Hello,
i have to measure an AC voltage (0 to 240V) and get a DC equivalent of smaller amplitude (0 to 24V DC). The DC output should vary linearly with AC.
Do you have suggestions?

 

Is this un-isolated main's voltage?
If so, it could be run it through a step-down transformer and then rectified with an op amp configured as a precision (ideal) rectifier.

 

Yes it is, I'm measuring the voltage of a wind-turbine phase.
And I need to have a lower DC equivalent of it.
Thank you for the reply!
I'm going to see what is a precision rectifier configuration for an op amp.

 

Below is the LTspice simulation of one of my favorite, low parts-count, full-wave, precision rectifier circuits.
Its filtered output is the full-wave average of the sinewave input.

 

I reproduced the circuit to simulate it at diferent voltages, and the output is still 12v even without input. Do you have the same kind of behaviour in your circuit?

 

Step-down the voltage with a small signal transformer then use an " AC RMS - to - DC " converter IC like the LT1966, or equiv.

https://www.analog.com/media/en/technical-documentation/data-sheets/1966fb.pdf

 

Do you have the same kind of behaviour in your circuit?

No.
Below is my sim with a 1Vpk and 10Vpk input signal:

What is the blue trace in your simulation that is offset by about +20Vdc?
The circuit is sensitive to any DC input or offset.
What do you mean by "no input"?

 

Step-down the voltage with a small signal transformer then use an " AC RMS - to - DC " converter IC like the LT1966, or equiv.

That converter a more expensive solution, but is good if the signal is distorted enough that you need true RMS rather than the average value of the signal.

 

Thanks a lot for your time,
what I meant with “no input” was with a 0v Amplitude signal.

That converter a more expensive solution, but is good if the signal is distorted enough that you need true RMS rather than the average value of the signal.

Yes, I think I’ll try to understand and simulate in LTspice your solution, because I need the cheapest solution.
The blue trace in my simulation was the voltage after the diode D1, I was trying to figure out what was my problem in proteus (if it was a mistake from me when I rebuilt your circuit or a problem with the simulation).

Is the first op amp in your circuit rectifying the voltage, and the second one a simple follower op amp with a filter (R+C)?

 

The blue trace in my simulation was the voltage after the diode D1

I see the problem with your simulation.
On the OP727, pin 4 is minus power and pin 8 is plus.
With power reversed, the output will go to near the plus supply rail.

Is the first op amp in your circuit rectifying the voltage, and the second one a simple follower op amp with a filter (R+C)?

U1 inverts and rectifies the negative half of the sinewave, with a gain of -1, as determined by R1 and R2.
The forward drop of the diode is reduced by the open loop gain of the op amp, so the diode looks essentially ideal.
The result is a positive half-wave at U2's input, with no diode drop, for the negative half-wave input.

When the input goes positive, the diode blocks the U1's output, so the signal goes directly through R1 - R2 to the high impedance input of unity-gain follower U2.

The end result is a full-wave rectified signal at the output of U2.

Yes, R3 and C1 filter the rectified signal to give its average value.
If a faster response (rise) time is desired for a given ripple output voltage then can be provided by an RC single-pole filter, a multi-pole active low-pass filter can be used instead (a single op amp can provide up to 3-poles).
I can provide a design for that, if interested.

 

Thank you for the explanation of the circuit! I Modified my circuit in proteus and it works now. I also did it on LTspice and it works well.

If a faster response (rise) time is desired for a given ripple output voltage then can be provided by an RC single-pole filter, a multi-pole active low-pass filter can be used instead (a single op amp can provide up to 3-poles).
I can provide a design for that, if interested.

I dont need a faster response, the final aim of my circuit is to use the dc voltage to feed a comparator with hysteresis.
When the AC voltage goes above 110V I need to activate a relay, and when it goes below 100V unactivate the relay.
This relay will be used to power a pump, so the response don't have to be quick.

Below is the simulation with a step down transformer, the problem is that I haven't found many web sites that sell transformers in general, so do you think I can use a voltage divider with huges resistors to adapt the voltage and limit the power dissipation (because the phase I'm measuring will be max 240V 7Amps)?