A current transformer (CT) is a passive instrument transformer that produces a reduced, accurately proportional alternating current in its secondary winding to safely measure high primary AC currents without breaking the main circuit. In a real installation, it changes a dangerous 100A+ mains line into a safe milliamp-level signal that microcontrollers and metering ICs can read, while providing critical galvanic isolation. Beginners commonly confuse CTs with voltage (potential) transformers or Hall-effect sensors, but CTs rely strictly on AC magnetic induction, cannot measure DC, and act as current sources rather than voltage sources.

The Physics Inside the Core: What Actually Changes

When an alternating current flows through the primary conductor (the wire passing through the center of the CT), it generates a fluctuating magnetic field in the transformer's ferrite or silicon-steel core. This changing magnetic flux induces a proportional current in the secondary winding, which consists of hundreds or thousands of tightly wound copper turns.

The relationship is governed by the inverse turns ratio. If your primary conductor is just a single straight wire, it acts as a 1-turn primary. A secondary winding with 2,000 turns yields a 2000:1 ratio, meaning 100A on the primary becomes exactly 50mA on the secondary.

Crucially, a CT does not output a voltage on its own; it outputs a current. It behaves as a high-impedance current source pushing against whatever load is connected to it. To interface this milliamp signal with the voltage-based Analog-to-Digital Converter (ADC) of an Arduino, ESP32, or dedicated metering IC like the ADE9000, you must convert the current back into a measurable voltage. This is done by placing a precise 'burden resistor' across the secondary terminals.

Worked Numeric Example: Sizing a Burden Resistor for an ESP32

Let's calculate the exact burden resistor needed for the most popular DIY energy monitoring sensor: the YHDC SCT-013-000. This is a split-core CT rated for 100A primary current and 50mA secondary current (a 2000:1 turns ratio). We want to interface this with an ESP32 DevKit v1 to monitor a 240V split-phase residential panel.

Step 1: Determine the ADC Voltage Limits
The ESP32 ADC operates on a 3.3V reference. However, the ESP32's ADC is notoriously non-linear near the rails (0V and 3.3V). To get clean, accurate sine wave readings without clipping, we should limit our peak-to-peak voltage to roughly 3.1V. This gives us a maximum peak voltage ($V_{peak}$) of 1.55V above and below our DC bias midpoint.

Step 2: Calculate the Secondary Peak Current
The CT outputs 50mA RMS at full 100A primary load. AC waveforms are sinusoidal, so the peak current is higher than the RMS value:
$I_{sec(peak)} = I_{sec(rms)} \times \sqrt{2}$
$I_{sec(peak)} = 0.050A \times 1.414 = 0.0707A$ (or 70.7mA)

Step 3: Calculate the Ideal Burden Resistance
Using Ohm's Law ($R = V / I$):
$R_{burden} = 1.55V / 0.0707A = 21.92\Omega$

Step 4: Select the Standard Resistor and Verify Power Rating
The closest standard 1% tolerance resistor is 22Ω. Next, we must ensure the resistor won't overheat. Power dissipation is calculated using the RMS current:
$P = I_{rms}^2 \times R = (0.050A)^2 \times 22\Omega = 0.055W$
A standard 1/4W (0.25W) through-hole resistor is more than adequate, though a 1/2W resistor provides extra thermal mass for continuous 100A loads in a warm breaker panel.

Step 5: Add the DC Bias Circuit
Because the ESP32 ADC cannot read negative voltages, the AC waveform must be shifted up. We do this by creating a 1.65V midpoint using a voltage divider (two 10kΩ resistors across the 3.3V and GND pins) and connecting the CT's burden resistor to this midpoint instead of raw ground. For a deep dive into the AC coupling math, the OpenEnergyMonitor CT sensor guide remains the definitive open-source reference.

Where You Meet Current Transformers in Practice

You will rarely see bare CTs in consumer electronics, but they are the backbone of modern AC power infrastructure and smart home energy tracking:

  • Residential Solar & Battery Inverters: Systems like the Enphase Envoy or SolarEdge use revenue-grade CTs clamped to the main service feeders to measure grid export/import, ensuring the inverter complies with utility anti-islanding and export-limit rules.
  • Smart Panel Monitors: Devices like the Emporia Vue or Sense energy monitor utilize arrays of 16 to 32 miniature split-core CTs to provide circuit-level granularity, identifying whether your EV charger or HVAC compressor is running based on current draw signatures.
  • Industrial Motor Protection: Variable Frequency Drives (VFDs) and motor protection relays (like the Schneider Electric TeSys series) use heavy-duty toroidal CTs to detect phase imbalance, ground faults, and thermal overloads before they destroy a 50HP 3-phase motor.

Common Confusions and Critical Safety Rules

The most frequent mistake hobbyists make is confusing a CT with a Hall-effect sensor (like the ACS712 module). While the ACS712 is cheap and can measure DC, it requires the mains voltage to pass directly through the IC. If the internal isolation barrier fails, 120V/240V mains is instantly routed into your 3.3V microcontroller, destroying it and posing a lethal shock hazard. A CT provides inherent physical galvanic isolation; the primary and secondary circuits never touch electrically.

Another confusion is mixing up CTs with Potential Transformers (PTs). PTs step down voltage (e.g., 7200V down to 120V for metering) and are wired in parallel. CTs step down current and are wired in series (by clamping around the line).

CRITICAL SAFETY WARNING: Never Open-Circuit a Loaded CT
If primary current is flowing and you disconnect the burden resistor (open-circuiting the secondary), the secondary current drops to zero. Without the secondary magnetic field to oppose it, the core instantly saturates. The rapid collapse of the magnetic flux induces a massive voltage spike—often reaching several thousand volts—across the open secondary terminals. This can cause lethal arcing, destroy the CT's internal insulation, and permanently magnetize the core, ruining its accuracy. Always short the secondary terminals before removing a meter or burden resistor. In professional panels, this is done using dedicated shorting blocks as mandated by standard instrument transformer practices.

Frequently Asked Questions

Can a current transformer measure DC current?

No. Current transformers operate entirely on Faraday's Law of Induction, which requires a changing magnetic field to induce a secondary current. Direct Current (DC) produces a static, unchanging magnetic field, meaning zero voltage is induced in the secondary winding once the initial connection transient settles. To measure DC current, you must use a shunt resistor (which measures voltage drop) or a Hall-effect sensor (which measures static magnetic flux density).

What happens if a current transformer secondary is left open?

As detailed in the safety warning above, leaving the secondary open while primary current flows is highly dangerous. The core saturates, and the secondary winding acts like the ignition coil in a car, stepping the voltage up to thousands of volts. This will arc across the terminals, potentially electrocute anyone nearby, melt the winding wire, and permanently degrade the core's permeability. Always ensure a burden resistor or a shorting switch is connected before energizing the primary line.

How do I choose the right current transformer ratio for my panel?

Match the CT's primary rating to the breaker size or the expected maximum continuous load of the circuit you are monitoring. If you put a 200A CT on a 15A lighting circuit, the secondary signal will be so small that the ADC's noise floor will ruin your resolution. Conversely, if you put a 20A CT on a 50A EV charger circuit, the core will saturate during peak draw, clipping the top and bottom of the sine wave and resulting in wildly inaccurate RMS calculations. The industry rule of thumb is to select a CT where your normal operating current falls between 60% and 80% of the CT's primary rating, while ensuring the absolute peak load does not exceed the CT's maximum rating. For deeper theory on core saturation limits, refer to Electronics Tutorials on CT design.