Reed switches can be handy little devices for detecting the proximity of a magnetic field. They draw zero current when open, and they can operate for at least hundreds of thousands, if not millions, of cycles. They are ideal for low current applications where the circuit merely checks for a reed switch contact closure to indicate a detection, but they can also directly drive moderate load currents, with some reed switches capable of handling greater than one amp.
However, some of our customers assume that they can just connect them to any load as long as the load does not exceed the maximum switch current of the reed switch. This is not true in the case of inductive loads, such as small motors or relays. One can still directly drive some of these loads, but measures must be taken to limit the voltage spike generated by the collapsing field of the inductive load when the circuit opens.
An inductive load, such as a motor or relay coil, stores energy when current passes through it. If the current is interrupted, the energy must be dissipated somehow. When the reed switch opens under such conditions, a large voltage spike will be generated across the coil, which can then arc across the contacts of the reed switch. This can significantly reduce the life of the switch, and even cause welding of the contacts in extreme cases.
For DC applications, one can minimize arcing by placing a diode in parallel with the load. This configuration allows the energy to dissipate through the coil by recirculating the current through the diode and back to the coil.
Reed Switch Powering DC Motor
This is a very effective way to protect the contacts of the reed switch, and it works well for inductive DC loads such as small motors.
However, for relays, this approach can significantly shorten the life of the relay contacts. This is because the diode slows the decay of magnetic flux within the coil, which reduces the energy available to forcefully open the armature of the normally-open contacts of the relay. A better solution for protecting both the relay and the reed switch is to place a bidirectional TVS (transient voltage suppressor) in parallel with the relay coil such as in the following circuit:
Reed Switch Powering Relay Coil
Select a TVS with a reverse standoff voltage higher than the nominal coil voltage of the relay.
When using a reed switch to energize an inductive load, one must protect the switch from an inductive kick-back voltage spike by adding an energy re-directing or dissipating device to the circuit as shown above.