This is a little current sensor we designed with the idea of providing an easy-to-use, electrically isolated current sensor for mains voltages (120/240 volts AC). It uses some Hall Effect Sensors with a lot of gain applied, followed by a peak detector, to sense the magnetic field generated by the current in a wire. The sensor can distinguish a 5 watt load at 120 volts, which means that it can sense a change in current of around 42 mA. Sensing down to 2 watts is also possible with long integration times (smoothing at 200ms). The sensor output is just an analog voltage which is proportional to the current sensed. It theory you shouldn’t be able to sense a current from two wires in close proximity, because the 180 degree out-of-phase fields should cancel one another. The trick then to making this work is getting the separate conductors as close as possible to the sensors with one wire closer to one sensor and the other conductor (neutral line) closer to the other sensor. The differential configuration of the op amp, then amplifies the difference voltage generated between the two sensors. The sensor also works fine for sensing current in only one wire (hot or neutral) alone, but you will need to orient the wire over only one of the hall effect sensors on the board. This sensor runs at either 3.3 or 5 volts and is easy to calibrate. Just plug in a known resistive load such as a lamp, and read the output voltage from the sensor. The output is very linear – although a very small non-linear region exists at the low power end of the curve. A gain control allows changing the gain over a range of about 3 to 1. Here is some data comparing the voltage output of the sensor with some various loads we have around the shop. You can see it is very linear except for extremely low power loads which are influenced by the “noise floor” of the sensor. 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. Just adjust the sensor’s position on the wire while monitoring the output for maximum response. There are also sketches in the blog posts below explaining very quick and simple methods for calibrating the sensor, one requires only gathering two data points.
The sensor should find uses in sensing whether appliances are plugged in and running and calculating energy usage. It interfaces seamlessly with any microcontroller ADC converter including Arduino, Tennsy and JeeNodes and their headers. You could also just use a voltmeter for quick measurements. 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||unlimited-depends on sensor-to-wire geometry||.05V to 2.8V||11 mA|
|5V||unlimited-depends on sensor-to-wire geometry||.05V to 4.5V||16 mA|
- 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