The Modern Device Current Sensor provides an easy-to-use, electrically isolated current sensor for mains voltages (120/240 volts AC). Just zip-tie it to a power cord, and read the current in DC volts. The sensor does need it’s own 3.3 or 5 volt power supply but this makes it entirely convenient for sensing AC currents with an Arduino or other microcontroller.
The sensor can distinguish as little as 2 watt loads at 120 volts, which means that it can sense a change in current of AC current of about 16 mA. The sensor output is just an analog voltage which is proportional to the current sensed. If you are looking for absolute values of current you will need to do a little calibration, but this is easy as applying two loads, hopefully a light load and a much heavier load, and just averaging beween the load points. As our testing shows, the sensor really does have impressive linearity.
How the current sensor works
It theory you shouldn’t be able to sense a current from two AC carrying wires in close proximity, because the 180 degree out-of-phase power fields should cancel one another. If you’ve ever used an AC clamp (current) meter, you know that it is important to put only one conductor inside the clamp. Putting both conductors inside the clamp meter will result in a zero, or very low reading as the fields cancel one another.
The trick the current sensor uses is to getting the separate sensors as close as possible to a cables separate conductors, one sensor closer to one wire and the other sensor closer to the other conductor. The current uses modern integrated Hall Effect Sensors which incorporate chopper op amps for high gain. The two hall effect sensors and then read deferentially, which amplifies the difference voltage generated between the two sensors.
The sensor can also work well for sensing current in only one wire (hot or neutral) alone if you wish to use it that way, but you will need to orient the wire over only one of the hall effect sensors on the board.
Simple calibration is necessary
It is important to note that the actual curve you will get (but not its linearity) is influenced by the geometry of your wire and the sensor’s position in relationship to the wire. For maximum sensitivity adjust the sensor’s position on the wire while monitoring the output for maximum response. The potentiometer on the board can also be used to adjust the gain by a figure of about 4. A sketch in the blog posts below explaining very quick and simple methods for calibrating the sensor, one requires only gathering two data points.
The sensor ships with two small zip ties that can be used to secure the sensor to a line cord, extension cord or power strip, and a 3 pin male header. In some cases you may want to add a drop of hot glue to keep the sensor locked to the wire in a stable configuration.
|Current Sensor Specifications|
|VCC||Sensor Measured Watts / Amps||Output Voltage Range||Sensor Current|
|3.3V||2 watts to 3000 watts||.05V to 2.8V||11 mA|
|5V||2 watts to 3000 watts||.05V to 4.5V||16 mA|
- An Arduino sketch for using the Modern Device Current Sensor
- Blog post on the current sensor
- Blog post on calibrating the current sensor
- Blog post the current Sensor at 3.3 volts
- Modern Device Current Sensor Schematic