User-programmable current transducers

The HO series of current transducers, from LEM, is setting a new standard of performance, programmability and ease-of-use for designers of the latest generation of motor drives and inverters. The HO series of open-loop ASIC based current transducers deliver better performance in areas such as thermal drift, response time, power supply and noise driven by technology advances in power electronics applications.

LEM's HO series are open-loop devices, based on Hall-effect current sensing technology, that measure AC, DC or pulsed currents with a nominal value of 8, 15 or 25 A_RMS, with a response time of 2 to 6 μsec. Both of these parameters, and several others, are user-programmable by a simple serial digital bit-sequence, generated by the system's host microcontroller. Other parameters that are user-programmable include reference voltage, over-current detection limits, fault reporting and low power mode.

This series delivers its output as a scaled analogue voltage; in most systems this will be converted to a digital value by an analogue/digital converter (ADC) which requires a reference voltage. The designer can program the LEM HO-series transducer to output a reference of 0.5, 1.5, 1.65 or 2.5V on a dedicated pin. Alternatively, the HO-series can be configured to make measurements relative to an external reference.

For the HO models, LEM designed a completely new ASIC (application-specific integrated circuit). The HO provides offset and gain drift figures twice as good (over the temperature range -25 to +85ºC) as previous-generation open-loop Hall-effect-ASIC based transducers. It achieves a typical accuracy of 1% and 2.8%, at +25°C and +85°, respectively, without offset, and with a high level of insulation between primary and measurement circuits.

An innovative feature of the series is programmable over-current detection, separate from the main current measurement. Using a single measurement range (for both detection and measuring functions), if the transducer must detect an over-current condition at (say) five times the nominal full-scale value, the measurement range must extend up to the over-current limit, reducing available accuracy within the nominal range. Separate over-current detection, with programmable threshold, ensures maximum resolution is maintained up to the nominal full-scale value.

LEM's HO-series transducer operates from a single supply voltage at 3.3 or 5V. It occupies a PCB-mountable, small (12 mm wide x 23 mm long x 12 mm high), lightweight (5g) package which incorporates three separate primary conductors to allow for multi-range measurement configurations (enabling nominal measurement as low as 2.67 ARMS when programmed at 8 ARMS for the nominal range). Versions of the HO-series transducers will be available for both
through-hole and surface-mount assembly. The construction provides long creepage and clearance distances (8 mm) and a CTI (comparative tracking index) of 600 for excellent insulation.

The high degree of programmability and flexibility of the HO series means that designers can use a unique reference in a wide range of circuit configurations, simplifying designs and reducing parts stock-holdings. As well as contributing to the efficiency of variable speed drives, LEM’s HO model is also ideal for a range of applications including solar-power inverters, solar combiner boxes, uninterruptible and switch-mode power supplies, air conditioning, home appliances, static converters for DC motor drives, and robotics.

The transducer is CE marked, conforms to the EN 50178 standard and is recognised for industrial applications with a wide operating temperature range of -40 to +115°C. LEM supplies example microcontroller code to configure the serial bit stream and program the HO series' parameters. LEM offers a five-year warranty for each transducer.