How to Measure AC Current Using CT Sensors: A Practical Guide

Current Transformer (CT) sensors are widely used for measuring AC current in industrial and embedded systems. They provide electrical isolation and allow high current measurement without direct electrical contact with the conductor.

This guide explains how to properly use CT sensors with microcontrollers for accurate current measurement.

How CT Sensors Work

A CT sensor operates based on electromagnetic induction. When AC current flows through the primary conductor, it induces a proportional current in the secondary coil.

Key points:

• Output is current (not voltage)
• Requires burden resistor to convert to voltage
• Only works with AC signals

Basic Connection to Microcontroller

A CT sensor cannot be connected directly to an ADC input. The output must be conditioned.

Required components:

• Burden resistor
• RC low-pass filter (optional)
• Voltage divider / biasing (for ADC protection)

Basic flow:

CT Sensor → Burden Resistor → Filter → ADC Input

Burden Resistor Selection

The burden resistor converts CT output current into measurable voltage.

Formula:

Where:

Selection considerations:

• Output voltage must stay within ADC range (0–3.3V / 5V)
• Too large → saturation
• Too small → low resolution

Proper selection is critical for measurement accuracy.

Signal Conditioning and Offset

Because CT output is AC (positive & negative), ADC input must be adjusted.

Common approach:

• Add DC offset (bias) to shift signal into positive range
• Use capacitor filtering to stabilize waveform

This ensures safe and stable ADC readings.

RMS Calculation Implementation

After signal acquisition:

1. Sample ADC values continuously
2. Convert to voltage/current
3. Apply RMS calculation
4. Display or log results

To improve accuracy:

• Use multiple sampling cycles
• Apply filtering
• Calibrate scaling factor

Common Mistakes

Avoid these common issues:

• ❌ No burden resistor (dangerous for CT)
• ❌ Direct connection to ADC
• ❌ Incorrect resistor value
• ❌ Ignoring signal offset
• ❌ Low sampling rate

These mistakes can cause inaccurate readings or hardware damage.

Practical Applications

CT sensors are commonly used in:

• Energy monitoring systems
• Industrial load measurement
• Smart meters
• Motor current monitoring
• Data logging systems

Conclusion

CT sensors provide a safe and effective method for measuring AC current in embedded systems. With proper signal conditioning, correct burden resistor selection, and accurate RMS calculation, reliable current measurement can be achieved for both industrial and IoT applications.

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