The chief benefit of reed switches is that they draw zero power when they’re closed. Here are the basics of employing these versatile components.
Contributed by Standex Electronics, www.standexelectronics.com
It’s easy to see why reed sensors and relays are good options for circuits that put a premium on energy efficiency. Normally closed (Form B) reed sensors and reed relays draw zero power in their normally closed states. A latching reed relay uses minimal power when ‘setting’ or ‘resetting’ its contact states (bi-stable). Similarly, latching reed switches and sensors (bi-stable) use the simple movement of a permanent magnet to change the state of reed contacts — no electrical power necessary.
The Form A reed switch
Reed switches are normally open. This is typically referred to as single-pole normally open, single-pole single-throw (SPST), or Form A. The two reed switch leads are ferromagnetic and are hermetically sealed in a glass capsule.
The contacts in the normally open reed switch close in the presence of a magnetic field. The contacts stay closed as long as the magnetic field remains. The contacts open once the magnetic field is removed. So if the magnetic field comes from an electromagnet, energy is expended the entire time the contacts are closed. This makes the closed state less than ideal from the view of power consumption.
The Form C reed switch
Another type of reed switch is the single-pole double-throw (SPDT) or Form C reed switch. It has one common lead, a normally open lead, and a normally closed lead. With no magnetic field present, the common contact maintains a connection to the normally closed contact. The reed switch draws no power while in its normally closed state. When a magnetic field is applied, the common reed element swings from the normally closed to the normally open contact. Once the magnetic field is removed the common contact swings back to the normally closed contact.
The normally closed (Form B) reed switch and sensor
Recall that the natural state of a form A reed switch is normally open. It can be turned into a normally closed switch by applying a permanent magnet with a field strong enough to close the reed contacts. This biasing magnet must be greater than the pull-in field or operate field that closes the contacts in the normally open condition.
The polarity of the magnet does not matter. But to open the contacts, a stronger permanent magnet with an opposite polarity must be brought near the biasing magnet.
Normally closed (Form B) reed relays
Numerous applications demand switch contacts that are closed for long periods, only opening when a fault condition arises. The normally closed (Form B) reed relay was designed for just such a situation. It has a biasing magnet so in the closed position, the relay coil draws no power. Applying power to the coil cancels out the biasing magnet to open the contacts.
The Form B sequence
It can be useful to review the step-by-step sequence of operation for a Form B reed relay. The nearby graph shows the sequence for a reed switch having an operate (pull-in) field of 4 mT and a release (drop-out) field of 2 mT. The biasing magnet has a field strength of 5 mT incident on the reed switch. This field strength exceeds the pull-in point of the reed switch, so the contacts close (point 1). Next the coil applies an opposing magnetic field of 4 mT. The net result of the two magnetic fields is 1 mT. This net field strength is below the drop-out of the reed switch, causing the contacts to open (point 2). Finally the coil is turned off and the contacts close because the magnetic field strength is back to 5 mT (point 3).
The voltage polarity of the coil applied to the Form B relay determines the magnetic polarity of the coil. This voltage polarity is determined by design and the polarity is marked on the relay. The relay will malfunction if it sees a reverse voltage polarity.
Also, applying a voltage above the specified nominal voltage can force the contacts to re-close. Generally, the re-close voltage is specified at 50% above the nominal. Essentially this means that applying more than 7.5 V for a 5-V nominally rated Form B relay could cause contact reclosure. If this is a concern, relay designers can adjust the magnetic design to boost the specified re-close voltage.
Latching reed relays/reed sensors
A latching reed relays/reed sensor, by definition, can exist in two states — its unlatched/open state or its latched/closed state. There’s no power required to keep the reed switch in either state.
Latching is possible because of the natural hysteresis between the operate (pull-in) and release (drop-out) points of the reed switch. The higher the operate point, the larger the hysteresis. The larger the hysteresis, the easier it is to establish the latch and unlatch points from a design standpoint. A permanent magnet is necessary to bias the reed switch, allowing it to operate in the latching mode.
Latching reed relay
A latching reed relay uses a Form A reed switch in conjunction with a permanent magnet. The reed switch may be latched in its normally open state or its normally closed state. Its state depends upon the
magnetic field it has experienced last. Applying the correct magnetic polarity to latched-open contacts will change them to a closed state. The reed switch will remain in its closed state until another magnetic pulse with the opposite magnetic polarity is applied.
It takes negligible power to pulse the coils of latching reed relays. Generally a 2-msec pulse supplied at the relay nominal voltage is enough to change the state of the relay contacts. Thus the power consumed closing and opening the relay contacts is minimal and it produces minimal heating.
The latching and unlatching sequence
For a better understanding of latching and unlatching, consider the operation of a reed switch that has an operate point (contact closure) when a field of 4 mT is applied and a release point (contacts open) at 2
mT or below. Assume the biasing magnet has a magnetic field strength of 3 mT. In the nearby figure we have sequentially chosen a full operate cycle, showing all the operate states. As can be seen the pull-in and drop-out points do remain constant and appear as constant lines.
The five stages and reed switch contact state are:
Stage 1: Here the biased magnetic field (BMF) which is always present and applied to the reed switch is shown at the 3 mT level. The contacts are open.
Stage 2: An external magnetic field (EMF) from a coil or permanent magnet is applied producing a 2 mT magnetic field that adds to the biasing magnet’s field. The combination of the two fields puts the magnetic field applied to the reed switch at 5 mT, exceeding the 4 mT level, and closing the contacts.
Stage 3: Now the EMF is removed leaving only the BMF. But the field strength is still above the drop out level, so the contacts stay closed.
Stage 4: The EMF is again applied, but this time the field opposes the BMF, reducing the net magnetic field strength to 1 mT. The net field is below the drop out level and the contacts open.
Stage 5: The opposing EMF is removed leaving only the BMF, and the reed contacts stay in the open state.
The cycle can either be accomplished using two coils or by reversing the polarity of a single coil. The former case costs more because there are two coils; the latter case takes more circuitry to change the polarity for each contact state change.
Using latching reed switches
Latching reed switches work in the same manner. However, another permanent magnet is used with a different polarity instead of a coil. Here, the contacts stay closed when the permanent magnet is removed. They remain closed until a permanent magnet with a polarity opposing the biasing magnet is brought close to the reed. The permanent magnet uses no electrical power so there’s no need for power supplies, electronics and timing circuits. And as with latching reed relays, one or two magnets can be used for changing the contact state.
• Using one magnet: Once a permanent magnet has been brought close to the reed switch the contacts close. When the permanent magnet is then withdrawn the contacts stay closed. The permanent magnet then must be rotated, reversing its magnetic polarity. When it is then brought close to the reed and biasing magnet again, it will open the contacts.
• Using two magnets: To latch, one magnet approaches from one direction to close the contacts and is then withdrawn. To unlatch, the opposing magnet approaches from the other direction showing an opposite polarity and thereby opens the contacts. This action can take place in several ways depending upon the type of movement an application requires.
Latching reed switches can require a fine balancing of the magnetic system, particularly when ferromagnetic materials are nearby. It’s often helpful to work with component application engineers because there are many ways to accomplish latching. For a given set of circumstances, application engineers can often come up with professional, simple, and economic approaches.
To summarize, Form B reed sensors or reed relays may be the best option when the contacts are expected to be closed for long periods. When power consumption in both the open and closed state is a consideration, a latching reed switch or latching reed relay may be best. The latching reed switch is the only sensor technology that needs no power for operation and release of the contacts. With the rising demand for low-power components, the latching or normally closed aspect of a reed switch can be an advantage.