rms AC to DC conversion

[zac]

What is the easiest way to create a DC voltage that is the rms value of an AC voltage?  I'm putting together a remote wireless voltage monitor for a generator from this:

https://www.ebay.com/itm/274617207090

The module will accept an input of up to 500V DC.  If I use a rectifier and capacitor, I would get root 2 (1.414x) times the rms value and could use a resistive divider to get the correct value.  But, that would waste electricity (even a 100K pot would dissipate 0.14 watts @ 120 VAC) and input impedance of this device may not be that high and require a lower resistance divider for decent accuracy. 

PS.  These device use a nRF24L01 based 2.4 ghz radio module which can be a swapped with a drop-in replacement version that includes a bidrectional rf amp to increase range.  (search for NRF24L01+PA+LNA)

[Sideshow Bob]

Have looked at using a transformer?

[pickit2]

you have a generator, so putting a meter to check the Voltage/Amprage is wasting power?
The device you want to use is a non-contact meter,
it still needs power to use, as does the wireless device.
you need to evaluate what your trying to achieve.

[Sideshow Bob]

It is hard to say very much based on the information you have given us. As we are not sitting beside you at your workbench. Posting things like any schematic or PDF files would be very helpful. It is up tou you, the ball is in your court

[Johnny]

Hey Zac,
might be easier to find something else or try rolling your own solution than to try adapt that one to do what you want.

If you're open to other solutions:

• Maybe look at energy monitors (the ones you plug into your power outlet). A lot of these will display voltage also. There are wireless models out there.
  Similar to this maybe? https://www.ebay.com/itm/402019637726
• Wireless multimeters? Although would have to find one that doesn't switch itself off. Might struggle with that.
  

Failing that:
If all you want to do is read the voltage, convert to RMS, and transmit it wirelessly to be displayed somewhere else, you could design your own system; wouldn't be too hard, not a quick fix though.

E.g. AC Voltage -> (Maybe a transformer to bring voltage down to safer level or isolation) -> Bridge Rectifier -> Smoothing -> Voltage Divider (set to say 3V3 at max Vin) -> Over Voltage Protection -> Op-amp/Buffer (for low impedance) -> ADC (PIC/arduino/or similar) -> Convert Value to Volts -> Apply Software Calibration -> Convert to RMS -> Send Over Wireless -> Display.

You could use an NRF52832/ESP8266/ESP32 for bluetooth or wifi and read the voltage from your phone/tablet/computer if you like, or use some sort of display like an OLED or character display.

Don't attempt this if you're not used to or confident working with mains voltage though.

[zac]

you have a generator, so putting a meter to check the Voltage/Amprage is wasting power?
The device you want to use is a non-contact meter,
it still needs power to use, as does the wireless device.
you need to evaluate what your trying to achieve.

I was trying to find a more elegant way to do it besides using a rectifier/capacitor and resistive divider.  Transformer instead of a divider would work too, but hard to come by a 1.414:1 ratio off the shelf.  I don't want to spend days or weeks on this project which is why I selected an off the shelf device to accomplish most of what I wanted.  I just thought perhaps someone had a simple idea that I overlooked to generate the DC equivalent of and rms AC voltage.    

[Sideshow Bob]

Have you tried google something like RMS circuit. If your source is a low frequency good sinewave. The circuit can bee quite simple. You can also google rue RMS To DC Converte. It is some ICs that can do all the job for you. If you use a transformer It can both insure safety but also be used to power your circuit. Do some work on google. Find some circuits alternatives. When you can come back with questions regarding the latter circuits. It is more easy to help that way

[kreutz]

use a 10 Mohm voltage divider with an opamp half or full wave rectifier, another opamp  as a buffer to feed the low impedance monitoring device. the 0-5 Volts output will be the input rms value Divided by 100. But you will need a power supply for the opamps anyways. 

[Manuel]

  LTC1966

take care,
Xo!

[Johnny]

Another option, modify your voltage monitor. Do a quick reverse engineering of the input. The input on that will have a voltage divider. Replace upper resistor with a suitable trim pot and adjust the measured value.

[Sideshow Bob]

You are wrong about the input range. It is not 500 volt. Read the manual
http://img.banggood.com/file/products/20180718053121SKU973245-Instructions.pdf

[solutions]

You say "up to" but you fail to spec a dynamic range or accuracy.

Just use a high impedance resistor divider to get 100:1 reduction and clamp the input to the ADC on a micro so it doesn't exceed positive and negative voltage limits. Then 0.707 all day in C

[bvrakgvnay]

The best way is to use phase locked loop to obtain the frequency and then getting samples periodically in a period accordingly . Thus you get a very precise RMS value since some wave forms might not be pure sine wave maybe a little distorted and composed of %90 percent harmonic like modified sine wave signal ... For the resistor divider you don't need to worry about that when you use a very big dividing ratio like 300:1 and you can even use DC resistor offset method .