# To read 120 V using Arduino

1131 Views 6 Replies 4 Participants Last post by  splatz
I have been involved in a project where I am reading the 120 V coming from a supply. Since input is AC, firstly I converted it to DC using full bridge, now that I used a voltage divider. I am using two resistors one of 20 k and otherone with 800 ohm, As per this VDR calculator for input of 120 there is very small current flowing towards the load, tell me what modification should I do so as to develop a 120 V meter through the Arduino :vs_sad:
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· Hackenschmidt
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hardworkingstiff

#### Electrical-EE

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Thank you for comment

Thank you very much for comment, but one last question, the current is very small in my case will it still work?

#### splatz

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Thank you very much for comment, but one last question, the current is very small in my case will it still work?
The Arduino input is not measuring the current, it's measuring the voltage 0-5V coming from the divider. You're using the voltage divider to scale the voltage coming from the bridge . Your leads to the Arduino inputs will be in parallel with the load in the diagram on the page you linked.

#### RePhase277

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I think your resistor values are bad. The drop across the 800 ohm resistor will be 4.6 volts when your supply is 120 exactly, and that's too close for comfort. I think you should shoot for something like 200 volts max and adjust your code accordingly.

splatz

#### oliquir

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are you able to measure rms value directly with the arduino (microseconds samples and calculated rms) or you will pas the ac through an opamp to make the rms value

#### splatz

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are you able to measure rms value directly with the arduino (microseconds samples and calculated rms) or you will pas the ac through an opamp to make the rms value
It's my understanding the arduino has a surprisingly high sampling rate, but maybe the opamp would be the better way to go...

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