An AC current transformer (CT) is a passive electromagnetic device that steps down high alternating current in a primary conductor to a safely measurable, proportional low current in a secondary winding while maintaining galvanic isolation. In a real circuit or installation, a CT changes a dangerous, unmeasurable 100A+ mains line into a milliamp-level signal that a 3.3V microcontroller can read safely without any physical connection to the high-voltage source. Hobbyists and engineers commonly confuse CTs with Hall-effect sensors (like the ACS712), which can measure DC but introduce insertion loss and heat, or with Potential Transformers (PTs), which step down voltage rather than current.
How an AC Current Transformer Changes Your Measurement Circuit
When you need to monitor the power consumption of a 240V electric vehicle charger or a home subpanel, you cannot feed that current directly into a multimeter or an Arduino. The CT solves this by using magnetic induction. The primary 'winding' is simply the single mains wire passing through the center of the CT's ferrite or laminated steel core. The secondary winding consists of hundreds or thousands of turns of fine enameled copper wire wrapped around that core.
Think of a CT like a mechanical step-down gearbox: it trades high primary current for a manageable secondary current, scaled strictly by the physical turns ratio. Because the measurement circuit is magnetically coupled rather than electrically connected, the CT provides vital galvanic isolation. If a voltage spike hits your microcontroller, the mains side remains completely unaffected, and vice versa. This isolation is why OpenEnergyMonitor and commercial systems like the Emporia Vue rely exclusively on CTs for non-invasive panel monitoring.
The Math: Turn Ratios and Burden Resistor Sizing
Microcontrollers read voltage, not current. To convert the CT's secondary current into a readable voltage, we must place a 'burden resistor' across the secondary terminals. The value of this resistor dictates your measurement range and resolution.
Let's walk through a worked numeric example using the most popular DIY sensor on the market: the YHDC SCT-013-000. This is a split-core CT rated for 100A primary current, with a secondary output of 50mA. This implies a turns ratio of 2000:1 (since 100A / 2000 = 0.05A).
ADC Linear Range: ~0.1V to 3.1V (biased at 1.65V)
Max Allowable Peak Voltage: 1.65V (to prevent clipping the AC waveform)
Step 1: Calculate Maximum RMS Voltage
Since the AC waveform is centered around our 1.65V bias, the peak voltage cannot exceed 1.65V. To find the maximum RMS voltage the burden resistor can develop:
V_rms = V_peak / √2 = 1.65V / 1.414 = 1.167V
Step 2: Calculate the Ideal Burden Resistor
At the CT's maximum rated primary current (100A), the secondary current is 50mA (0.05A) RMS.
R_burden = V_rms / I_secondary = 1.167V / 0.05A = 23.34 Ω
Step 3: Select a Standard Resistor Value
The nearest standard E12 resistor value is 22 Ω. Let's verify the peak voltage with a 22 Ω resistor at full 100A load:
V_rms = 0.05A × 22 Ω = 1.1V
V_peak = 1.1V × 1.414 = 1.55V
Since 1.55V is safely below our 1.65V headroom limit, 22 Ω is the perfect choice. For power dissipation, P = I²R = (0.05)² × 22 = 0.055W. A standard 1/4W (0.25W) resistor is more than adequate.
Where You Meet AC Current Transformers in Practice
You will encounter CTs in almost every modern power-monitoring application. In residential smart panels, arrays of split-core CTs are clamped directly onto the THHN insulation of branch circuits. Because they are split-core, the installer does not need to disconnect the wire or cut the circuit to install them.
In solar PV installations, solid-core CTs are often hardwired inside the combiner box to monitor individual string output, ensuring no shading or cell degradation is occurring. Industrial motor drives (VFDs) use high-precision, multi-ratio CTs for closed-loop torque control and instantaneous overcurrent fault protection. According to Electronics Tutorials, protective relaying CTs in industrial settings are specifically designed to maintain accuracy even during massive fault currents, unlike metering CTs which intentionally saturate to protect downstream instruments.
Sizing and Specifying: Common CT Models Compared
Choosing the right CT depends on your maximum expected current, whether you need a built-in burden resistor, and your physical installation constraints. Here is a comparison of standard models available to makers and integrators:
| Model | Primary Rating | Secondary Output | Core Type | Best Use Case |
|---|---|---|---|---|
| YHDC SCT-013-000 | 100A | 50mA (Current) | Split-Core | DIY ESP32/Arduino energy monitors (requires external burden) |
| YHDC SCT-013-030 | 30A | 0-1V (Voltage) | Split-Core | Direct plug-in to analog ADCs (internal burden installed) |
| Magnelab SCT-0500 | 50A | Current / Voltage | Solid-Core | High-accuracy commercial metering and solar inverters |
| Talema AS-100 | 100A | Current | Toroidal (Solid) | Custom PCB integration where wire is routed through the board |
For permanent, high-accuracy installations where the wire can be disconnected during assembly, solid-core CTs (like those from Magnelab) are superior. The continuous ferrite ring eliminates the air-gap flux leakage inherent in split-core designs, yielding vastly better linearity at low currents.
Frequently Asked Questions About AC Current Transformers
Can I use an AC current transformer to measure DC battery current?
No. CTs operate strictly on Faraday's Law of Induction, which requires a changing magnetic field to induce a secondary current. Direct current produces a static magnetic field, resulting in zero secondary output. To measure DC battery current or solar charge controller output, you must use a Hall-effect sensor (like the ACS758) or a precision shunt resistor with an isolated amplifier.
What happens if I leave the secondary wires of a CT open-circuited?
If the secondary circuit is open while primary current flows, the CT acts as a step-up voltage transformer. The secondary voltage will spike until it arcs across the terminals or breaks down the internal wire insulation, permanently destroying the sensor and creating a severe shock and fire hazard. Always ensure a burden resistor is soldered in place, or short the secondary wires together if the CT is installed but not yet connected to a measurement circuit.
Why does my CT reading fluctuate when the wire is moved inside the split core?
Split-core CTs have a tiny air gap where the two halves of the ferrite core meet. This air gap introduces magnetic reluctance. If the primary wire is pushed right up against the hinge or the mating surface of the split core, the magnetic flux path becomes asymmetrical, causing measurement errors of up to 5-10%. For the most accurate readings, route the primary wire directly through the physical center of the CT window.
Do I need a burden resistor if my CT has a built-in 3.5mm audio jack?
It depends on the exact model suffix. If you are using a 'voltage output' CT (like the SCT-013-030), the manufacturer has already soldered a surface-mount burden resistor inside the plastic housing, and the 3.5mm jack outputs a 0-1V AC signal. You do not need an external resistor. However, if you are using a 'current output' CT (like the SCT-013-000), the jack simply breaks out the raw secondary coil wires. You must add your own external burden resistor across the analog input and ground on your breadboard or PCB.
