075582814553
Electromagnetic relays and reed relays both use a coil-generated magnetic field to operate mechanical contacts, but their contact structures and switching characteristics differ. Conventional armature-type electromagnetic relays are widely used for power control, while reed relays are often selected for low-level signal switching and measurement systems. This article explains their operating principles, key differences, applications, and selection considerations.
Figure 1. An Overview of Electromagnetic Relay and Reed Relay
A relay is a switch operated by an external control signal. In an electromechanical relay, current through a coil produces a magnetic field that changes the state of one or more contacts. The coil and contacts belong to separate electrical circuits, allowing a low-power controller to operate a load without a direct conductive connection between the control and contact paths.
| Feature | Conventional Electromagnetic Relay | Reed Relay |
| Switching mechanism | Coil moves a separate armature | Coil moves flexible magnetic reeds |
| Contact enclosure | Open, protected, or sealed packages | Contacts sealed inside a glass capsule |
| Switching speed | Generally slower | Generally faster |
| Load capability | Broad selection for power loads | Commonly signal or lower-current switching, specialist high-voltage types |
| Low-level signals | Requires suitable contact material and rating | Well suited when correctly specified |
| Contact life | Strongly dependent on load and design | Long at light loads, reduced by electrical stress |
| Size | Wide range, often larger for power switching | Often compact and suited to dense layouts |
| Coil power | Varies widely | Low-power options widely available |
| Typical use | Motors, heaters, power control | Test equipment, measurement, signal routing |
For non-latching relays, a normally open (NO) contact remains open when the coil is de-energized and closes when operated. A normally closed (NC) contact is closed when the coil is de-energized and opens when operated. A changeover contact selects between two paths. These arrangements are commonly described as SPST (single-pole, single-throw), SPDT (single-pole, double-throw), and related configurations.
Not every relay contains mechanical contacts. Solid-state relays use semiconductor devices. Within the electromechanical family, conventional armature relays and reed relays share magnetic actuation but use different contact structures. That distinction largely determines where each type performs best.
A conventional electromagnetic relay (EMR) uses a movable armature to open or close electrical contacts. Its construction supports a broad range of contact arrangements and load ratings, making it a familiar component in power-control and industrial circuits.
A typical armature relay includes a copper coil, magnetic core, movable armature, return spring, and fixed and moving contacts. The armature transfers magnetic force into mechanical motion, while the spring returns the mechanism to its resting position.
Silver alloys, including silver-nickel and silver-tin-oxide systems, are used in many power relays because of their electrical and arc-resistance characteristics. Gold-clad contacts appear in relays designed for low-level signals. Thin gold flashing is damaged by repeated arcing.
Conventional EMRs come in open, dust-protected, flux-resistant, and sealed designs. Their contacts are not necessarily exposed to ambient air, and environmental protection must be checked for the selected package.
When the rated voltage is applied to the coil, current generates a magnetic field that attracts the armature toward the core. The armature moves the contacts, completing or interrupting the load circuit. In a non-latching relay, removing coil power weakens the magnetic field, allowing the return spring to restore the contacts to their resting state.
Operating time begins when the coil is energized and ends when the specified contacts operate. Release time measures the response after the coil is de-energized. Contact bounce can follow the first contact impact, so a circuit that requires a stable connection must account for more than the nominal operating time.
Figure 2. Operating Principle of a Conventional Electromagnetic Relay (Normally Open Example)
The main strength of a conventional EMR is its broad selection of power-switching ratings and contact configurations. Properly rated models control heaters, motors, solenoids, and other AC or DC loads. Multiple poles, changeover contacts, and different mounting options simplify power and control wiring.
Its limitations follow from the moving mechanism and switching load. Armature travel adds operating time and audible clicking. Contacts bounce and gradually wear, especially when opening inductive loads or closing into high inrush currents. Coil power and package size also vary by design.
A reed relay combines an electromagnetic coil with a reed switch: a pair of flexible ferromagnetic metal blades enclosed in a hermetically sealed glass capsule. The blades act as both the moving elements and the electrical contacts, eliminating the separate armature mechanism used by conventional EMRs.
The reed switch typically contains a pair of overlapping nickel-iron blades with contact surfaces treated for the intended duty. Depending on the switch design, the glass envelope contains an inert gas or a vacuum. The sealed enclosure limits contact exposure to moisture and airborne contamination.
A coil surrounds or sits alongside the switch capsule within the relay package. Normally open contacts are common. Normally closed and changeover versions are also available. Some reed relays incorporate magnetic shielding to reduce interference from nearby coils, while others are designed for low thermal electromotive force (thermal EMF) or low leakage.
In a normally open reed relay, energizing the coil magnetizes the two reed blades. Opposite magnetic poles develop near their overlapping tips, attracting the blades until the contacts meet. When coil current is removed, the field collapses and the blades’ elasticity separates the contacts.
Because the moving elements are small and light, many reed relays operate faster than conventional armature types. Contact bounce can still occur in reed relays, so operating time and contact bounce time must be evaluated separately in timing-critical applications. The sealed contact path remains electrically isolated from the operating coil.
Figure 3. Internal Construction and Operating Mechanism of a Reed Relay
Reed relays excel at switching low-level signals because their sealed contact surfaces resist environmental contamination. Compact packages fit densely populated test boards, and low moving mass supports rapid, repeated operation. Under light-load or cold-switching conditions, suitable reed relays achieve very long service lives.
Their limitations arise mainly from contact size and electrical stress. Many standard reed relays cannot interrupt the currents handled by power EMRs. Capacitive inrush and lamp loads can weld contacts even when steady-state current appears acceptable. The glass-to-metal seals also require protection against excessive mechanical stress, and closely packed unshielded relays can affect one another magnetically.
An armature relay must accelerate a relatively large mechanical assembly. A reed relay moves only its slender contact blades. This structural difference generally gives reed relays shorter operating and release times. However, coil construction, magnetic sensitivity, drive voltage, and suppression components also affect timing.
Both types experience contact bounce as the metal surfaces strike and rebound. A digital controller can interpret those transitions as multiple events, while a measurement system can capture unstable readings immediately after switching. Datasheets may list operate time separately from bounce time. Wait for contact settling before taking measurements or applying the stimulus.
Figure 4. Illustrative Relay Timing, Contact Bounce, and Application-Dependent Measurement Delay
Power relays are designed around the thermal and arc stresses associated with substantial load current. Standard reed relays generally have lower switching-current limits, but specialized versions can withstand or switch high voltages. The limits are distinct:
• Maximum switching voltage: the voltage the contacts can safely make or break under specified conditions.
• Maximum switching current: the current permitted while opening or closing the contacts.
• Maximum switching power: an additional limit on the permitted combination of voltage and current.
• Maximum carry current: the current permitted through contacts that are already closed.
A relay must satisfy all applicable limits simultaneously. A high carry-current rating does not mean the device can interrupt that current, and a high open-contact withstand-voltage rating is not the same as a high-voltage switching rating. AC and DC ratings also differ because DC arcs do not benefit from periodic current zero crossings.
Load type changes the actual stress. Inductive motors and solenoids generate voltage transients when interrupted. Capacitive circuits draw charging surges, and incandescent lamps have high cold-filament inrush current. A resistive-load rating must not be applied automatically to these loads.
Hot switching means the contacts open or close while voltage or current is present. Cold switching means the electrical stimulus is applied only after contact closure and removed before opening. Cold switching greatly reduces contact arcing and is common in automated test equipment, but it requires coordinated timing and does not remove the need to respect carry-current and insulation ratings.
Figure 5. Comparison of Hot-Switching and Cold-Switching Sequences
For low-voltage or low-current signals, contact stability matters as much as current capacity. Films that form on some power-relay contact materials can raise contact resistance when the switched signal is too weak to break through them. Low-level-rated contacts, often using suitable gold surfaces, address this problem. Using those contacts beyond their specified power rating can damage the plating and compromise later low-level operation.
A reed relay’s hermetic seal protects its contacts from external contamination, supporting stable low-level switching. Precision measurement also introduces requirements beyond contact resistance: insulation leakage can distort high-impedance measurements, while thermal EMF created by temperature gradients can add offset to microvolt-level signals. For radio-frequency paths, contact capacitance, impedance, and insertion loss also matter. These properties depend on package and circuit design. Not every reed relay is an RF or low-thermal-EMF component.
Mechanical life describes how many operations a relay can perform without an electrical load, under specified conditions. Electrical life describes operation while switching a stated load. The two ratings are not interchangeable.
In conventional EMRs, repeated arcing erodes contacts, while the armature, spring, and contact assembly undergo mechanical fatigue. Reed relays avoid a separate armature mechanism but still suffer contact wear or welding under overload. Severe shock or poorly controlled mounting stress can damage the glass capsule or its seals.
It is therefore misleading to say that a reed relay always lasts longer. A reed device used for cold-switched measurement signals can outlast a power relay switching a motor, but those are different operating conditions. Compare datasheet life curves or test results at the required voltage, current, load type, and switching frequency.
Reed relays are often attractive in high-density boards because compact packages allow many switching channels within a limited area. Conventional power relays generally need more space for contact separation, insulation, and current-carrying structures. Both categories include compact designs.
Coil power affects driver selection and thermal management. For a DC coil, steady-state current and power can be estimated using and . At 5 V, the OMRON G5LE’s nominal 63 Ω coil draws and dissipates (about 400 mW). The Pickering 109-1-A-5/2D has a nominal 500 Ω coil, drawing and dissipating (50 mW). These nominal figures apply at the specified coil resistance; actual resistance changes with temperature.
Conventional electromagnetic relays are widely used in industrial automation, appliances, and power-control equipment. Depending on their contact ratings and configurations, they switch heaters, motors, pumps, and solenoids or change between circuit paths. Their broad range of load ratings makes them suitable for many power-control applications, although inductive loads and high starting currents require particular attention. Larger loads may require contactors rather than small PCB relays.
Reed relays are commonly used in automated test equipment (ATE), data-acquisition systems, precision instruments, and signal-routing matrices. Their hermetically sealed contacts support stable low-level switching, while compact packages allow high-density channel layouts. Specialized versions also serve high-voltage insulation testing and radio-frequency (RF) switching, subject to their specified voltage, leakage, and frequency characteristics.
The OMRON G5LE-1A DC5 conventional electromagnetic relay and Pickering 109-1-A-5/2D reed relay both use 5 VDC coils and SPST-NO contacts, but their switching and operating characteristics differ. The Pickering part includes an internal suppression diode, so coil polarity must be observed. The OMRON operate-time figure excludes contact bounce. Pickering’s figure includes bounce.
| Specification | OMRON G5LE-1A DC5 | Pickering 109-1-A-5/2D |
| Contact configuration | SPST-NO | SPST-NO |
| Rated coil voltage | 5 VDC | 5 VDC |
| Contact switching rating | 8 A at 30 VDC (resistive) | 0.5 A maximum, 10 W maximum |
| Maximum switching voltage | 125 VDC maximum (30 VDC when UL/CSA requirements apply) | 200 V maximum |
| Maximum carry current | 10 A | 1.2 A |
| Nominal coil resistance | 63 Ω at 23 °C | 500 Ω |
| Built-in coil diode | No built-in diode in this model | Yes (D suffix, observe coil polarity) |
| Operate time | 10 ms max. (excluding bounce) | 0.5 ms max. (including bounce) |
| Release time | 5 ms maximum | 0.2 ms (datasheet value) |
| Electrical life | ≥100,000 operations at rated load, 1,800 operations/hour | ≈10⁹ typical at 10 V/10 mA resistive or cold switching (1 Ω end-of-life criterion) |
A 30 VDC, 5 A resistive heater draws 150 W (). The OMRON G5LE-1A DC5 meets the stated 8 A at 30 VDC resistive-load rating. Although 30 V is below the Pickering part’s 200 V maximum switching voltage, the heater exceeds its 0.5 A maximum switching current and 10 W switching-power limits.
For a 10 V, 10 mA signal-routing application, the Pickering relay is a more suitable starting point. Its low-level contact design supports precision switching, while its compact package and short operating time suit dense measurement systems.
The life ratings use different test conditions and must not be compared as unconditional guarantees. OMRON lists at least 100,000 electrical operations at rated load with a test frequency of 1,800 operations/hour. Pickering reports approximately 10⁹ typical operations when switching a 10 V, 10 mA resistive load or under cold switching, and approximately 10⁷ typical operations at its maximum resistive load, using a contact-resistance limit of 1 Ω as the end-of-life criterion.
Verify rated coil voltage, operate voltage, release voltage, coil resistance, and the driver’s current capability. If the controller cannot supply the required coil current, use a transistor or dedicated relay driver. Check whether the relay already contains a suppression diode and whether the coil is polarity-sensitive.
When DC coil current is interrupted, the collapsing magnetic field produces a transient that can damage the driver. A reverse-biased diode across the coil provides a simple suppression path, but delays current decay and contact release. A series rectifier-and-Zener clamp allows a faster release while limiting the transient, as shown in Figure 6. For the illustrated low-side driver, its collector voltage at turn-off rises to approximately the supply voltage plus the Zener voltage and the rectifier forward drop. The transistor must be rated to withstand this voltage with appropriate margin. This suppression approach is discussed in TE Connectivity’s technical note on DC relay coils. AC coils require an appropriate separate suppression design.
Figure 6. DC Relay Coil Flyback Suppression Circuits
Contact protection addresses the load circuit, not just the relay coil. Use suitable suppression across inductive DC loads, and consider properly rated RC snubbers or varistors for applicable AC loads. Limit capacitor-charging or lamp inrush current where necessary. Select protection components for the actual voltage, energy, and safety requirements. A snubber placed across contacts can also introduce off-state leakage.
Contact ratings should be checked at the expected worst case, including startup, fault conditions, and switching transients. Do not place contacts in parallel simply to double their current rating: unequal make and break timing prevents reliable current sharing.
Nearby unshielded reed relays can interact magnetically, changing the coil drive needed for reliable operation. Choose magnetically shielded packages or verify spacing and orientation under the intended operating pattern. Keep sensitive measurement traces away from noisy coil-drive paths, and use electrostatic shielding where measurement requirements justify it.
Temperature also matters. Copper coil resistance rises as temperature increases, affecting current and operate margin. Sustained coil power creates local heating that can increase thermal EMF in precision circuits. Follow the relay’s specified temperature range and drive conditions. Avoid bending relay leads or flexing the PCB in ways that stress a reed capsule’s glass-to-metal seals. Finally, observe the specified contact-to-coil isolation, creepage, clearance, and applicable equipment-safety requirements.
Conventional electromagnetic relays and reed relays share magnetic actuation but differ in contact construction and intended electrical duty. Armature-type relays offer extensive choices for power switching and control. Reed relays combine sealed contacts, compact construction, and strong low-level switching performance, with specialized options for demanding measurement tasks.
The best choice follows from the load profile, switching sequence, signal requirements, and verified datasheet limits. Assess those conditions together, then check the design under realistic operating stresses before finalizing the relay.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other 
