Current transformer [/ˈkɑrənt trænsˈfɔrmər/] n - A current transformer, or CT, is a sensor that measures the current coursing through a wire without interrupting the circuit. It works by feeling the magnetic field about the conductor and rendering it a small, measurable current or voltage upon a secondary winding.

Of Its Working

The conductor bearing the load current passes through the heart of a magnetic core. The core gathers the magnetic field and induces a proportional current in a secondary winding; that secondary current, being far the smaller, may be safely read by a microcontroller or an analog-to-digital converter.

Of Its Kinds

• Solid-core CT. [/ˈsɒlɪdkɒrɛ ˈkt/] n High accuracy, but the conductor must be disconnected to seat it. Good for permanent points of monitoring.
• Split-core CT. [/ˈsplɪtkɒrɛ ˈkt/] n Opens as a clamp and closes about the conductor. Easier to install in a live panel, though somewhat less accurate and dearer.
• Rogowski coil. [/ˈrɒgɒwskɪ ˈkɒɪl/] n Pliant, and apt for large or irregular conductors. It yields a voltage proportional to the rate of change of current, and so wants integration.

What Bears Watching

• Burden resistor. [/ˈbʌrdɛn ˈrɛsɪstɒr/] n The CT requires a known load upon its secondary side; wanting it, the output may be wrong or unsafe.
• Direction and phase. [/ˈdɪrɛktɪɒn ˈænd ˈphæsɛ/] n For the measure of power one needs the voltage also, and the phase relation between voltage and current. A CT alone measures apparent current, not real power.
• Safety. [/ˈsæfɛti/] n CTs sit commonly upon the mains side. Though the secondary be low voltage, the primary is not; installation belongs to qualified hands.

In These Pages

Current transformers appear in On the Making of Bespoke Devices, and the field note The Watts Hidden in Volt-Amps shows why measuring the right quantity matters. See also Temperature sensor for the other common sensor of a custom build.