RMS to ADC - measure ac voltage using Arduino.

Thread Starter

Randa Mohamed

Joined Jun 19, 2015
25
Hello everyone

I'm trying to measure ac voltage using Arduino, I'm using a step down transformer (12Vrms output) as a voltage sensor, i'm having trouble in conditioning this signal to the ADC pin. Please give me your suggestions.

Thanks
 

MikeML

Joined Oct 2, 2009
5,444
What aspect of the AC voltage are you trying to measure? Vpp, Vrms, Vave???

@MaxHeadRoom 's method is rife with errors. If you step down the voltage, the forward drops of the rectifiers will make the voltage at the ADC input very non-linear with respect to the actual amplitude of the AC waveform. You would actually be much better starting out with hundred volt AC signal, full-wave rectifying it, filtering it, and then using a voltage divider to adapt it to the allowable range for the ADC. This method makes the forward drops of the rectifier an insignificant fraction of the total voltage...

The alternative is to start with a small AC voltage, and then use an opamp precision rectifier circuit that does not suffer from the rectifier forward voltage drops.
 
Last edited:

Thread Starter

Randa Mohamed

Joined Jun 19, 2015
25
@MikeML
I'm trying to measure Vrms only, I just don't get how would the ADC measure the voltage if i rectify it and smooth it using the capacitor. It looks more like im building a DC source just without the voltage regulator IC.

I'm actually trying to measure mains voltage and i need isolation that's why im using a transformer.

I will research about precision opamps, thanks a lot
 

MikeML

Joined Oct 2, 2009
5,444
It is quite difficult to measure true RMS. If the waveform is sinosoidal with not much harmonic distortion, it is much easier to use an opamp precision rectifier followed by a filter to get the average value, and then just multiply by the appropriate scaling factor and call it good...

In that case, use your existing transformer, followed by a resistive voltage divider to reduce the amplitude of the signal. Use modern rail-to-rail opamps, and you can likely power them from your Arduino 5V...
 

Thread Starter

Randa Mohamed

Joined Jun 19, 2015
25
@MikeML

Which software did you use here ?

From the picture you posted, i should do the following:

Transformer(12V) to Voltage Divider(5V) to Precision Rectifier then to Low Pass Filter to spans matching circuit to ADC
 
Top