Only transducers using the closed loop technology require special care when defining the power supply and its limitations. Due to the working principle of closed loop current and voltage transducers, the current consumption IC can be spit in two parts: a fixed one at primary nil plus the part which is the function of the current/voltage to be measured (I_S). The second part can be calculated as follows:

Sometimes also called “continuous or rated” current (voltage), it is the maximum permanent thermal current (voltage) that the transducer can carry.

Another definition is: the maximum rms current (voltage) which may flow through the transducer under specific conditions, so that the temperature during continuous operation does not exceed the specified value. This is measured with a 50Hz sinusoidal signal.

Used to characterize dynamic behavior of a transducer, step response time is the delay between the primary current reaching 90% of its final value and the transducer’s output reaching 90% of its final amplitude. The primary current shall behave as a current step, with a given di/dt slope (usually 100A/µs) and with the amplitude close to the nominal current value I_PN .

LEM defines the reaction time (tra) as the delay between the rise time of the output signal and the rise time of the applied signal taken at 10% of the total variation of I_PN .

The bandwidth is the frequency comprised between 0 Hz and the cut-off frequency corresponding to an attenuation of 3dB , unless otherwise specified. It is the measure how rapidly the amplitude and phase of the signal fluctuate with respect to time. Hence, the greater the bandwidth, the faster the variation in the signal parameters may be.

The vast majority of LEM closed loop transducers are specified for use with bipolar supply voltages (e.g. ±15 V). However, most transducers can also be operated from an unipolar supply for the measurement of unidirectional currents. In such cases the following must be taken into account (solution is not valid for DV and DVL family):

Depending on the type of transducer and the magnetic material used, the residual flux (magnetic remanence) of the magnetic core induces an additional measurement offset referred to as ‘magnetic offset’. Its value depends on the previous core magnetization and is at a maximum after the magnetic circuit has been saturated. Magnetization might occur:

• After a high overload current
• Due to the interruption (lack) of power supply

The offset created by the magnetization will disappear:

To measure the sensitivity and linearity, the primary current DC is cycled from 0 to I_PM then to –I_PM and back to 0 .

The sensitivity G is defined as the slope of the linear regression line over the whole current range (the cycle between ±I_PM).

The linearity error is the maximum positive or negative difference between the measured points and linear regression line, expressed in percent of the maximum measured value.

The elimination of magnetic offset requires demagnetization. A degauss cycle requires driving the core through the entire B-H loop with a low frequency AC source, then gradually decreasing the excitation returning the B-H operating point to the origin. As a minimum, provide 5 cycles at full amplitude and then decrease the excitation smoothly no faster than 4 % per cycle, requiring 30 cycles or 500 ms at 60 Hz.

Voltage sensors and current sensors giving a current output need to have a burden resistor (RB or RM - also called measuring or load resistor) connected to their output in order to obtain the correct measurement.