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Reading AC Voltages with a Microcontroller

Posted onSeptember 11, 2011bypaul

This article shows an op-amp based circuit to transform a high-voltage AC waveform intosomething that a micro-controller can measure directly.

DISCLAIMER: This article is provided as-is, with no guarantee that the circuit will work

in your situation, or that it will be safe. If youre going to attempt to measure mains

voltages, do so at your own riskif you dont understand the math or circuit diagram,

then you probably shouldnt be working with mains anyway. I am not responsible for your

mistakes. You have been warned.

Background

A few years ago, I was involved in a project that needed to measure mains voltages in NewZealand, which has a 230 V, 50 Hz supply. My circuit needed to be as cheap as possible, and

didnt need to be electrically isolated from the supply. I needed to measure the voltage using amicrocontroller that had an input range of 0 to 5 volts, and needed to measure voltages from -375

V to 375 V (or 265 VRMS).

To convert this large, alternating voltage range to something compatible with the micro, Ive

seen several approaches on the web. Most do something along the lines of rectifying the AC

voltage before feeding it into the micro. While that may be fine for some, it creates ambiguitybetween positive and negative voltages. I needed something better. It would be great to have a

circuit that multiplies the input voltage by a constant, then adds half the supply voltage of the

micro. It turns out you can do that with a single op-amp, and three resistors (five if you donthave a handy half-supply reference). Heres how:

The Circuit

AC voltage measurement system withAVR microcontrolleror any other microcontroller.

This task can be divided into two sub tasks.

1)Attenuating Line voltage to the level that can be applied to microcontroller pin.Here is the circuit used to measure AC line voltage (230V,50 Hz) this circuit converts line

voltage to the level of 0 to 5 volts. This can be understood by most of microcontroller and ADCs.

Here is an op amp based circuit to do the job.

This a simple adder circuit to add some offset to your signals.

2)Software to measure AC value

You can directly measure DC value with ADC if you take a reading but that is not the case with AC

signals.
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To calculate RMS value of AC signals you should sample at least 3-4 cycles and then calculating RMS

value.

If youve used op-amps before, this should look reasonably familiarit is pretty much the

standard inverting amplifier. However, the key differences are that the non-inverting input is tiedto half of Vcc instead of ground (so if you have a 5 V microcontroller, tie this to 2.5 V), and the

extra resistor pulling the inverting input up to Vccthis resistor should be exactly the same

value as the other resistor called Ri.

The operation that this circuit performs is:

This is precisely what we want. Say we would like to measure mains voltage in New Zealand(remembering the disclaimer above), with a range of -375 V to 375 V, and the microcontroller

can accept voltages from 0 V to 5 V. By choosing Ri to be 150 k and Rf to be 1 k, the gain

from the formula is 1/150. IMPORTANT: unless something is wrong, the voltage at the inverting

input will be 2.5 V (since both inputs will be the same, unless the op-amp is saturated). Thatmeans that the resistor Ri will need to be large enough that very little current flows when Vin is

large.

Heres a quick table for Ri = 150 k, Rf = 1 k, Vcc = 5 V.

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Vin (Volts) Vout (Volts)

-375 5.0

0 2.5

375 0.0

As you can see, an input waveform centred around 0 V is nicely converted to a much smallerwaveform, centred around 2.5 Vjust right for a microcontroller.

Proof

From Kirchoffs Current Law, the current into the inverting input node is, remembering that no

current flows into the op-amp:

This is simply the voltage across each resistor connected to the node, divided by that resistors

resistance.

Simplifying:

And we get what we expect.

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Notes

Remember that an op-amp is not perfect. For best results, get a rail-to-rail one (that can output

voltages near the supply rails), and make sure youre not driving too much current through Ri.Most importantly, do not connect anything to mains unless you know what youre doing! Dont

blame me if you blow something up.

set the values of R1 & R2 by measuring Vx across capacitor

5(R1 + R2) = R2 Vx

0 volts means 0v main supply

5 volts means 220v main supply

measure the value of vx via digital volt meter

for eg if vx = 7v

then select

R1 = 1k , R2 = 2.5k

here it has assumed that the Maximum Main AC voltage is 255v

u can change the max limit according to ur requirments

Main_max_volt = 255

(5/Main_max_volt) x present_main_volt = volt_to_ADC

for eg::

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if present_main_volt = 0v then

5/255x0 = 0.0v & ADC = 000


5/255x160 = 3.137v & ADC = 160


5/255x200 = 3.921v & ADC = 200


5/255x220 = 4.313v & ADC = 220


5/255x250 = 4.901v & ADC = 250

now u don't need any calibration just simply convert the ADC Register value to ASCII & display it on ur

screen thatz it

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