How can I measure AC voltage in the range 0-250 VA

How can I use the Arduino to measure a variable AC voltage? The range would be 0-250 VAC.

You probably need to make a circuit to reduce the 0-250vac to 0-5vdc, and probably would be good to incorporate some type of electrical isolation depending on the useage.

Any idea where I can find the circuit that would do the voltage reduction and ac/dc conversion?

Is the 250V RMS?

Isolated transformer. Bridge rectifier. Capacitor. Voltage divider for the rest?

Here's my take:

You could use a multi-tap transformer so you can have selectable ranges for better accuracy...

DMM with serial interface:

Accurate measurement, no isolation/safety problems, no hardware development.

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You may use this HCPL-7520 Isolated Linear Sensing IC is designed for current sensing applications

Should you be messing with what I'm guessing is mains AC supplies when you have such a limited understanding?

Please be careful...

Being creative, the schematic for the below multimeter might show some components to able to take an ac voltage and develop a +5v dc output. I haven't checked to see what the operating voltage is internal to the meter (0-3v, 0-5v, etc), but if one is careful and knowledgable, one probably modify the multimeter for adaption to the arduino.

Thanks to everyone for their input. This definitely points me in the right direction. While I have very limited experience with the Arduino I do have considerable experience with high voltage AC systems. I regularly work with marine HVAC control panels that use 115-1-60, 230-1-60, 230-3-60, 460-3-60 and 380/415-3-50 power circuits.

I don't see where in that hand-drawn schematic an AC voltage would be applied.. Am I missing something?

Your probes would be at COM and V on the lower left side of the drawing

Ok.. I see the "AC" point next to "V". I was hoping for some insight as to how it measure AC voltage, but it doesn't look like there is any special circuitry other than a diode and some resistors. Oh well.

Is it mains you're measuring (experience noted, by the way!), or is it general AC at various frequencies?

If it's just mains (50Hz/60Hz depending where you live I guess) then simply recitfying the output of a step-down transformer as earlier post should be fine (as long as you calibrate the variable resistor etc.).

If it's a huge frequency range you're after then converting to rms becomes more tricky.

Most likely I have not understood the problem...

The straightforward solution is to reduce the voltage to a ADC acceptable range, e.g. +/- 1 V. This can be done by a voltage devider of - say - 2.2M and 10 k and connect it to the ADC. The negative voltage will do no harm, as the current flowing will be < 100uA. You will see the half waves of a sinus in the ADC. Use any integration algorithm that suits your need to get something you can call RMS...

Ok.. I see the "AC" point next to "V". I was hoping for some insight as to how it measure AC voltage, but it doesn't look like there is any special circuitry other than a diode and some resistors. Oh well.

I think an interesting point might be that it might be possible to attach an arduino ground to the 7106 chip "common" and the an arduino analog input to the pin 31 input. If the input is always in the 0-5v range to pin 31, then the multimeter possibly could be made into an easy arduino data logging interface.

7106 chip having 200mV or 2V full scale and not 0-5V range at pin 31.

All you circuit are "LIVE", make sure not connect to computer USB if you connected it to your arduino analog input. if you do, it may damage your computer.

For the lower ac voltages (like 125vac) and the multimeter one might be able to use a small isolation transformer like below for the multimeter connection to the ac. Another issue would be to set the reference voltage on the arduino to 2v to get full reading range capability. Not sure how easy that would be.

It would definitely be best to get an isolation transformer made for ac line conditions, as the nominal ac voltage is usually an average value and not the peak voltage encounered.

I would like to know if anyone knows where transformers can be had(for medium to high power applications) for less than a small fortune. I've also heard of toroids that you can wind yourself, but what if you need 20A @ 100V for a DC supply?? This should probably be its own thread, but I'm lazy. I would REALLY APPRECIATE any advice on this.

One could relatively easily wind a multitap transformer so they could have a selectable ranges of AC measurement voltages (10V,100V,1000V...). Perhaps not.

Are the transformers found in microwave ovens easily re-wired? I'm not sure if it works this way, but those things can be rated for upwards of 1500W. I'm sure that they are wound for a different voltage, though. I don't know much at all about magnetic flux density/saturation...stuff. All I have learned about that stuff is the effects of fields on charged particles (not really pertinent to transformers) and electronics.

Question: If you have a toroid(or whatever) with a 20-turn primary winding and a 10-turn secondary.. Is it the same as a 200-turn primary with a 100-turn secondary? Is there a trade-off? How do you choose the number of windings?