Insight: How a current transformer works

A transformer is an electrical device that operates on Faraday’s law of electromagnetism and is used to step up or down the input voltage. Faraday’s law describes how a magnetic field interacts with an electric circuit to generate an electromotive force. This principle explains electromagnetic induction, which serves as the foundation for many electrical devices and technologies.

Several types of transformers exist, each with unique characteristics:

Current transformer: Reduces or multiplies an alternating current.
Power transformer: Transfers electrical power from one circuit to another without changing the frequency.
Potential transformer: Converts voltage from a higher value to a lower value.
Pulse transformer: Transmits electrical pulses or high-frequency signals with minimal distortion.
RF transformer: Transfers energy between circuits using electromagnetic induction.
Audio transformer: Modifies input electromagnetic signals into output signals via inductive coupling.

This article focuses on current transformers, which reduce high-voltage currents to a lower, measurable value, providing a safe method for monitoring electrical current. These transformers also offer galvanic isolation in current-sensing applications, such as switched-mode power supplies (SMPS), motor control systems, and electronic lighting ballasts.

Now that we understand the basics of how a current transformer works let’s take a look inside this small yet essential device.

Image Showing A Typical Current Transformer

Figure 1. A typical current transformer.

The markings on a current transformer are important, as they indicate the number of turns in the secondary coil. Manufacturers use unique formats for labeling each device.

For example, the transformer shown above is marked as 54XXXC, where the three X’s are replaced by digits representing the number of turns in the secondary coil. So, 54050C would indicate that the secondary coil has 50 turns.

The packaging

Closer View of Current Transformer Packaging

Figure 2. The view of the transformer’s packaging.

This transformer has a polymer housing made from UL94 V-0 material, which complies with RoHS standards. RoHS (Restriction of Hazardous Substances) compliance means the product has been tested by an independent authority for certain banned substances.

UL 94 is a plastics flammability standard established by Underwriters Laboratories. It classifies plastics commonly used in electronic devices based on their burning behavior in different orientations.

The last two alphanumeric characters in the UL 94 rating indicate flame resistance. V-0 means that flames dissipate within a maximum of ten seconds without continuing to burn the material.

The windings and pin structure

Picture Showing Windings Inside A Current Transformer

Figure 3. The windings inside a current transformer.

Copper wires serve as current-carrying conductors on the underside of this current transformer. The transformer has six terminals — two connected to the secondary winding and four to the primary winding.

The primary winding has a larger cross-sectional area because a wire’s resistance is inversely proportional to its cross-sectional area. Since the input current in the transformer is greater than the secondary current, the secondary coils have a smaller cross-sectional area to help minimize magnetic flux density.

This device contains two primary coils with a 1:1 ratio, which enhances the transformer’s safe current capacity.

The pin structure

Figure 4. The structure of the current transformer.

This current transformer is a six-pin device, with the pins serving as input and output terminals. A circular groove at the bottom left corner of the transformer helps identify the pin structure.

The primary windings are connected to pins 1 and 6, then 2 and 5, while the secondary winding is connected to pins 3 and 4. The device has a turn ratio of 1:1:200, where 200 represents the number of turns in the secondary coil.

The internal structure

Internal Structure of Current Transformer

Figure 5. The internal structure of the current transformer’s wiring.

The transformer’s polymer housing was carefully cut open to view how the copper windings overlap. The windings are placed concentrically to reduce flux leakage in the transformer.

Primary Winding and Primary Winding Insulation In A Current Transformer

Figure 6. The primary windings in a current transformer.

The core and coils

There are two primary single-turn windings, which overlap the secondary winding. Insulators made from varnish or enamel separate the primary and secondary winding. The transformer also has insulation inside to prevent the conductors from shorting.

Figure 7. The core and coils of the current transformer.

After removing some of the copper wires, it’s possible to view “the core” of the transformer. This is a toroidal design, wherein the conductor tightly surrounds the center core without an air gap.

The ferrite toroidal core transfers energy through a magnetic field. It’s used to reduce losses in high frequencies.