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AC main voltage measurement

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cheetha

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Assalam-o-alekum

I am in need of measuring AC main voltages varying from minimum to 500V. I required to convert the AC main voltages into linear equivalent DC low voltage to feed the ADC of microcontroller.
i tried with voltage divider followed by instrumentation amplifier, the output of in-amp has capacitor with ground, this output is then feed to microcontroller. but this o/p is too non-linear and too unstabel i dont know why ....?
plz help!



thanks in advance
 
Obviously, an active "precision" rectifier will be required. Different AC quantities are available, averaged rectified value, peak value, RMS value. Averaged rectified value is giving best results if no true RMS circuit is required.
 
A full circuit of what your preliminary circuit is required, including component values, to help you determine what is wrong.

There are MANY possibilities that could cause unstable readings.

Additionally, we can give you hints how to measure 500V safely.
 

thanks alot
plz your hints are welcomed and I am in need of transformerless measurement with as simplified as possible circuitry..
 

Your requirements are still a little vague.

The simplest circuit consists of a single diode, a pair of resistors in a divider and a single capacitor. However, this circuit has several limitations.

For starters, the RC time constant to obtain a low ripple, would have to be at least ten periods. Meaning an RC constant of 200 msecs (assuming a 50 Hz powerline), which would have a long settling time. Quick line surges or sags would be effectively "filtered out".
The resultant DC voltage would be the peak of the waveform, whose ratio conversion of 1.414/peak = RMS value for a perfect sine wave you could perform on software....however any waveform distortion would create errors.

Last but not least, is the lack of isolation from lethal voltage levels.

But these limitations could or could not, be of consequence for your requirements, which have not yet been fully outlined.
 

Thanks alot
I am in need of a constant DC voltage that varies linearly with the i/p AC RMS voltages. So to use this voltage for AC voltage measurement through ADC of microcontroller
I tried with the voltage divider a capacitor but the voltage is not readable with multimeter it is too unstable, and I am unable to carry out my calculations.
Plz suggest design with component values that could work on distorted waveform efficiently…..

Plz help…!
 

thanks again
plz suggest op amp based circuits as cost and availability is concerned..
 

i have tried with voltage divider followed by peak dectector ciruit using op-amp, diode, capacitor and resistor, so the peak value can be measured, the output voltage is stable below 1.17V, After this voltage the output become unstable and starts oscillating continously, then i increased the value of resistor so make the values stable, i was successful for a instant but due to may be frequency fluctuations or other reasons (which i dont know) the output of circuit is become unstable, where as the multimeter reading is still quite stable.


plz help
 

Without knowing your circuit, we can hardly determine why it's "oscillating".
 

Yes, please post a schematic...hand drawn is sufficient. Indicate ALL COMPONENT VALUES, and where you are measuring the 1.17Volt, how are you powering the opamp and microcontroller.
 

I am powering the microcontroller from 7805 at 5V and op amp with 7812 at 12V. I am measuring voltage at the output of peak detector circuit,....plz find the attached circuit diagram


thanks.....
 

Attachments

  • Peak detector circuit.bmp
    2.3 MB · Views: 175

The circuit is unstable "by design", you'll need at least a small series resistor before the capacitor.

There may be also problems caused by negative input voltage to the OP.

I generally think that an averaged rectified value would give a more stable and reliable AC voltage measurement than a peak detector because it's less sensitive to switching transients and similar interferences.
 
Last edited:

FvM said:
I generally think that an averaged rectified value would give a more stable and reliable AC voltage measurement than a peak detector because it's less sensitive to switching transients and similar interferences.
Click to expand...

can u suggest efficient and reliable circuit op amp based

thanks
 

Any help ???????????
 

hi,
I suggest to use a transformer, with the adequate transformer ratio and a half graetz bridge , you can measure directly with the ADC module, and compute the voltage ( a 5 v zener diode can be added on the mcu's input pin for protection ........ and maybe a small low pass filter ..... )
 

i cannot use transformer for measurement circuit.
i am in need of opamp circuit that can give stable DC voltage through which input ac voltage can be measured accurately..
plz help!
 

let me see if I can do it with ASCII symbols

INPUT o--->|---^^^^-----.----^^^^----]
|
|
.-----|(-------]
|
|
o
OUTPUT

Diode: 1N4007
First resistor: 990k
Second resistor: 10k
Capacitor: 10uF
350 VAC input for 5VDC output

edit; the symbols are moved, the capacitor connects to the dot of the first branch
 
Last edited:

Hello cheetha
Did you connect your circuit ground to AC power system ? (example: circuit ground connected to neutral or connected to one of the three phases etc.....)
 

yes the DC ground and AC neutral are connected.... i tried with the diode and voltage divider circuit but it is unstable as i want constant stable DC voltage for accurate measurement purpose which changes linearly with AC voltages.... plz help
 

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