Reed switches
Reed switches are widely used for signal switching, particularly at the lower signal frequencies. The main attraction of the reed type of switch is that the contacts are enclosed, and cannot therefore be damaged in adverse environments. In addition, the switch is magnetically operated either by a solenoid coil or by a permanent magnet, so that remote control is possible and variable capacitance to the hand of an operator is not a problem. In this respect, the reed switch is normally used as a secondary switch or relay in the sense that the operating current to its coil will normally be controlled by another switch. Reed switches are generally classed as relays for the purposes of cataloguing electronic components. The reed switch can therefore be mounted directly on a PCB deep inside the circuit, possibly in an inaccessible place, and operated from any conventional type of low- voltage switch on a panel. This avoids the problems of bringing a signal cable out to a panel-mounted switch and back again.
The basic reed principle is illustrated in Figure 12.6. The thin metal reeds are sealed into a glass tube, and bent so as to make contact because of the effect of a magnetic field. The usual arrangements of contacts are normally open or changeover, and the reed portion can often be specified separate from the actuating coil or magnet. The use of the solenoid coil allows a relay type of operation, but the use of a permanent magnet allows mechanical operation (by moving the magnet to or from the reed tube) which can be custom-made to whatever pattern is needed. The typical magnet distances are of the order of 7-11 mm to operate the reed and 13-16 mm to release, so that a movement of around 5-20 mm would normally be used to ensure reliable opening and closing of the reed circuit. The predominant use of reeds, however, is with the solenoid, using up to 100 ampere-turns to operate the reed.
One important feature of reed-switch use is the switching time which is fast by the standards of mechanisms. This will be of the order of 1-2 ms to make, and can be as low as 0.2 ms to break, with reed resonant frequencies in the order of 800-2500 Hz. Fast switching speed is not necessarily an important feature of signal switches, but if signals have to be sampled, or if there has to be switching from one signal source to another at frequent intervals, then the reed type of switch has considerable applications.
An important further consideration for high-frequency or fast rise-time signals is the very low capacitance between open contacts, which can be less than 1 pF for the normally-open type of reed. The use of silver or gold- plated contacts allows low contact resistance values of around 100 mN, and for applications that demand very low and stable contact resistance, mercury-wetted reeds can be obtained. The mercury-wetted type also has the advantage of providing very low-bounce operation, a point that will be taken up later.
The other features of reed switches are high insulation resistance of the order of 100 000 MQ (because of the use of glass encapsulation), and high breakdown voltage of at least 200 V. Note that the use of nylon as an insulator can affect these figures adversely, because the insulating properties of nylon deteriorate as the temperature rises. The operating currents are also quite surprisingly high for a device with small contacts and limited contact force, of around 0.25-2 A. The mechanical life, depending on type of use and reed construction, ranges between one million and one hundred million operations.
The normal operation of the reed relay by its solenoid coil can raise one problem which is common to any device that uses an inductor. ;\’hen the current through the coil is broken, the usual back-EMF is generated, and this can cause sparking between switch contacts (of the switch used to operate the solenoid) or breakdown of a transistor if this is used as the coil driver. ;\’hen a reed switch and solenoid are constructed as one unit, the coil will usually incorporate a protection diode to avoid the effects of the back-EMF, and this implies that the polarity of connections to the coil will have to be observed. This in turn means that such a coil should not be used if you are operating the coil from an AC supply (as a 100 Hz sampling circuit, for example). If you are using a separate coil and reed, you will need to incorporate the diode for yourself (Figure 12.7) if you are using a DC supply to the coil.