A current transformer (CT) is an instrument transformer that steps down high AC line current to a safe, proportional secondary current for metering and protection. In a real installation, it galvanically isolates your low-voltage measurement circuit from lethal mains voltage while scaling 100A+ line currents down to milliamp levels that standard microcontrollers and panel meters can safely read. Beginners commonly confuse CTs with Hall-effect sensors (which can measure DC) or shunt resistors (which require breaking the circuit and lack galvanic isolation).
The Core Mechanism: Scaling Current Without Direct Contact
Unlike a standard voltage transformer that connects in parallel, a current transformer connects in series with the load. The primary 'winding' is often just the single mains wire passing through the center of the CT's toroidal core. The secondary winding consists of hundreds or thousands of turns of fine enameled copper wire wrapped around that same core.
Think of the primary wire as a high-volume water main, and the CT secondary as a narrow bypass tube that siphons a precisely proportional trickle to spin a small meter turbine, completely isolated from the main water pressure. The governing physics rely on Ampere's Law and the turns ratio:
Where I is current and N is the number of turns for primary (p) and secondary (s).
If your primary wire carries 100A (1 turn) and the secondary has 2000 turns, the secondary will output exactly 50mA (100 × 1 / 2000 = 0.05A). This scaling allows you to measure massive industrial loads using cheap, low-voltage electronics.
Worked Example: Sizing the Burden Resistor for an ESP32 ADC
A CT outputs a current, but microcontrollers like the ESP32 or Arduino measure voltage. To convert the secondary current into a readable voltage, we must place a 'burden resistor' across the CT's secondary terminals. Let's calculate the exact resistor value for a common DIY energy monitoring scenario.
The Goal: Measure up to 30A RMS on a 120V AC branch circuit using an ESP32 DevKit v1 (which has a 3.3V ADC limit) and a YHDC SCT-013-000 current transformer.
- Identify CT Specs: The SCT-013-000 has a 100A:50mA ratio (2000 turns). At our 30A maximum primary current, the secondary RMS current is:
30A × (0.050A / 100A) = 15mA RMS. - Calculate Peak Current: AC is a sine wave. The ADC needs to read the peak, not the RMS.
15mA × √2 (1.414) = 21.21mA peak. - Determine Target Voltage: The ESP32 ADC reads 0V to 3.3V. Because AC alternates positive and negative, we must bias the signal to sit at half the ADC range (1.65V) so the waveform swings from 0V to 3.3V without clipping. Our target peak voltage above the bias is 1.65V.
- Calculate Burden Resistor: Using Ohm's Law (R = V / I):
1.65V / 0.02121A = 77.79Ω.
The nearest standard E12 resistor value is 75Ω or 82Ω. We will select 75Ω to ensure we don't exceed the 3.3V ADC ceiling during minor grid voltage swells.
Where You Meet Current Transformers in Practice
You are likely already interacting with CTs daily, even if they are hidden inside commercial enclosures:
- Smart Home Energy Monitors: Systems like Emporia Vue and Sense use arrays of 16 to 32 micro-CTs clamped onto individual branch circuits inside your main panel to provide real-time appliance-level disaggregation.
- Solar Inverters: Grid-tied inverters (like Fronius or SolarEdge) use high-accuracy revenue-grade CTs to monitor grid export/import, ensuring anti-islanding protection and compliance with utility interconnection agreements.
- Industrial Motor Protection: Overload relays on 3-phase 480V motors use heavy-duty solid-core CTs to detect phase imbalance or ground faults, tripping the main contactor before the motor windings melt.
- DIY IoT Power Logging: Makers use split-core CTs paired with ESP32s and MQTT to build custom dashboards for tracking server rack power draw or EV charger consumption.
Current Transformers vs. Shunts and Hall-Effect Sensors
Choosing the wrong sensing topology is the most common cause of failed power-monitoring projects. Here is how CTs stack up against the alternatives.
| Feature | Current Transformer (CT) | Shunt Resistor | Hall-Effect Sensor (e.g., ACS712) |
|---|---|---|---|
| Measures DC? | No (AC only) | Yes (AC & DC) | Yes (AC & DC) |
| Galvanic Isolation | Yes (Inherent magnetic) | No (Direct electrical contact) | Yes (Magnetic) |
| Insertion Loss | Zero (Clamps over wire) | High (Creates heat/voltage drop) | Zero (Clamps or IC package) |
| Accuracy / Drift | Excellent (No offset drift) | Excellent (Tempco dependent) | Poor (Prone to thermal/offset drift) |
| External Power Needed? | No (Passive) | No (Passive) | Yes (Requires 3.3V/5V VCC) |
For pure AC mains monitoring, the CT wins on safety and zero insertion loss. If you need to measure DC battery current from a LiFePO4 bank, you must abandon the CT and use a shunt-based monitor like the INA226 or a Hall-effect module.
Decision Path: Choosing the Right CT for Your Next Build
Use this decision tree to select the exact part number for your workbench. Stop guessing and buy the right sensor the first time.
| If your project requires... | Then choose this topology... | Concrete Part Number / Pick |
|---|---|---|
| AC mains logging (up to 100A) feeding a microcontroller ADC | Split-core CT with current output (no internal burden) | YHDC SCT-013-000 (Pair with calculated external burden resistor) |
| AC mains logging feeding an analog panel meter or PLC analog input | Split-core CT with internal burden (voltage output) | YHDC SCT-013-030 (Outputs 0-1V AC directly, no resistor math needed) |
| Measuring high-frequency switching currents (SMPS or VFD outputs) | Rogowski Coil (Air-core CT) | PEM CWT Mini or generic Rogowski with integrator circuit |
| Measuring DC solar/battery current or bidirectional AC/DC flow | NOT A CT. Use a Shunt Monitor or Hall IC. | Texas Instruments INA219 (I2C Shunt) or Allegro ACS712 (Hall) |
The Default Recommendation: For 90% of hobbyist and prosumer AC energy monitoring projects, buy the YHDC SCT-013-000. Its 3.5mm audio jack termination, split-core design (no need to disconnect the mains wire to install), and well-documented 2000:1 ratio make it the undisputed standard for ESP32 ADC integration. Just remember to wire your burden resistor and DC bias network before applying power.
