Technical Introduction

Open Loop current transducer operation principle

The magnetic flux created by the primary current Ip is concentrated in a magnetic circuit and measured in the air gap using a Hall device. The output from the Hall device is then signal conditioned(amplified) to provide an exact representation of the primary current at the output.

The strength of magnetic flux is proportional to the primary current and the hall element driving current Ic (mostly constant current drive). The voltage of the hall element while magnetic flux applied can be indicated by VH ={K/d(gap of core)}x Ic x a x Ip. To be more simplied, VH = b( constant) x Ip. Then we can measure the input current.


  • Small package size
  • Extended measuring range (3 times of nominal current while under 50A)
  • Quick response (3uS generally)
  • Reduced weight
  • Low power consumption
  • No insertion losses

Output characteristic:

  • While positive primary current applied on/through CT, it will
  • output +4V(stardard type, dual power supply); on the contrary,
  • when negative current applied, it will show -4V.

Close Loop current transducer operation principle

The magnetic flux created by the primary current lp is balanced by a complementary flux, which is produced by driving a current through the secondary windings. A hall element and associated electronic circuit are used to generate the secondary (compensating) current that is an exact representation of the primary current.

The whole system is likely a transformer characteristic. The multiply of primary current Ip and the primary turns Np on the core will be the same as secondary current Is and the secondary turns Ns. It can be indicated by: Ip*Np=Is*Ns. There is no magnetic flux remain on the core, so it also called compensated type or zero magnetic flux current transducer. This make the close loop operated on batter performance especial on temperature effect.

Features :

  • Wide frequency range( about 2 times of nominal current load)
  • Good overall accuracy
  • Fast response time ( 1uS possible)
  • Low temperature drift
  • Excellent linearity
  • No insertion losses

CT Application in Inverter

CT Application in UPS

CT Application in High Frequency Welder