Sensor Sense: Load dump proximity sensors for vehicles

Feb. 3, 2005
A special class of inductive proximity sensor has been developed for use in battery-operated systems of large vehicles.

Though sometimes dubbed a load-dump sensor, it does not detect dumped loads. The term load dump refers to special circuitry protecting the sensor from high-voltage spikes in the vehicle wiring. The spikes arise when high-power loads disconnect from electrical power sources like alternators. The disconnects are called load dumps.

As load current drops, alternator output voltage rises faster than the voltage regulator can correct. This places higher than normal voltages on vehicle power lines for periods of up to half a second. While the battery should limit the amount of voltage increase, bad grounds, broken wires, and corroded contacts limit battery voltage-clamp effectiveness. The Society of Automotive Engineers has developed a specification for "Immunity to Conducted Transients on Power Leads" known as SAE J1113-11. The SAE specification defines five pulse types of differing intensity, duration, and repeatability a device should withstand before use in an auto or mobile vehicle.

Load dump sensors differ from other sensors by the protective circuitry and shielding to meet J1113-11 specs. The entire sensor body is shielded. Zenerdiode voltage clamps are on input wiring to limit harmful transients from exceeding device ratings. And radio interference filters prevent RFI pickup in the sensor coil.

Load dump sensors work like all inductive sensors detecting the presence of a metallic object within an electromagnetic field. An electronic oscillator excites a coil of wire wound on a ferrite core. The electromagnetic field generated by the coil extends beyond the front of the sensor creating the detection region. Eddy currents are induced in any metallic object entering the field. The eddy currents drain power from the oscillator/coil circuit and reduce the amplitude of the oscillations. A detector circuit senses the lower amplitude and triggers the output circuit either ON or OFF.

Turck Inc. provided information for this article (

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