AC Voltage Measurement Error

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bilal shareef

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Hello Every Body,

I am making a voltmeter that can measure the AC voltages from 0 VAC to 115 VAC accurately using microcontroller. Below is the circuit for converting AC to DC for microcontroller.

I am using my multimeter to cross checking my voltmeter. Now I am having a problem that when I increase the AC voltages like around 90 to 115 VAC the error occurs in the reading of my voltmeter it actually give less value from the actual voltages that my multimeter shows and when I decrease the AC voltages like around 0 to 15 VAC my voltmeter gives more value from the actual value that my multimeter show.

Please tell me how can I remove this error completely and get actual voltage values with out any error ?
 

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What is the frequency of the voltage because improper value of capacitors can screw up whole things. just in your case for example the input impedance from source side is 176.8359 arg(90.28) & not even close to k\[\Omega\] range..
 

Your circuit should give out ~ 1/20 X Vpeak, V peak = 110 X 1.4 = 154, output voltage = 154/20 = 7.7 V. I presume the input protection diode in the micro are conducting and pulling the voltage down. Change the 1.8K to a 1K with a 1K variable resistor in series so you can set the range to some thing convenient with an output less then 5V. Say, 160 Vpeak :5 V, 32:1 attenuation. (you can work out the exact value )
With these circuits they will read LOW at LOW input voltages, as the the diodes drop a fixed voltage of about 1.2 -1.6V, you will get a zero reading for 1V input, .4 for 2V input 2.4 for 3V...
Frank
 

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