Reed contacts, although rugged, can require protection from certain loads

Contact Protection

Reedswitches can exhibit a high degree of reliability. Their contacts are sealed in an ultra clean environment at a predetermined pressure or vacuum, specifically chosen for their intended areas of application. The contact material, the size of the switch and the mechanical form may also be developed with some particular application in mind. Reed can be used at low level in circuits passing pico and micro amps, with associated switch insulation resistance in the order of 1014 if required, through to high voltage versions capable of switching 15kV or power versions that can handle several amps. Careful attention should be paid to the manufacturers’ published technical literature, to ensure the correct switch type and sensitivity range within that switch type is selected. It is assumed, in this application note, that the most suitable switch has been selected for the intended application and that the remaining problems are associated with the external factors that influence switch life, particularly external load applications.

Capacitive loads: Long wires

Capacitance across a can dramatically reduce life and even cause early contact sticking. This can occur even with low values of capacitance associated with circuit wiring. Damaging current surges occur at make or closure, in capacitive circuits, that will exceed the rating of the contact. Measures should be taken to reduce the inrush current to a minimum. The most common solution is to fit a in series with the switch.

Cable

R1 RS

V C Load R2

Lamp Loads

It is important to note, with lamp loads, that cold filaments have a resistance approximately 10 times smaller than already glowing filaments. This means that, when turned on, the lamp filament draws a current 10 times greater than when hot. This high inrush current can be reduced to an acceptable level, through the use of a current limiting resistor in series with the switch.

RS

RS R1 R2 V Lamp V Lamp

Lamp load with parallel or current limiting resistor across the switch

Inductive Loads: Relays, Solenoids and Motors

Many applications involve interfacing the main load through a system, in order to handle larger values of voltage or current. Coil operated relays or solenoid control valves possess considerable inductive values. The significant factor to guard against is the release of energy, temporarily stored in the coil, at the time the reed switch opens. The collapsing magnetic field, when an inductive circuit is broken, causes a voltage transient or back EMF. This will eventually lead to complete welding of the switch. There is a choice of devices that can be used for contact suppression that includes , RC circuit, metal oxide or transient voltage suppression . The two most common methods of suppression are diode and RC (resistor ) suppression.

Diode suppression (DC circuits only)

The diode is excellent for removing the inductive voltage spike, as the back EMF is directed through the diode. Contact erosion is reduced to a minimum but, when placed across a relay coil, release time of the relay contacts will be increased by several milliseconds while the stored energy is being dissipated. The diode chosen must have a forward current rating equal to, or greater than, the steady state current of the circuit and the diode must be connected cathode to positive.

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