Why Review the LEM Closed-Loop Family

LEM has built current transducers for decades, and its closed-loop Hall family is among the most widely used in motor drives, solar inverters and battery systems. The datasheet promises high accuracy, low drift and a fast response, but the datasheet only tells part of the story. This review examines the closed-loop family from the perspective of an FAE who supports customers through design-in and bring-up, focusing on accuracy and drift, response, single versus bipolar supply, and the practical layout that makes the performance real.

Accuracy and Drift

The defining feature of a closed-loop transducer is that it holds the magnetic core at zero flux by driving a secondary current that balances the primary ampere-turns. Because the core never saturates, the transfer characteristic is linear and stable, and the offset stays low. On evaluation hardware the measured offset tracked the datasheet closely, and over temperature it remained far below that of an open-loop part. That stability is exactly what a current loop needs, because an offset becomes a torque error in a drive and a charge error in a battery system. The low nonlinearity also keeps the gain error small across the measuring range, which matters when the sensor must cover both the rated and the overload current.

Offset and the Zero Crossing

One detail that separates a good design from a marginal one is behavior when the current crosses zero, which happens constantly in a battery system as it moves between charge and discharge. A low offset keeps that transition accurate, so the state-of-charge estimate does not accumulate error each time the current reverses. This is where the closed-loop family earns its place in electric-vehicle battery measurement.

Response and Bandwidth

A closed-loop transducer responds quickly because the compensation loop corrects the output as soon as the primary current changes. On the bench the step response was clean, with no significant overshoot, and the bandwidth was wide enough for field-oriented control. The fast response preserves the phase information that the control loop depends on, so a drive can run at higher current-loop bandwidth and produce smoother torque. For a motor drive this is a direct performance benefit, not just a datasheet number.

Single versus Bipolar Supply

The bipolar-supply models, such as the LA range, suit conventional analog control boards and deliver the highest accuracy. The single-supply models, such as the LTS range, bring closed-loop performance to modern digital boards that only provide a +5 V rail. In practice the single-supply parts are the ones we recommend most often for new designs, because they simplify the power architecture without giving up meaningful accuracy, and their wide temperature range suits industrial and outdoor equipment. For higher currents or where a bipolar rail is already present, the LA range remains the natural choice.

Packaging and Layout

Both families use compact, factory-calibrated modules that are easy to mount, but the performance still depends on the layout. The primary conductor geometry is part of the calibration, so keep the conductor centered and symmetric and follow the recommended copper. Keep the secondary output away from switching nodes and bring it to the ADC with a clean ground return. In a three-phase bridge, lay out the three phase sensors identically so their errors are consistent. These simple disciplines are what turn the datasheet accuracy into board performance.

Verdict

The LEM closed-loop current transducer family earns its reputation. It combines accuracy, low drift and a fast response with the isolation that modern power electronics demand, and the single-supply models make that performance accessible to digital control boards. It is the sensor we recommend for motor drives, precision converters and battery systems where the current signal is central to performance. BeiLuo holds the mainstream models in regional stock and ships them with import declaration, certificate of origin and RoHS documents, so you can evaluate a sample and move to production without a supply gap.