INSTRUMENTATION • ELECTRONICS
BOTH TECHNOLOGIES HAVE THEIR OWN SPECIFIC USES “Demand for reed-based sensors has risen considerably recently against the backdrop of Germany’s energy transition and the requirement to prioritise energy effi ciency in the manufacture of end devices,” says Martin Reizner from sensor manufacturer Standex. Reizner does not see this trend coming to an end any time soon. “More and more manufacturers are switching to reed technology, particularly in the case of white goods such as dishwashers and fridges or battery-powered devices.” Sensor manufacturers and customers
alike are focusing on minimising the application’s electricity consumption, which can be optimised and/or reduced in accordance with the device’s requirements through the use of reed technology. T e reality is that, “Both technologies
Hall eff ect sensors function due to the Hall eff ect. Electrical current fl ows through a conductor, with a magnetic fi eld positioned perpendicular to the direction of current. A diff erence in current is created perpendicular to the electrical and magnetic fi eld planes
consumption to zero when the switch is in a passive state. T at is why reed switches are an attractive alternative to Hall eff ect sensors for applications in which energy effi ciency and low power consumption are key.
D LEFT: The comparison with a
Euro cent coin shows the size of the switches. The smallest reed sensor
(right) made by Standex Electronics, with a length of 4mm (glass
envelope) reaches the dimensions of the Hall eff ect sensor (left)
Reed and Hall eff ect technologies are based on diff erent construction concepts. Both are controlled and activated by means of an external magnetic fi eld – however a Hall eff ect sensor still requires an electrical circuit to operate, which needs power even when the sensor is in a passive state as its construction principle is based on the provision of an output signal. T e reed sensor, on the other hand, is a mechanical switch that is only activated by a voltage pulse to switch a load. If it does not receive this pulse, it does not consume any power. T is key diff erence has a variety of consequences for specifi c applications of both forms of technology.
emand for reed switches has risen considerably in the recent past. T is is primarily due to their mechanical structure, which reduces energy
have their own specifi c uses,” Reizner says. He believes that Hall eff ect sensors are better suited to high-speed applications in the range of 1kHz and above, because this is where the reed contact meets its physical operating boundaries. For applications with frequencies lower than 1kHz, Reizner recommends reed switches. T ese applications include fl ow meters, which are used in white goods. Even though there are not yet any safety standards for reed switches and each application must go through a separate authorisation and safety approval process, they off er one key advantage over Hall eff ect sensors: switch hysteresis.
SWITCH HYSTERESIS Due to its physical characteristics, the mechanical reed switch features switch hysteresis, which describes the diff erence between the pull-in and drop-out points of a reed switch. In other words, if a passing magnet reaches a pre-defi ned pull-in point 5mm away from the reed switch, for example, its blades will close and the reed switch is now active. If the magnet continues to move until reaching a pre-defi ned drop-out position of 7mm, it will only turn off at this point. And so the switch hysteresis equals 2mm. T is feature may be required in certain applications, such as a reed-based water fl ow meter with a paddle wheel. T is switch is resistant against random movements of the water’s waves, which may slightly move the paddle wheel. Manufacturers do not want the switch to be activated, which would cause a measurement to be taken, due to such
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