A current transformer (CT) steps down high primary AC current into a safely measurable, proportional secondary current—known as CT current—for metering and protection circuits. In a real installation, this changes everything by allowing a standard 5A panel meter or a 3.3V ESP32 ADC to safely measure a 200A service feeder without any galvanic connection to lethal mains voltage. The most common mistake makers and junior techs make is confusing AC CTs with Hall-effect sensors (which can read DC) or misunderstanding how to calculate the burden resistor required to convert that secondary current into a readable voltage.
The Core Physics: How CT Current Steps Down Mains
Unlike a voltage transformer that steps down potential, a CT is designed to step down current while maintaining a strict proportional relationship. The primary winding is simply the mains wire passing through the center of the toroidal core (effectively a 1-turn winding). The secondary winding consists of thousands of turns of fine enameled copper wire wrapped around that same core.
Think of a CT like a highway toll plaza funneling 10 lanes of fast-moving traffic (primary current) into a single, heavily monitored exit lane (secondary current). The ratio of the traffic remains constant, but the volume in the exit lane is small enough to safely count.
If you have a CT with a 2000:1 turns ratio and 100A flowing through the primary wire, the secondary CT current will be exactly 50mA (0.05A). This secondary current is strictly alternating (AC) and is entirely isolated from the high-voltage primary circuit, which is why CTs are the backbone of safe electrical metering. For a deeper dive into the magnetic flux mechanics, the Electronics Tutorials guide on Current Transformers provides excellent cross-sectional diagrams of the core saturation limits.
Worked Numeric Example: Sizing a Burden Resistor for an ESP32
Microcontrollers cannot measure current directly; they measure voltage. To use a CT with an ESP32 or Arduino, you must pass the secondary CT current through a burden resistor to generate a proportional AC voltage. Let's calculate the exact resistor value for the popular YHDC SCT-013-000 (the raw 50mA output version, not the 1V internal-resistor version) paired with an ESP32.
The Parameters:
- Primary Max Current: 100A RMS
- Secondary Max CT Current: 50mA (0.05A) RMS
- ESP32 ADC Max Voltage: 3.3V (but we use 3.1V to avoid non-linear clipping at the rails)
The Math:
- Find Peak Voltage: The ADC reads peak-to-peak. Max peak-to-peak is 3.1V. The peak voltage (from zero) is half of that:
3.1V / 2 = 1.55V. - Convert to RMS Voltage: AC RMS is peak divided by √2 (1.414).
1.55V / 1.414 = 1.096V RMS. - Calculate Burden Resistance: Using Ohm's Law (R = V / I).
1.096V / 0.05A = 21.92Ω.
The closest standard E12 resistor value is 22Ω. Because the CT current will dissipate power as heat, check the wattage: P = I² × R = (0.05)² × 22 = 0.055W. A standard 1/4W (0.25W) metal film resistor is more than adequate.
The DC Bias Requirement:
The ESP32 ADC cannot read negative voltages. The AC wave from the burden resistor swings from +1.096V to -1.096V. You must build a voltage divider using two 470kΩ resistors across the 3.3V and GND pins to create a 1.65V DC offset, and use a 10µF capacitor to filter it. This shifts the AC wave so it oscillates cleanly between 0.55V and 2.75V, perfectly inside the ESP32's linear reading window. The OpenEnergyMonitor CT theory documentation provides the exact breadboard schematic for this bias circuit.
Where You Meet CT Current in Practice
You interact with CT current measurements constantly, even if you don't see the sensors. In the residential space, smart home energy monitors like the Emporia Vue and Sense use arrays of split-core CTs clamped onto individual branch circuits inside your breaker panel to track appliance-level usage.
In solar installations, grid-tied inverters (like Fronius or SMA models) use precision CTs installed on the main service feeder to monitor export limits. If the CT current indicates power is flowing backward to the grid beyond the utility's allowed threshold, the inverter instantly throttles its DC-to-AC conversion. In industrial settings, motor protection relays use CT current to detect phase imbalances or ground faults, tripping the main contactor in milliseconds if the secondary current deviates from the expected vector sum.
Sensor Showdown: CTs vs. Hall Effect vs. Shunts
Choosing the right sensor depends entirely on your isolation requirements, AC/DC needs, and physical space. Here is how CT current stacks up against the alternatives.
| Feature | Current Transformer (e.g., SCT-013) | Hall Effect (e.g., ACS712) | Shunt Resistor (e.g., INA219) |
|---|---|---|---|
| Current Type | AC Only | AC and DC | AC and DC |
| Galvanic Isolation | Excellent (Magnetic) | Good (Magnetic) | None (Direct electrical contact) |
| Insertion Loss | Zero (Clamps around wire) | Zero (Clamps or inline) | High (Generates heat/voltage drop) |
| Mains Safety | Very High | Moderate (IC breakdown risk) | Dangerous at high voltage |
| Best Use Case | Home energy monitoring, AC panels | DC battery banks, low-voltage AC | Low-voltage DC PCB routing |
Frequently Asked Questions About CT Current
Can I use a CT current sensor to measure DC battery current?
No. Current transformers rely on Faraday's Law of Induction, which requires a changing magnetic field to induce a secondary current. DC current creates a static magnetic field, meaning the secondary CT current will be exactly zero. For DC battery banks (like a 48V LiFePO4 server rack), you must use a Hall-effect sensor (like an HTFS 200-P) or a high-side shunt monitor (like the Victron SmartShunt).
What happens to the CT current if I disconnect the burden resistor?
How do I calculate actual amps from a microcontroller ADC reading?
Once you have the raw ADC value, you must convert it back to primary amps using your specific CT ratio and burden resistor. First, convert the ADC reading to RMS voltage at the pin (subtracting your DC bias offset). Then, use the formula:
Primary Current = (V_rms / Burden_Resistance) × CT_Turns_Ratio
For our ESP32 example with a 22Ω resistor and a 2000:1 CT, if you measure 0.5V RMS at the ADC pin: (0.5 / 22) × 2000 = 45.45A. Keep in mind that the ESP32 ADC is notoriously non-linear near 0V and 3.3V, which is why keeping your signal centered at 1.65V via the DC bias circuit is critical for accurate readings.
