What type of transformer is characterized by using only a secondary coil?

A current transformer is specifically designed to measure the current flowing through a conductor by using only a secondary coil. When current flows through the primary winding (which is usually a single conductor passing through the transformer's core), it induces a current in the secondary coil proportional to the primary current. This allows for safe measurement of high currents at lower levels, suitable for standard measuring devices.

The unique operation of a current transformer relies on its ability to produce a secondary output current that is a fraction of the actual current flowing in the primary circuit, making it essential for applications in electrical metering and protection devices. Other transformer types such as step-down, step-up, and isolation transformers involve both primary and secondary coils for varying voltage levels or electrical isolation but don't operate solely on the principle of using only a secondary coil for current measurement.