Measuring 220 AC with an ADC

[abahler]

I need a simple way to measure 220AC with an ADC with a five volt range.

 

[mr_byte31]

you must use a step down transformer
ex 220/5 then use ur ADC but it will measure instantaneous values
is that what u need?

 

[A.Anand Srinivasan]

i dont understand your question properly..... r u planning on a circuit which gives the AC value of the signal in the range of 0-220V using a ADC circuit.........

 

[ronyutp]

You must be very carefull. Your need a step down transformer (a 220VAC/5VAC), but also you need to avoid any negative voltage in your ADC input.

Please refer to:

Measurement Systems: Application and Design

Ernest Doebelin

 

[abahler]

you must use a step down transformer
ex 220/5 then use ur ADC but it will measure instantaneous values
is that what u need?

What do you mean by instantaneous values?

 

[A.Anand Srinivasan]

by instantaneous value he means the value at the particular instant of time..... he just means that the ADC will sample the AC signal and give the digital value corresponding to each sample and is that what u need........

 

[abahler]

Well here is some things I've come up with.
The voltage is more like 240 than 220 and it is made of two 120 lines with a ground.
(The ground is a center tap in the transformer. It's US 240 if you know what I mean.)
So how do I read the voltage? I'm thinking of a rectifier and voltage divider circuit to take it to the range of 5VDC and then do a little math in my software to get the AC value. Will this work?? The accuracy only needs to be within a volt or so.

 

[shortcircuit]

Seems u want to measure the rms value of the sine wave input.. the method of rectifier followed by a voltage divider and C filter would work..once u read the dc voltage from rectifier from ADC, u can use a little math to get the rms.....
but please note that u will not have isolation between the power circuit(220 V AC ) and ur adc circuit.. .. so it depends on the application u use it for.. and be carefulll....

Assuming that the ADC doesnt load the rectifier+C filter output, The DC o/p(Vdc) is equal to the peak of the AC input.... so the rms voltage of input AC is....

Vrms =(Gain factor calculated from Voltage divider)* Vdc/sqrt(2)

 

[vaibhav.k]

U have to stepdown transformer then.Dc voltage adder then u can connect it to unipolar ADC with protection of
transorbs and MOV.

 

[A.Anand Srinivasan]

abahler
that idea of your idea of your will surely work.... take care of stepping down the voltage of input to the required level of below 5V and also take care of rectifying the signal because if you rectify it after stepping down and feed it to the ADC the voltage drop of about 0.7V of a ordinary diode can cause a large error in the output obtained....

 

[abahler]

So what is the isolation needed for?
And also I'm hearing two options, one is just run it through a rectifier and voltage divider and the other is use a step down transformer.
So I don't know what to do.
Does the transformer act like a voltage divider with a ratio?

 

[A.Anand Srinivasan]

the requirement of the isolation is that 220V is a little high and hence any problem in that like spike, short circuiting etc is gonna burn out your circuitry(semiconductors)....
also it ensures your safety from shock hazards....

the transformer can act a voltage divider with a ratio and it is basically the turns ratio.... you can run the input through a rectifier and then use a voltage divider but the voltage is high and hence there would be a lot of power dissipation reducing the efficiency of the circuit...

 

[abahler]

Ok I'll use a transformer. the question that leaves me with though is how do I know what the measured voltage is in relation to the ADC reading.
Say I have everything set up so the input to the ADC is 2.5VDC when the
input to the transformer is 240VAC. Now when the ADC reads at 2VDC how do I know what the trans input is.
See what I mean?

 

[Rishi2530]

hi
take samples between two zero cross of input signal.
then add all samples value and divided them with no of samples between two zerocross.

 

[A.Anand Srinivasan]

ADC has a uniform resolution... so just measure the output of the ADC when the transformer input is zero volts.... now dividing the difference between output for 240V and 0V divided by 240 gives the resolution..... now divide your output by the resolution to the value of your input voltage....