Arduino Non-Invasive AC Current Sensor SCT-013
2018 FEB by Gene Casanova
Measuring Power And AC Current
The SCT-013 series of sensors function as transformers. The current circulates through the cable functions as the primary winding (1 turn), and internally has a secondary winding. The number of turns in the secondary winding, depends on the model of the sensor.
The number of turns represents the relationship between current flowing through the cable and the current delivered by the sensor. The ratio between the two currents, is the difference between the different SCT-013 sensor models; additionally they can have a load resistance at the output. In this way, instead of current, a voltage output is used.
These type of sensors, are known as CT (Current transformers) sensors.
The core must only cross a single line. If both single-phase AC hot and nuetral wiresare passed thorugh the sensor, the reading will be 0; since the two wires have opposite currents.
An advantage of a SCT-013 sensor is in not needing to interrupt (cut or undock) the electrical lead wire to measure. Measure is done through a split-core clamp.
The SCT-013-100 sensor, enables measuring a current up to 100A, having 50mA output for a current of 100A; the ratio is 100A / 50mA.
The SCT-013-30 sensor, enables measuring a current up to 30A. This model has an internal load resistance, delivering a voltage output. The ratio is 30A / 1V.
The output of this sensor is an alternating signal, and the values are not within the range of the Arduino analog inputs (0 to + 5V). The negative part of the signal can damage the Arduino Analog I/O pin internal circuit.
Ideally, if the sensor has an output of -50mA to + 50mA, condition it to an output of 0 to 5V. This could be done converting from current to voltage, (to a range of [-1V + 1V]), then amplify it (to [-2.5V + 2.5V]) and finally an 'adder' to eliminate the negative part ([0 5V] ).
Another way to condition the signal is rectifying the input and working with the positive part, this assums the signal is symmetric.
Diodes cannot be used to rectify the siganl, since the voltage drop in the diode is very large, compared to the voltage of the signal.
Rectifying the signal can be done using an operational amp, configured in a voltage follower. The opamp LM358 will be used, which works with positive polarity; eliminating the negative part of the signal. THe LM358 is not a full wave rectifier, but with a rectification of half wave is useable.
Power the LM358 with 5V; saturates with 3.5V approximately. Work with the internal reference of 1.1V and take advantage of the full range of analog reading.
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