# Linear AC voltage measurement using controller without transformer.

#### ralph stephenson

Joined Feb 12, 2019
1
Hello everybody,
I'm trying to make a voltage monitor to continuously display the voltage variation in my place. I tried using a transformer but it is not giving a linear output, also I tried the capacitor based circuit but it is efficient for continuous working. What should I do how can I measure AC voltage linearly using arduino? I want to convert 230V AC to 5V DC so that take bit values using arduino.

#### ericgibbs

Joined Jan 29, 2010
19,071
hi ralph,
Welcome to AAC.
Use a mains isolation step down transformer , say 5Vac output, full-wave rectify that to give approx 7Vdc and use a resistive divider.
Do you have an existing circuit that you could post.?
E

#### DickCappels

Joined Aug 21, 2008
10,245
If you use a Control Transformer the nonlinearity should be very small.

The nonlinearity is probably high magnetizing current (too little copper and too little core) causing voltage drops across the resistance of the primary winding (too little copper). Control transformers are designed to minimize this effect.

From the web:
A control transformer is an isolation transformer that provides good voltage regulation, and is also designed to provide a high degree of secondary voltage stability (regulation) during a brief period of overload condition (also referred to as “inrush current”). Control transformers are also known as Machine Tool Transformers, Industrial Control Transformers or Control Power Transformers.

Joined Jan 15, 2015
7,570
Hello everybody,
I'm trying to make a voltage monitor to continuously display the voltage variation in my place. I tried using a transformer but it is not giving a linear output, also I tried the capacitor based circuit but it is efficient for continuous working. What should I do how can I measure AC voltage linearly using arduino? I want to convert 230V AC to 5V DC so that take bit values using arduino.
Without a transformer things can get a little expensive. There are AC Voltage Transducers where a transducer takes a signal you have like AC mains Voltage and converts it into a signal you want like 0 to 5 VDC. They work great and can be extremely accurate. Unfortunately they come with a cost. If your AC mains is a nice clean sine wave then a simple Average responding transducer is fine but if you want a True RMS response they get expensive. Just as an example, The CR4800 Series, Average RMS Voltage Transducers and Transmitters are designed to provide a DC output proportional to the AC voltage input. It can be had with a 0-250 VAC input range and a 0 to 5 VDC output making scaling, for example on an Arduino, an easy task. The downside is they cost about \$100 USD. A Google of "AC Voltage Transducers" will bring up plenty of similar AC Voltage Transducers. When you start getting into True RMS the cost really goes up. They are accurate and reliable but not practical for someone who just wants to measure their home AC mains voltage but they are out there.

I would stick with some of the mentioned solutions. You can get reasonably close measurements. There are also plenty of inexpensive data logging shields for Arduino so you can measure and data log your mains voltage over a period of time. Here is an AC Voltmeter using Arduino example. Basic transformer designs are the least expensive way to go.

Ron

#### AnalogKid

Joined Aug 1, 2013
11,190
I like the AH suggestion in #3. The uC can calculate the peak, RMS, or whatever value complete with long-term averaging, brownout detection, etc.

ak