Some special field applications, for the sake of circuit logic, security concerns, or a power saving necessity, require a sensing signal which is normally closed (on) to go off at certain times. This could be for example, a product which comes sealed with a battery inside that should not be used until the product is installed. Another example would be, a type of sensing where it is not convenient to install an actuating magnet and only a Ferro-magnetic vane can be used. Such applications can be classified as Magnet Biasing type Applications.
Such situations require a biasing type normally closed function trigger that needs to change the state of a bi-stable or normally closed reed switch. Normally closed or bi-stable Reed Switches are constructed by using a combination of a Reed Switch and a biasing magnet, near it. The biasing magnet keeps the Reed Switch in a closed state.
The “opening” of a normally closed Reed Switch is usually achieved by actuation with the opposite pole of magnet or by a ferro-magnetic shunt which comes in between the magnet and the reed switch. All Biasing type applications are also classified as position sensing applications but not all position sensing type applications use biasing magnets.
A basic normally open for Form A type of Reed Switch can be modified with an external magnet to work as a normally closed or a latching (bi-stable) device. The best method is to use a permanent magnet close to the reed switch to keep it “closed”, and to bring a larger magnet with an opposite polarity to “open” the reed switch. The same function can also be achieved by bringing a Ferro-magnetic part between the reed switch and the biasing magnet which will shunt the field away from the reed switch and “open” it.
As Reed Switches undergo a special heat treatment or annealing prior to assembly, it ensures that the magnetic remanence of the reed blades is extremely low. This helps in applications where the reed switch may need to be closed for months or years by ensuring that the blades spring back and open up as soon as the magnetic field is removed or shunted.
For such biasing type applications, it is also important that the release AT of the reed switches being used are within a tight tolerance of the Operate AT to maximize the production yield of the biased reed switches. The only care that needs to be taken when using biased reed switches is that the actuating magnet should not come closer than the actuating point where the reed switch opens as this could lead to a pole reversal which in reed switch parlance is called re-closure.