INSTRUMENTATION • ELECTRONICS
movements. T e water fl ow must reach a certain rate before the switch is activated and the meter takes a measurement. In other words, if the reed switch moves within its hysteresis, it will not measure units of water. By contrast, Hall eff ect sensors do not have a switch hysteresis and therefore lack diff erent pull-in and drop-out points. T ey are activated or deactivated as soon as the water moves.
SMALLEST REED SWITCH T e mechanical structure of a reed switch also has an eff ect on its cost. Reed switches are cheaper to produce than Hall eff ect sensors, with the latter requiring features such as additional external switches, amplifi cation circuitry, temperature stabilisation, short-circuit protection and electricity consumption. T e reed switch may not be able to match the Hall eff ect sensor in terms of its minimum size, but Standex produces what it believes is the smallest reed switch in the world with a glass envelope that is just under 4mm long. T is enables compact reed design that can compete with Hall eff ect sensors on size. T e reed switch itself is enclosed in a glass envelope that is fi lled with protective gas, usually nitrogen. T is unit is also protected by a stable housing element. T e glass and the housing ensure that the reed switch is hermetically sealed from atmospheric infl uences such as dust, oil, water and chemical substances, which could impair the sensor’s functions. T e hermetic seal has the added benefi t of protecting the sensor from corrosion. Reed switches also off er a number of benefi ts when used in extreme thermal conditions, such as in very
hot or cold
temperatures. Hall eff ect
sensors begin
to suff er in terms of performance and reliability in such conditions,
whereas reed switches function perfectly
in temperatures from -65to 150°C (Hall eff ect sensors: -55 to 125°C).
T e reed switch’s mechanical
characteristics also make it immune to electronic disturbances, meaning that it does not require any protection against electromagnetic discharge (ESD), unlike a Hall eff ect sensor. As a result, reed switches have a high level of electromagnetic compatibility (EMC) with neighbouring system-relevant devices. T e Hall eff ect sensor off ers no such EMC, because it must emit an output signal and therefore requires a constant supply of power. T is means that particular EMC precautions must be taken to isolate the Hall eff ect sensor from neighbouring devices. T e most problematic area here is dealing with leakage current; special electrostatic discharge (ESD) protection is essential. T e maximum isolation of a reed switch is 1,015 Ohms, which is many times higher than that of a Hall eff ect switch. Leakage currents still occur with reed switches. However, these currents are in the fempto- amp range, which is so insubstantial that it is within the tolerances applied to medical devices. T e mechanical principle on which a reed switch is based also allows it to measure tiny voltages. T is is due to the low contact resistance of 50 mOhms, which is lower than Hall eff ect sensors’ contact resistance.
T e fundamental range of potential loads that can
LEFT: A reed switch consists of two ferromagnetic separators that are hermetically sealed in a glass tube. The two separators overlap. If a magnetic fi eld is applied to the switch, the two blades move towards each other and the switch closes
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Each solution has its own merits
be switched using a reed switch is huge. It extends from the nanovolt to kilovolt range, from fempto amps to amps and enables frequencies of up to 10GHz. Even the smallest of reed switches are capable of isolating voltages of up to 1,000V. No ESD protection is required for this.
APPLICATION-SPECIFIC CONFIGURATION Reed switches are available in a variety of sizes with diff erent ampere turns and hystereses regarding magnet distance and magnet size. T anks to the operating principle of the switch, these reed parameters can be adapted to application-specifi c requirements, in other words the special requirements of energy effi ciency classes. T is puts the Hall eff ect sensor at a disadvantage. A Hall chip can be programmed to compensate for this weakness, but it still lags behind the reed switch when it comes to application-specifi c confi guration. Hall eff ect sensors are more suited to applications with frequencies higher than 1kHz. T ese include high-speed sensors for measuring revolutions per minute. Due to the lack of switch hysteresis, the Hall eff ect sensor also off ers much higher repeatability when it comes to signal measurement. T is also means that they have longer life cycles. T e Hall eff ect sensor and its 500 million switch cycles is far superior to reed technology in this respect. However, if the reed switch is required to switch predominantly small loads below 5V, as in the case of meter applications, switch cycles into the billions can be achieved using reed switches.
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