APCS the Signal Conditioning Specialists

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Sensor and Probe Functions

Current Transformer


APCS sells two type of current transformers, ac Current Transformers and hall effect current transformers.


ac Current Transformers

Basic Current Transformer Theory

The ac current transformer is the traditional item where the name comes from as it is a transformer in the true seance. Being a traditional transformer it operates correctly over a small frequency range typically 50 or 60 Hz with a sine wave shape.

Most current transformers will specify the primary and secondary current 400/5. This means that when 400A passes through the cable in the measurement window there will be a 5A current on the secondary. The secondary of the current transformer should always be connected to the measurement device. If 400 A were passing through the primary and the secondary is un-terminated then the output voltage will rise to an extremely high level and may damage the insulation of the CT.

APCS also sells a range of small CT's with mA outputs. These CT's have zenar diodes mounted internally to prevent over voltage if exterminated. There will be a load resistance specified that is mounted inside or on the input terminals of measurement device.


Hall Effect Current Transformer

The current conductor passes through a magnetically permeable core that concentrates the conductor's magnetic field. The Hall effect device is mounted in the core at a right angle to the concentrated magnetic field. When the energised Hall device is exposed to a magnetic field from the core, it produces a potential difference (voltage) that can be measured and amplified.

It can be seen that the Hall Effect CT is a complex transducer that requires a power supply to operate is circuits and produce an output. The big advantage is that its output is the same wave shape as the input current and operates on direct and alternating currents. This means complex currents produced by complex machines can be measured and processed without using a shunt.

Hall Effect Sensor


A shunt is a small known resistance that all load current must pass through. The voltage drop is then measured across the shunt and the load current is determined by the use of ohms law. The measurement terminals are elevated or have a common mode offset making measurement more difficult but sill practical.

50A Shunt



The sensor category will select various third party probes that APCS sells. For more information on the output from these sensors please refer to the input selection guide.