Volt Meter Design using Pic or Arduino

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what does digital voltmeter measure when we apply any kind of voltage at its input.


Suppose we apply sine wave .
does he measure rms value .
if rms then how .

When we apply random wave like triac based dimmer output then what do they measure.
When we apply square wave at its input then what do they measure.


Basicall i want to design a voltmeter using pic microcontroller which should behave same as Market available voltmeters .
like
 

It measures the peak value (for ac) and scales it down to the RMS value for a sine wave. So the reading is perfect for sine waves, very wrong for square waves and somewhat wrong for triangle waves.
 

Hi,

It measures the peak value (for ac) and scales it down to the RMS value for a sine wave. So the reading is perfect for sine waves, very wrong for square waves and somewhat wrong for triangle waves.
Click to expand...

For sure not every volt meter works this way.
Every voltmeter of some quality should have written in it's datasheet how it measures this voltages.

There are "true RMS" meters but also others.

What calculation method you need depend son your applications... "true RMS" is not always the best...

Klaus
 

KlausST said:
There are "true RMS" meters but also others.
Click to expand...

You are right, of course. One of meters shown in #1 has true RMS written on it. What I mentioned will be then called as 'false RMS' !!

If the OP wants to compute RMS values (with a PIC microcontroller), it will be necessary to sample at least 10-20 points in a given cycle. If the frequency is high, sampling can become difficult. A 220V "square wave" with 20% duty cycle will give a RMS value of 44V but it may mean very little.
 

Hi,

A 220V "square wave" with 20% duty cycle will give a RMS value of 44V
Click to expand...
This is not true

A DC of 220V will give a true RMS value of 220V
The same signal with 50% duty cycle (0V, 220 V) will give 110V average and 155.5V RMS
20% duty cycle (0V, 220V) will give 44V average, but 98.4V RMS.

Klaus
 
Hi,

I forgot to mention...

It's not true that RMS is more correct than average.
..some applications need average...or better say the average value makes more sense
..and with some applications the RMS value makes more sense.

Formerly we used analog instruments... those with a needle to read the value.
* moving coil instruments show the average value
* moving iron instruments show the RMS value

Klaus
 

Thanks for your guidance.
I just want to make copy of digital VOLT meter suppose this one .
what ever this meter shows against pwm, AC dimmer output , sine , square , saw tooth, my meter should be equal.
what should i follow
RMS OR AVERAGE OR SOMETHING ELSE ?

You can choos another meter as well
 

Hi,

I assume only the manufacturer can answer this.
Or you open it an try to copy the circuit. (As long as you don't sell yor "copy")

Klaus
 

Is "Read the manual" asking too much? For the said UT33A, the AC measurement is specified as
Displays effective value of sine wave (mean value response)
Click to expand...

That could be expected for a non "true rms meter", it's measuring averaged rectified value, result is corrected to be equal with rms for sine voltage. Example, average rectified value of 230 Vrms sine is 230*√8/pi = 207 V. Value multiplied with 1.11, displays 230 V.

No commercial instrument is measuring peak value because it would be too sensitive to waveform distortions.
 

i have made volt meter using this method .
1. find out Vmax of sine wave by taking 500 samples .
2. V=Vmax * 0.707

it works on sine wave but
when i try to measure ac dimmer output or square wave of ups then it fails.
 

Hi,

it works on sine wave but
when i try to measure ac dimmer output or square wave of ups then it fails.
Click to expand...

When you read all our posts before, what else did you expect?

****
If you need true RMS then implement true RMS. --> The result is you have a true RMS measuremnt tool. The results are true for a wide variation of input signal.
If you need average, then implement average. --> the result is you have built an averaginv voltmeter. The results are true for a wide variation of input signal.

Now you built the "peak" solution, that is only true for exactely one input waveform: undistorted sine.
But dimmer output and UPS output is not undistorted. --> Obviously you will see wrong readings.


Klaus
 

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