Help! Regarding AC voltage amplification(mV to V)

[mananshah93]

Hi! I want to convert or amplify AC voltage in the range of mV(2 to 20 mV)to 0 to 5 V , how can I do that??????
Another querry is that AC voltage amplified from the amplifier can be converted into DC????/How?

 

[ericgibbs]

Hi! I want to convert or amplify AC voltage in the range of mV(2 to 20 mV)to 0 to 5 V , how can I do that??????
Another querry is that AC voltage amplified from the amplifier can be converted into DC????/How?

hi,
Is the mV signal +/-mV about 0V.?
also do you mean an output of +/-2.5V centered on +2.5Vdc.?

The amplified voltage can rectified into dc.

What OPA's do you have access too.?

 

[mananshah93]

hi,
Is the mV signal +/-mV about 0V.?
also do you mean an output of +/-2.5V centered on +2.5Vdc.?

The amplified voltage can rectified into dc.

What OPA's do you have access too.?

Hey!
the voltage is AC so no +/- in that.
My querry is that I have 2 mV or above AC voltage as a INPUT. Firstly, I want to amplify it and then convert it to the D.C in range of 0-5V(DC), which can given to microcontroller.

Basically I am making circuit which can sense the AC current in the line., so I have attached one low value resistor across it which is giving me mV drop starting from 2 mV.

 

[ericgibbs]

Hey!
the voltage is AC so no +/- in that.
My querry is that I have 2 mV or above AC voltage as a INPUT. Firstly, I want to amplify it and then convert it to the D.C in range of 0-5V(DC), which can given to microcontroller.

Basically I am making circuit which can sense the AC current in the line., so I have attached one low value resistor across it which is giving me mV drop starting from 2 mV.

If you think there is no +/-V swing in an 'ac' signal I suggest you open your text book at page #1.

If you are sensing current by using a resistor in an ac line, the voltage will swing above and below zero volts.
If you tried to amplify such a signal you would require to offset the ac signal before amplifying or use an ac amplifier able to swing above below zero on the output, then rectify it.

I'll ask again, what OPA's do you have available.?

 

[MrAl]

Hi,

My question would be what kind of accuracy you are looking for.
Can the design put up with 10 percent, 5 percent, or does it
have to be more like 1 percent or even 0.5 percent.

 

[mananshah93]

Hi,

My question would be what kind of accuracy you are looking for.
Can the design put up with 10 percent, 5 percent, or does it
have to be more like 1 percent or even 0.5 percent.

Actually I want a voltage as a reference, so it should be linear rather than be accurate, actually it should be such that I can discriminate that it's x volt after amplification of 2mV and y volt after ampl. of 5mV (e.g.), means it should follow some equation.

 

[mananshah93]

If you think there is no +/-V swing in an 'ac' signal I suggest you open your text book at page #1.

If you are sensing current by using a resistor in an ac line, the voltage will swing above and below zero volts.
If you tried to amplify such a signal you would require to offset the ac signal before amplifying or use an ac amplifier able to swing above below zero on the output, then rectify it.

I'll ask again, what OPA's do you have available.?

you have taken it wrongly about that 'ac' signal, my meaning was there may be no terminal polarity when you check with multimeter.(rms value) and there must be positive and negative cycle in 'ac' thats why its(alternating current).

But i dont understand the term 'opa' you have referred...

 

[ericgibbs]

you have taken it wrongly about that 'ac' signal, my meaning was there may be no terminal polarity when you check with multimeter.(rms value) and there must be positive and negative cycle in 'ac' thats why its(alternating current).

But i dont understand the term 'opa' you have referred...

OK, no problem.
You understand that we have no idea of your project other than what you tell us.

OPA [operational amplifiers], some of the older types are not very suitable for this job.
So if you know whats available in your location a design could be drawn.

The other important question what is the 'ac' line, is it part of the mains wiring.? If its not mains what is the frequency.?

It is possible to amplify a 20mV ac signal to 5Vppk then rectify it to give about +5Vdc suitable for a PIC's adc.

 

[mananshah93]

OK, no problem.
You understand that we have no idea of your project other than what you tell us.

OPA [operational amplifiers], some of the older types are not very suitable for this job.
So if you know whats available in your location a design could be drawn.

The other important question what is the 'ac' line, is it part of the mains wiring.? If its not mains what is the frequency.?

It is possible to amplify a 20mV ac signal to 5Vppk then rectify it to give about +5Vdc suitable for a PIC's adc.

ya! you got it, it is mains wiring having 50Hz frequency (in India).
So my job is to monitor the value of current flowing through the line.

 

[ericgibbs]

ya! you got it, it is mains wiring having 50Hz frequency (in India).
So my job is to monitor the value of current flowing through the line.

Thats going to be an mains isolation problem.! unless you use a transformer of some kind.
Are you planning to measure the voltage drop across a resistor or use a current transformer.?

 

[mananshah93]

Thats going to be an mains isolation problem.! unless you use a transformer of some kind.
Are you planning to measure the voltage drop across a resistor or use a current transformer.?

My preference is to use voltage drop across a resistor,because current transformer is too bulky and too costly for our company budget.

 

[ericgibbs]

My preference is to use voltage drop across a resistor,because current transformer is too bulky and too costly for our company budget.

As you are not using an isolation transformer could you tell me what the full application is.? Whats its purpose.

How is the PIC going to be powered and enclosed.?

 

[Willbe]

voltage drop across a resistor
current transformer is too bulky and too costly for our company budget

The resistor can be the wire itself, depending on the accuracy needed.

#10 AWG gives 1 mΩ/ft at 20°C. At 20A and 10' this would be a drop of 200 mV (uncorrected for ambient temp. and the temp. rise due to the current through it).

The voltage sensing lead connections have to be separate from the current carrying connections (a four terminal Kelvin method). **broken link removed**

 

[Hero999]

Do you need true sine wave RMS value or not?

If so your best bet is to build a procision rectifier amplifier to get a 5V unfiltered DC waveform and use the microcontroller's processing power to convert it to a true RMS value.

See Wikipedia for information on doing this.

 

[mananshah93]

As you are not using an isolation transformer could you tell me what the full application is.? Whats its purpose.

How is the PIC going to be powered and enclosed.?

How is the PIC going to be powered and enclosed.?[/QUOTE]

See, My aim is to measure the current which is flowing in the mains( 0amp to 1000Amp AC 50 Hz freq.). without transformer.

So, Initially I put a low value resistor of 15 ohm across the line, and according to that value of current flowing in phase I got voltage drop across resistor.
(3 mV for 25 Amps, 3.4 for 30 Amps, 3.6 for 32.5 , 3.8 for 35, 11mV for 100 amps.)
So I thought by amplifying and converting into DC , I can give it to ADC of controller.

 

[ericgibbs]

How is the PIC going to be powered and enclosed.?

See, My aim is to measure the current which is flowing in the mains( 0amp to 1000Amp AC 50 Hz freq.). without transformer.

So, Initially I put a low value resistor of 15 ohm across the line, and according to that value of current flowing in phase I got voltage drop across resistor.
(3 mV for 25 Amps, 3.4 for 30 Amps, 3.6 for 32.5 , 3.8 for 35, 11mV for 100 amps.)
So I thought by amplifying and converting into DC , I can give it to ADC of controller.

[/QUOTE]

hi,
Do you mean 15 milli ohms.??? and a maximum of 100A not 1000A.?

Look at this circuit, it could be modified to give what you are asking.

 

[ericgibbs]

hi,
Tidied up the circuit diagram.

Two stages of ac amplification, followed by a precision rectifier.

Do you follow OK.?

EDIT: increase the gain of the amps to give the required dc out.

 

[Hero999]

What power rating is the resistor?

For a continious current of 100A it needs to be rated to at least 150W.

 

[mananshah93]

hi,
Tidied up the circuit diagram.

Two stages of ac amplification, followed by a precision rectifier.

Do you follow OK.?

EDIT: increase the gain of the amps to give the required dc out.

It's 15 ohms, but I am trying with other combination so I can improve the mV drop, and for current range, it is generally limited to 70-100 amps max, but accidently due to short circuit (between phase or neutral) it can carry more than 1000amps, so it should detect it.

By the way which op-amp I should prefer......LM series or traditional 741..


Heartly Thanks for your support.

 

[Willbe]

https://en.wikipedia.org/wiki/Arc_Flash

And, don't you want to sense the high side current?