Non-Invasive Current Sensor - (100A Max) Current Output CT Sensor เซ็นต์เซอร์วัดกระแส

 Data Sheet :openenergymonitor.org/emon/sites/default/files/SCT013-000_datasheet.pdf

CT sensors - Interfacing with an Arduino

 http://openenergymonitor.org/emon/node/58

 

Resistors R2 and R1 form the voltage divider that scales down the power adapter AC voltage and resistors R3 and R4 provide the voltage bias. Capacitor C1 provides a low impedance path to ground for the a.c. signal.

R1 and R2 need to be chosen to give a peak-voltage-output of around 1V, for an AC-AC adapter with an AC 9V RMS output a resistor combination of 10k for R1 and 100k for R2 would give a suitable output:

peak-voltage-output = R1 / (R1 + R2) x peak-voltage-input = 10k / (10k + 100k) x 12.7V = 1.15V

Improving the quality of the bias source

The relatively simple voltage bias source illustrated in “Measuring AC Voltage with an AC to AC power adapter” does have some limitations if high accuracy is required, it can be improved by adding an operational amplifier. The op.amp functions as a voltage follower, and buffers the mid-point voltage that appears at the junction of R3 & R4. This significantly reduces the impedance of the voltage source, resulting in enhanced performance.

Only one op.amp is needed, the current sensors can share the bias supply and connect to amplifier output (“mid-point”) with little fear of interaction between the inputs (which can be troublesome in some circumstances).

Notes:

1. this modification might not be suitable where the power source is batteries,
2. any pair of equal resistors up to 100 kΩ may be used for R3 & R4, and C1 can be reduced to 100 nF for resistor values at the high end of the range.

Primary peak-current = RMS current x sqrt(2) = 100A x 1.414 = 141.4A

Secondary peak-current = Primary peak-current / no. of turns = 141.4A / 2000 = 0.0707A

 If your using an arduino running at 5V: AREF / 2 will be 5V / 2 = 2.5V and so the ideal burden resistance will be

Ideal burden resistance = (AREF/2) / Secondary peak-current = 2.5V / 0.0707A = 35.4 Ohms

Tool for calculating burden resistor size, CT turns and max Irms - thanks to Tyler Adkisson for building and sharing this.

 

 

 

2) Adding a DC Bias

 

If you were to connect one of the CT (+burden) wires to ground and measured the voltage of the second relative to ground the signal would oscillate from positive to negative voltage. The Arduino requires a positive voltage only and so by connecting the end of the CT that we just connected to ground to a 2.5V level (half of the supply voltage) instead, the signal voltage will now oscillate around 2.5V and remain positive.

 

The resistors Rvd in the circuit diagram above make up a voltage divider that provides this 2.5V level.

 

Choosing a suitable value for Rvd resistors:

 

Higher resistance lowers quiescent energy consumption but also increases noise.

 

We have been using 10k resistors for mains powered monitors.  The emontx uses 470k resistors to really keep the power consumption to a minimum as it is intended to run off batteries for several months.

 

 

Arduino sketch - current only

1) Download EmonLib from github and place in your arduino libraries folder.

Download EmonLib:github.com/openenergymonitor/EmonLib

2) Upload the current only example:

#include "EmonLib.h" // Include Emon Library
EnergyMonitor emon1;                   // Create an instance

void setup()
{  
  Serial.begin(9600);
  
  emon1.current(1, 111.1);             // Current: input pin, calibration.
}

void loop()
{
  double Irms = emon1.calcIrms(1480); // Calculate Irms only
  
  Serial.print(Irms*230.0); // Apparent power
  Serial.print(" ");
  Serial.println(Irms); // Irms
}

EX.openenergymonitor.org/emon/buildingblocks/measuring-voltage-with-an-acac-power-adapter
EX.openenergymonitor.org/emon/buildingblocks/report-yhdc-sct-013-000-current-transformer
EX.openenergymonitor.org/emon/
EX.openenergymonitor.org/emon/buildingblocks/ct-sensors-introduction