Current Transformer

current transformer

A Current Transformer detects electric currents in circuits to generate signals proportional to that current and the signals can be displayed, measured or stored for further analysis. Current sensors will measure AC and DC current. While the physical principles are the same, the details of a “current” sensor compared with a voltage transformer will differ owing to different requirements of the application. Shop now and see our wide ranges of current sensing and voltage sensing products.

Current Transformers are of two main types, open or closed loop. The open-loop transformers measure AC and DC currents and provide electrical isolation between the circuit being measured and the output of the sensor (the primary current is measured without electrical contact with the primary circuit, providing galvanic isolation). Open loop current transformers are usually preferred in battery-powered circuits for their low power needs and small footprint features. Open-loop current transformers are also typically cheaper than closed loop CT’s.

Current Transformer types

  • Hall Effect – Hall Effect current transformer devices have a core, Hall Effect device and signal conditioning circuitry. When the Hall device is exposed to magnetic field from the core it produces a potential difference  which can be measured and amplified. Current transformer work when a current conductor passes through a magnetically permeable core that concentrates the conductors magnetic field.
  • Inductive – This type of current transformer uses coils which current-carrying wires pass. Making power that is proportional to the current flow in the coil. The current flow then can be measured or transformed.
  • Magnetoresistive – The property of certain materials changes the value of its resistance when a current flows through the plate. When currents flow for a longer distance, resistance is increased and decreased at the shorter distance.

Closed Loop

Closed-loop transformers provide electrical isolation and measure AC and DC currents. They ideal for their fast response, low temperature drift and high linearity. The closed-loop sensor current output is relatively immune to electrical noise. Due to its Hall-Effect sensor feeds back an opposing current into a secondary coil, the Closed- Loop sensor is sometimes called a ‘Zero-Flux’ sensor. The secondary coil is wound on the magnetic core to zero the flux produced in the magnetic core by the primary current. When high accuracy is priority, closed-loop sensors are a good choice of sensor.

current transformer

Open Loop

Open loop current transformers have a Hall sensor mounted in the air gap of a magnetic core. A conductor gives a magnetic field that is comparable to the current. The magnetic field is measured by the Hall sensor and concentrated by the core. The signal from the Hall generator is amplified as it is low, this amplified signal becomes the sensor’s output. Normally open-loop sensors have circuitry that provides temperature compensation and calibrated high-level voltage output. While they are cheaper than their counterparts, they can be prone to saturation and temperature drift. However by injecting a positive coefficient in the control current to reduce the drift in sensitivity over temperature the drift can be minimised.

Current transformer

High Frequency Current Transformer (HFCT)

Frequency Response up to 500 MHz – Measurement range up to 20kA

The Magnelab High Frequency Current Transformer – designed to provide a very accurate, non contact, non destructive measurement of either a single or a repetitive bipolar or unipolar pulse. Furthermore, it contains a solid toroid core and will measure input ranges of 200 to 20,000 amps DC or 2 to 71 amps AC. It comes with opening sizes from 1/4 to 2 inches. In addition, it’s voltage output ranges from 0.025 to 5.0 V/A. It also works for the continuous wave.

Broad Frequency Response

The AC current sensors in this series measures current from just microamperes up to 20,000 amperes. These are broad frequency response CTs that can operate from just 0.5 Hz to 500 MHz. The devices come in five different case sizes and you will choose between an SMA or a BNC connector type.

Another high accuracy Current Transformer is the HT-300M Hall effect current transducer. This has a fast response and provided with a 3 kV proof stress. Unlike the HFCT, which measures AC current, the HT-300M measures DC current of up to 300 amperes.


  • Higher frequency cutoff, assuring faithful rendering of fast transients
  • Up to five times more sensitive for low intensity signal measurement
  • Operation temperature -20 degree C to plus 120 degree C
  • Higher nominal accuracy ±0.5% over a wider bandwidth

Primary Winding

The Current Transformer is different to the voltage and power transformer as its primary winding has only one or few turns. This primary winding can be:

  • A coil of heavy duty wire wrapped around the core
  • A conductor or bus bar placed through a central hole or
  • A single flat turn, or just as shown.

As the primary winding is in series with the current carrying conductor supplying a load the current transformer is also known as a “series transformer”. However the secondary winding may be wound on a laminated core of low-loss magnetic material however and have more coil turns. So that the density of the magnetic flux created is low using smaller cross-sectional area wire, the core has a large cross-sectional area. This is dependant upon how much the current needs to be stepped down as it attempts to output a constant current which is independent of the connected load.

Different to the voltage transformer, the primary current of a current transformer is controlled by an external load and is not dependent of the secondary load current. The rating of the secondary current is usually at a standard 1 Ampere or 5 Amperes for larger primary current ratings. Accurate and small instruments can be used with CT’s being insulated away from any high-voltage power lines because Current transformers can reduce or “step-down” current levels from thousands of amperes down to a standard output of a ratio to 5 Amps or 1 Amp for normal operation.

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