurate results for non-sinusoidal waveforms..."

Answer 2

A simple non-RMS AC voltmeter will report the average value of the rectified voltage multiplied by 1.11, which is the conversion factor to go from the rectified average value of an AC voltage to the RMS value OF A SINEWAVE. In other words, the value reported will only be the RMS value if the AC voltage is a sinewave. If the signal is not a sinewave, then the value reported will not be the RMS value, but simply the average value multiplied by 1.11.

A true-RMS voltmeter will use a different approach. It will contain circuitry that will:

Sample the voltage thousands of times per second-
Square that value and sum it with previous squared samples-
Divide by the number of samples used (average the squares)-
Take the square root and report it.

This is RMS conversion, and represents the power (rather than the weighted average) of the signal, that is, if the signal was a power source, how much power is it delivering to a load.

A common chip for this is the AD637 RMS-to-DC converter IC -
http://www.analog.com/en/prod/0%2C2877%2CAD637%2C00.html

The data sheet reveals the details of this process

Answer 3

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Answer 4

Vrms or Veff is the measure of effectiveness of the voltage sourse to deliver powerto the resistor or load


Vrms = Vm / root of 2(for sinosoidal function) , where Vm is the magnitude of the voltage
or

Vrms= whole root of (1/T integral v^2 dt)

Answer 5

True Rms Voltmeter

Answer 6

rms stand for root mean square voltage => the measure voltage = Peak voltage /root 2 ... since it work on heat disipated principle = i2R