can i connect ADC or microcontroller ground pin to mains neutral?

[shaha20111]

i have this wattmeter circuit

it is directly powered by mains and neutral lines via 15V zener diodes, pin 7 is the output pin, this circuit can sense up to 2200W of power. now my question is, i want to connect the pin 7(output pin) to ADC input, and ADC output to the 8051 microcontroller, but my problem is that where to connect the ADC ground pin and microcontroller ground pin?? can i connect it directly to the neutral line?? please friends help me in this circuit.

 

[ronsimpson]

Yes you need to connect the ground of the 8051 to pin 4 of the AD633.
BUT
Now your 8051 is at neutral (or line) and not at earth ground. Neutral is not at ground. Line and neutral are some times mixed up.
If you 8051 is only diving a LCD display and is isolated from humans then no problem.
If the 8051 is connected to USB and to a PC then you just shorted neutral to ground through the PC. You probably shorted out "0R4 40W".

 

[crutschow]

Run the ADC from the mains power with the common connected to mains neutral but connect the ADC digital output through optical isolator(s) to the micro. That way you can power the micro from a normally isolated DC supply and avoid the electrocution danger of connecting the micro directly to the power line.

 

[shaha20111]

THANK YOU ronsimpson andcrutschow, i have create a diagram from above solutions... is this correct???

 

[NorthGuy]

If you already have an MCU, you could dramatically decrease part count if you measure voltage and current directly and multiply them at a frequency of few kHz. Many MCUs will have built-in ADCs to do the measurements. In this case, you only need few resistors ans an op amp to amplify current sensing.

 

[crutschow]

THANK YOU ronsimpson andcrutschow, i have create a diagram from above solutions... is this correct???

That looks okay. But the circuit is live to the mains and the complete circuit must be well isolated from any possible external contact. You also must use an isolation transformer to power the circuit for testing and probing.

 

[kubeek]

I would rather use an oridnary tranformer to measure voltage and a current transformer to measure current. The circuit will be much safer that way because you will have it galvanically isloated from the mains voltage.

 

[ronsimpson]

Looking at your diagram:
The ADC seems to be I2C so; Use a ISO1540 IC to isolate the ADC from the 8051. The Isolation IC would allow the 8051 + LCD to be at ground and the AD633 + ADC to live on the power line with no ground connection between them.

As NorthGuy said; you can use two ADC inputs and in software multiply the numbers to get the same output as the AD633.

 

[shaha20111]

Thanks to all of u friends... but i am still confused, as suggested by Kubeek i am planning to put CT(current transformer) and a normal potential transformer to sense current and voltage, and then i will apply this ti AD633, and i will power up AD633 with DC suppply so that the circuit will be completely isolated from main.. but i am not getting how to connect current tranformer to AD633, can anyone help me with circuit diagram?? please friends , can anyone show me the ckt diagram for it?
Thanks friends

 

[kubeek]

You will need a dual supply with +/-8 to +/-18V to power the mutliplier, then simply connect pins 2 and 4 to ground, and apply signal to pins 1 and 3. Also ground pin 6, or apply some offest to it to null the output.
The CT needs a burden resistor connected across it, so the ratio of the CT times the input current times the resistor value will give you voltage that goes into the input pin.
For the potential transformer just use a normal voltage divider to get the appropriate voltage at the input of the AD633. The circuit seems to have inbuilt 1/10 divider, so if you apply 1V to both inputs you should get 0.1v out.

 

[shaha20111]

Thank you Kubeek... please hav a look at the circuit. is it ok?? or any changes?? one terminal of CT is connected to pin 3, and another terminal to 0V, similarly 1 terminal of PT is to pin 1 of AD633 and another to 0V. Do i need a voltage rectifier(AC to DC) after PT ?? or no need??

 

[kubeek]

PT doesn´t need to to have dual winding, a single secondary is enough, so the divider goes simply between 12 and 0 taps.
The CT is ok like that, only I would add another divider to it so that you have some option to trim the total gain.
On the output you might want come RC filter to get the average value of the output, this could depends on what meter you are using.
2x18V transformer is overkill, since the rectified voltage will be 18*1,414=25V, so a 2x15V transformer will be enough.
Add a connecttion from pins 2 and 4 to the supply ground.

 

[shaha20111]

hi kubeek i made a ckt using CT and normal potential transformer and some voltage dividers, and its working fine and showing accurate results.. but the problem is that voltage is comming in -ve (minus voltage), how to convert this into +ve volt??? so that i can apply it to the ADC. Or is there any ADC which can read from -1.2mV to -3V...??

 

[kubeek]

Glad to hear it worked
Try reversing either the polarity of CT or PT. One of them is phased backwards so with resistive load you have allways one wave going positive and the other negative.

 

[shaha20111]

thanks u kubeek for ur reply.. but still it is showing -ve voltage at the output, any other way to make it positive??

 

[kubeek]

Do you have an oscilloscope to check what is happening on the inputs and outputs?

 

[shaha20111]

No kubeek i dnt have oscilloscope

 

[kubeek]

Well then try putting one of the singal on both of the pins and disconnecting the other and see what happens.

 

[shaha20111]

by applying one signal to both line it is comming positive...

 

[NorthGuy]

thanks u kubeek for ur reply.. but still it is showing -ve voltage at the output, any other way to make it positive??

May be kubeek didn't explain it fully. Take the CT off the wire, rotate it 180 degrees, then pull the wire through it.