Current Transformers, Toroidal Core – Custom Built


Butler Winding designs and manufactures Current Transformers for a variety current monitoring applications. A Current Transformer is a transformer that is typically designed with the coil wound on a toroidal core. Current Current_Transformer_Design_Input_Formtransformers may also be designed on a bobbin that uses a split core structure.

Where are Current Transformers found?

  • Commonly used in current measuring applications.
  • Revenue metering for utility companies.
  • Monitoring the operation of a power grid.
  • Current_Transformer_Operating_TheoryWhere there is a need to provide isolation between metering and protection circuits.
  • Requirements for safe measurement of large currents, often in the presence of high voltages.

Current transformers may also be mounted to a surface mount “header” for printed circuit board connection. Current transformers can be designed to operate from 60Hz, or line frequency, well into the MHz range which are called wideband current transformers. These electronic transformers are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages.

What is the purpose of a current transformer?

  • To measure alternating current flowing through a conductor. The output from a current transformer may be either a voltage or a current.
  • For a voltage output type, the voltage drop across a known resistor is measured to give a voltage which is proportional to the measured current. This arrangement is okay for low current applications but is often impractical for high current applications. Unless the resistor is very low in value, the resistor consumes a lot of power lowering efficiency. If the resistor value is excessively large, the accuracy of the measurement may be affected.
  • For a current output type, the current transformer can accurately measure the alternating current and put out a reasonable voltage, which is proportional to the current being measured, but without as much heat and size that an appropriate resistor would require.
  • The current transformer can perform its function with very little insertion loss into the conductor current being measured.
  • The current transformer also provides voltage isolation between the conductor and the measuring circuitry.
  • Proper function of a current transformer requires use of a load resistor. The load resistor is often referred to as a “burden resistor”.

Current Transformer Upper Design Limit

What is the core structure of a current transformer?

  • The best core structure for a current transformer in terms of electrical performance is a toroidal coil.
  • Many toroidal current transformers have only one winding. This winding is usually a “high turns” winding which functions as the secondary winding.
  • In application, the toroidal current transformer is slipped over an end of a high current wire or buss bar, which conducts the primary current. The wire or buss bar constitutes a one turn primary winding.
  • Split core current transformers are designed so that they can be assembled around a buss bar without disconnecting the buss bar.
  • “C”- cores and “U” core structures are commonly used for split-core current transformers because they are relatively easy to take apart and put back together around the buss bar.
  • Some printed circuit board applications will utilize bobbin wound current transformers with two or more windings. One winding is an integral part of the circuitry, while the other winding acts the secondary.
  • Butler Winding can make (and has made) current transformers in a wide variety of shapes and sizes. This includes toroids, “U” and “C” cores for split-core applications; various standard types of “core with bobbin” structures (E, EP, EFD, PQ, POT, and others), and some custom designs.

Current Transformer Spec Sheet
Current Transformer Glossary of Terms
Split Core Current Transformer
VIDEO Potting Prep Current Transformer
VIDEO Finish Potted Current Transformer