Power-Efficient Switching Solutions Using Reed Switches and Magnetic Control
As portable electronics continue to grow, reducing power consumption is more critical than ever. Engineers are turning to energy-efficient components like Form B reed sensors and latching reed relays, which operate with little to no power. Reed switch technology enables low-power design by using magnetic fields instead of electricity to control switching functions.
If there are particular areas you need help with, feel free to skip to any of the following sections:
- Why Power Efficiency is Critical in Today’s Electronics
- Reed Switch Applications in Real-World Electronics
- Form A Reed Switch: Normally Open Operation
- Form C Reed Switch: Dual Contact Switching Works
- Form B Reed Switches & Sensors: Power Efficient Normally Closed Design
- Form B Reed Relays: Efficient Normally Closed Switching
- How Form B Reed Relays Operate: Step-by-Step Guide
- Latching Reed Relays & Sensors: Retain State Without Power
- How Latching Reed Relays Work: Internal Mechanism
- Magnetic Pulse Control: How Latching & Unlatching Work
- Designing with Latching Reed Switches: Using Magnet Control
- Summary: Choosing the Right Reed Technology for Magnetic Control and Power Efficiency in Low-Power Designs
Why Power Efficiency Is Critical in Today’s Electronics
In today’s energy-conscious world, engineers are under increasing pressure to design smarter, more efficient systems. Whether it’s battery-powered devices, smart appliances, or industrial sensors, the demand for low-power solutions is growing rapidly. That’s where reed switch technology shines.
Unlike Hall effect sensors, which require continuous electrical power even in standby mode, reed switches operate passively, consuming zero power until activated by a magnetic field. This makes them an ideal choice for applications where energy savings are critical. Reed switches are ideal for low-power design because they consume zero energy until activated by magnetic control, making them perfect for battery-operated and energy-conscious applications.
In this article, we’ll explore how reed switches deliver reliable, contactless switching with minimal power draw. We’ll compare them to other magnetic sensing technologies, highlight their unique advantages, and show how they’re transforming industries from home automation to medical devices.
What You’ll Learn
- How reed switch technology enables ultra-low power switching, ideal for battery-operated and energy-sensitive applications.
- The differences between Form A, B, and C reed switches and how each configuration impacts power consumption and contact behavior.
- Why latching reed relays and sensors are uniquely power-efficient, maintaining state without continuous power.
- Real-world applications of reed technology in medical devices, smart home systems, automotive safety, and industrial sensors.
- Design insights for selecting the right reed switch or relay based on magnetic field strength, polarity, and switching requirements.
Reed Switch Applications in Real-World Electronics
Reed switches are ideal for low-power design because they consume zero energy until activated by magnetic control, making them perfect for battery-operated and energy-conscious applications.
- Battery operated portable electronics requiring switching functions
- Safety circuits requiring a normally closed switch drawing no power that will only be activated (opening the circuit) when a fault condition develops
- Applications where the switching function is closed for a long time drawing no power
- Applications where the switching function is either open or closed for long periods of time and draws no power while in either state
- Low microvolt offset requirements
- Switching functions required in a ‘no power’ or ‘lost power’ requirement
- Automotive Systems – Position sensing and safety interlocks with zero standby power consumption.
- Consumer Electronics – Power-saving activation mechanisms in portable and battery-operated devices.
- Medical Equipment – Used in low-power wake-up sensors, such as a redesigned wearable device that eliminated the need for a charging cradle by switching to a normally-open reed switch and a primary battery. This reduced cost, improved usability, and extended battery life.
- Industrial Sensors – Contactless switching in harsh environments with high reliability and minimal power draw.
- Smart Home Devices – Reed switches enable magnetic control in smart home devices for energy-efficient automation.
To properly understand the functioning of reed technology in the configurations below, it is first important to understand the basics of the Reed Switch.
- Normally closed Reed Sensors
- Normally closed Reed Relays
- Latching (bi-stable) Reed Switches/Sensors
- Latching (bi-stable) Reed Relays
Magnetic control allows for contactless operation, ideal for low-power design environments.
Form A Reed Switch: Normally Open Operation
Reed Switches exist in their natural state in the normally open condition. This is typically referred to as single pole normally open, single pole single throw (SPST), or Form A (see Figure 1). This reed switch configuration supports power efficiency by minimizing energy draw during switching. Magnetic control allows for contactless operation, ideal for low-power design environments.
The two reed switch leads are ferromagnetic and are hermetically sealed in a glass capsule. When the normally open reed switch is subjected to a magnetic field the contacts will close (see Figure 2). The contacts will stay closed as long at the magnetic field remains. Once the magnetic field is removed the contacts will open. Here energy is expended the entire time the contacts are in the closed state, making it less ideal from a power consumption standpoint.
Form C Reed Switch: Dual Contact Switching Works
Another type of power efficiency reed switch configuration is the single pole double throw (SPDT) or Form C Reed Switch (see Figure 3).
In Figure 3, no magnetic field is present therefore the common contact maintains its 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 reed relay draws 100% power while the common reed element swings from the normally closed to the normally open contact (see Figure 4). Once the magnetic field is removed the common contact swings back to the normally closed contact.
Form B Reed Switches & Sensors: Power Efficient Normally Closed Design
Since the natural state of a form A reed switch is normally open, we need to apply a permanent magnet to the reed switch sufficient in its magnetic strength to close the reed contacts (See Figure 5). The biasing magnet must be greater than the Pull-in or operate milliTesla (mT) field that closes the contacts in the normally open condition.
At this stage, the polarity of the magnet does not matter. To open the contacts, you must bring a permanent magnet near the biasing magnet. This magnet must have opposite polarity and a magnetic strength equal to or greater than that of the biasing magnet (see Figure 6).
Form B Reed Relays: Efficient Normally Closed Switching
Certain applications require reed relays to maintain closed contacts for extended periods, only opening in response to a fault condition. The normally closed (Form B) reed relay is specifically engineered for such scenarios. In its default closed state, the relay coil consumes no power, making it ideal for battery-powered devices or systems with limited energy availability.
This design is particularly well-suited for fault detection circuits, where the relay remains closed under normal conditions and only opens when a fault occurs. Because no power is drawn while the contacts are closed, Form B relays support ultra-low-power applications. Figure 7 illustrates the schematic of a Form B reed relay in its unenergized, normally closed configuration.
Applying power to the coil producing a magnetic polarity that opposes the polarity to the biasing magnet, and of sufficient magnetic strength to overcome the field strength of the biasing magnet, will cause the contacts to open (see Figure 8).
Comparison of Reed Switch Configurations for Power Efficiency
| Configuration | Default State | Power Usage |
| Form A Normally Open (SPST) | Open (no contact) | Power or magnetic field needed to close |
| Form B Normally Closed (SPST) | Closed (contact made) | Power or magnetic field needed to open |
| Form C Changeover (SPDT) | One contact closed | Power or magnetic field switches contact |
How Form B Reed Relays Operate: Step-by-Step Guide
A step-by-step guide of how a form B reed relay operates can be seen in Figure 9.
Stage 1
We add a biasing magnet that produces a 5 mT magnetic field directly on the reed switch. Since this field strength is above the pull-in point of the reed switch, the contacts will close as shown at point 1 in the graph.
Stage 2
Now, a reed switch with a operate (Pull-in) of 4 mT and a release (Drop-out) of 2 mT is selected.
Stage 3
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, as illustrated at point 2.
Stage 4
We turn off the coil, and the contacts close as the magnetic field strength returns to 5 mT.
The voltage polarity of the coil applied to the Form B relay determines the magnetic polarity of the coil. The design determines the voltage polarity, and we mark this polarity directly on the relay. Applying reverse voltage polarity causes the relay to malfunction until the correct polarity is restored. Also, too high a voltage applied above the specified nominal voltage can cause the contacts to re-close. Generally, the re-close voltage is specified at 50% above the nominal. Essentially this means that applying above 7.5 volts for a 5 volt nominally rated Form B relay could cause the contacts to re-close. If this is ever a concern with a user where their circuit may produce voltages greater than 50% above the nominal, relay designers can adjust the magnetic design to increase the specified re-close voltage. This makes Form B reed relays a reliable and energy-efficient choice for fault detection systems.
Latching Reed Relays & Sensors: Retain State Without Power
Latching reed switches maintain their state without continuous power, offering unmatched power efficiency through magnetic control. A latching reed relay or sensor operates in two distinct states: unlatched (open) and latched (closed). It maintains either state without requiring power. The reed switch’s natural hysteresis, between its operate (pull-in) and release (drop-out) points, enables latching, as shown in Figure 10. A higher operate point increases hysteresis, which simplifies the design of latch and unlatch thresholds. To enable latching mode, we bias the reed switch with a permanent magnet.
The following discussion will explore this in more detail.
How Latching Reed Relays Work: Internal Mechanism
A latching reed relay uses a Form A reed switch in conjunction with a permanent magnet. (see Figure 11).
In this state, the reed switch may be in its normally open state or its normally closed state. The state it is in, depends upon the previous magnetic field it had last seen. If the contacts are open, applying a magnetic pulse with the correct polarity switches them to the normally closed state. (see Figure 12).
Generally, a 2ms pulse supplied at the relay nominal voltage is enough to change the state of the relay contacts. Therefore, heat generated when closing and opening the relay contacts is minimal thereby producing minimal thermal offset voltages.
Magnetic Pulse Control: How Latching & Unlatching Work
To clarify how latching and unlatching occur, we selected a reed switch that closes its contacts when exposed to a 4 mT magnetic field and opens them when the field drops to 2 mT or below. We set the biasing magnet to produce a steady 3 mT field. Figure 14 illustrates a complete operate cycle, highlighting each state sequentially. The pull-in and drop-out points remain constant, as shown by the fixed lines.
The five stages and reed switch contact state are outlined below:
Stage 1
This example shows the biased magnetic field (BMF), which we continuously apply to the reed switch, at the 3 mT level. The contacts are open.
Stage 2
Next, we apply an external magnetic field (EMF)—from either a coil or a permanent magnet—which generates a 2 mT field that reinforces the biasing magnet’s field (BMF). This puts the magnetic field applied to the Reed Switch at 5 mT, which is above the 4 mT level, therefore closing the contacts.
Stage 3
Now the EMF is removed leaving only the BMF. But as one can see, the field strength is still above the drop out field, so with the EMF removed the contacts stay closed.
Stage 4
The EMF is again applied, but this time the field is opposing the BMF bringing the net magnetic field strength to 1mT. Now the field is below the Drop out level and the contacts open.
Stage 5
The opposing EMF is now removed leaving only the BMF and the reed contacts stay in the open state.
In this manner one can latch and unlatch the contacts.
The cycle above clearly shows that latching reed relays require a reversal of magnetic polarity to change the contact state. So, you can achieve this either by using two coils, as shown above, or by reversing the polarity of a single coil. Consequently, using two coils increases the relay’s cost, while reversing a single coil’s polarity demands more complex electronic circuitry for each contact state change.
Designing with Latching Reed Switches: Using Magnet Control
Latching reed switches operate similarly to those shown in Figure 11, where a permanent magnet biases the switch. However, instead of using a coil as in Figures 12 and 13, we use a second permanent magnet with opposite polarity, as illustrated in Figure 15. By leveraging magnetic control with permanent magnets, latching reed switches eliminate the need for electrical power, supporting ultra-low-power design strategies.
In this case, removing the permanent magnet keeps the contacts closed. They remain closed until another permanent magnet with opposite polarity approaches the reed and biasing magnet. This is like the latching reed relay above. The use of permanent magnets uses no electrical power at all; and therefore, eliminates the need for power supplies, electrical circuits and timing circuits. The state of the reed switch uses no power (unlike Hall sensors) and relies strictly on the movement of magnets in and out of its influence.
Also like the latching reed relay, one or two magnets can be used in changing the contact state.
Using One Magnet
When a permanent magnet approaches the reed switch, it closes the contacts. Withdrawing the magnet leaves the contacts closed. To open them, you must rotate the magnet to reverse its polarity. Bringing it near the reed and biasing magnet again then opens the contacts (see Figure 16).
Using Two Magnets
You can accomplish latching and unlatching by moving a magnet toward the reed switch to close the contacts, then withdrawing it to open them. The opposing magnet then approaches from the other direction showing an opposite polarity and thereby opening the contacts. This can be done effectively in several ways depending upon the type of movement for a given application.
Latching reed switches can require a fine balancing of the magnetic system, particularly when there are other ferromagnetic materials nearby. An application requiring latching Reed Switches can undoubtedly be the best design choice; however, we recommend having Standex Detect engineers and sales engineers work closely with you for best results. There are many ways to accomplish the latching environment, so for a given set of circumstances, our engineers will produce a professional, simple, and cost-effective approach.
Summary: Choosing the Right Reed Technology for Magnetic Control and Power Efficiency in Low-Power Designs
When you expect the contacts to remain closed for long periods and need to minimize power consumption, consider using a Form B reed sensor or reed relay. If power efficiency matters in both open and closed states, a latching reed switch or latching reed relay may offer the best solution.
The latching reed switch is the only sensor technology where no power is necessary for operation and release of the contacts. With increased demand for low power components, the latching or normally closed aspect of a reed switch stands by itself. Whether using Form B for normally closed operation or latching reed switches for power-free state retention, reed switch technology is central to modern low-power design.
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