Relays essentially function as switches that govern the operation of other switches within electrical circuits. Employing a low-power input signal, they exert command over higher-power circuits. Activation of the relay's low-power signal triggers the energization of an electromagnet, initiating the movement of an armature. This motion, in turn, prompts the closure of electrical contacts, thereby facilitating the transmission of power to the controlled circuit.
One of the paramount advantages of this design lies in its ability to isolate the low-power control signal from the higher-power circuit. This isolation not only safeguards operators from potential hazards but also shields equipment from potential damage. Additionally, this arrangement facilitates remote control of devices or systems, enabling operation from a distance.
The genesis of electromechanical relays traces back to 1835, and despite significant advancements in their components and diversity over the years, their fundamental function has endured. One of the most well-recognized relays throughout history is the power relay. While all electrical relays inherently control power, not all relays are correctly termed “power relays”. This article will take a closer look at power relays, including their benefits, configurations, and key selection criteria.
Power relay basics
Power relays are known for their specialized capability to manage high-level current switching, ranging from several amps to substantially higher magnitudes. Their more robust construction and enlarged dimensions allow power relay contacts to withstand sizable currents, rendering them ideal candidates for applications demanding currents typically exceeding 10 amps.
They find increased usage across diverse industries, encompassing automotive systems, elevator controls, valve actuators, and various devices characterized by high initial current surges, such as motors, solenoids, power supplies, and electronic ballasts.
Much like other electrical components, relays come with inherent limitations regarding the power they can safely manage. Each model is assigned a maximum power rating, ensuring efficient matching with a diverse range of loads, from low-power entities like light bulbs to robust machinery such as large motors. However, exceeding the specified power rating can spell permanent damage for the relay.
Additionally, misalignment of contacts can induce contact arcing, characterized by current flow through the air gap between relay contacts when they are open but near each other. This phenomenon poses risks beyond sparks and heat generation, including erosion of contacts and generation of unwanted electrical interference, potentially harming nearby equipment.
Figure 1: Electromechanical relay contact arcing. (Image source: CUI Devices)
Enter power relays, engineered specifically to tackle the electrical loads of high-current devices such as heaters, motors, lighting arrays, and industrial equipment. The heightened current and voltage ratings of power relays stem largely from the utilization of switch contact materials distinct from those in regular relays. These materials are chosen for their ability to withstand the rigors of high-power applications, ensuring reliable operation and longevity in demanding industrial settings.
Power relay contact materials
When current flows through relay contacts, it encounters resistance, a factor contingent on the size and material composition of the contacts. Elevated resistance not only amplifies the power dissipated within the relay but also increases heat generation. One approach to mitigate contact resistance is the careful selection of contact materials.
Conventional relays typically feature contacts crafted from silver nickel, a metal renowned for its historical ubiquity in relay construction. Silver nickel contacts excel in switching resistive loads, where current and voltage are in phase with each other.
In contrast, relays designed for higher loads, like power relays, opt for contacts fashioned from materials such as silver cadmium oxide, silver tin oxide, or gold alloy. These materials are ideal for handling inductive loads, characterized by asynchronous current and voltage, which may yield substantial current or voltage spikes. Both silver cadmium oxide and silver tin oxide contacts offer diminished electrical resistance and mitigate the risk of contact welding stemming from high inrush currents. Notably, the adoption of silver tin oxide circumvents environmental concerns associated with cadmium-based alloys, thus aligning with regulatory standards upheld by certain nations.
Power relays compared to signal relays
Power relays and signal relays represent two popular variants within the realm of relays. While power relays prioritize handling higher voltages and currents, they typically endure fewer lifetime cycles. Conversely, signal relays are engineered for a higher lifetime cycle count but operate with lower voltages and minimal current.
The contact materials employed in power relays, while adept at managing high-power scenarios, are not ideally suited for low-power switching. This stems from the fact that at lower voltages, the physical connection between contacts assumes paramount importance, dictated by factors such as contact pressure and cleanliness, rather than the contact material.
Furthermore, employing a signal relay in a power application carries inherent risks, potentially culminating in catastrophic failure due to overvoltage or overcurrent. Even if such a relay were to survive, it would lack crucial features such as arc prevention and contact self-cleaning, compromising long-term reliability.
In the decision-making process between power and signal relays, adherence to a fundamental guideline is crucial: always match the power level being switched with the relay's power rating. This ensures optimal performance, mitigates risks of failure, and upholds the integrity of the relay and associated systems. More can be read about signal relays in CUI Devices’ other article titled, Signal Relays – Understanding the Basics.
Power relay types
Power relays, like regular relays, are available in two primary types: electromechanical and solid-state.
Electromechanical power relays rely on a combination of electrical coils, magnetic fields, springs, movable armatures, and contacts to regulate power delivery to a device.
On the other hand, solid-state relays utilize no moving parts. Instead, they leverage semiconductor devices such as silicon-controlled rectifiers (SCRs), TRIACs (triode for alternating current), or switching transistors to switch both AC and DC currents. Solid-state relays offer advantages such as faster switching speeds and enhanced reliability compared to electromechanical relays. However, their cost-effectiveness diminishes as power demands escalate due to the higher costs associated with robust power semiconductors and the inclusion of additional thermal management components.
Figure 2: Example of a solid-state relay combined with a heat sink. (Image source: CUI Devices)
Common configurations and ratings
Power relays, much like their non-power counterparts, are categorized based on their contact configuration, indicating the number of devices they can control simultaneously. The common classifications include:
- SPST (Single Pole, Single Throw)
- DPDT (Double Pole, Double Throw)
- 3PDT (Three Pole, Double Throw)
- SP3T (Single Pole, Three Throw)
Relay contacts are designated as either normally open (NO) or normally closed (NC), depending on their state when no power is applied to the relay.
Relay ratings denote the maximum power that can be safely and effectively switched by the relay. These ratings are typically expressed in terms of amperes for both AC and DC currents. It's crucial for the relay's rating to exceed the rating of the device being switched, with a safety margin considered.
Similar to non-power relays, power relays can also be described using the term "forms". Phrases like "1 Form A" or "2 Form C" provide insights into the relay's characteristics. The number preceding "Form" indicates the quantity of described contacts available in the relay. "Form A" signifies a normally open relay, while "Form B" denotes a normally closed relay. "Form C" and "Form D" apply to SPDT relays, indicating which position is considered normally closed and whether the relay is break-before-make or make-before-break, respectively. While there are numerous other forms, these four are the most commonly used.
- Form A – normally open
- Form B – normally closed
- Form C – break-before-make SPDT switches
- Form D – make-before-break SPDT switches
Additional considerations
Some additional considerations to take into account during device selection include:
- Input power surges: Certain devices may produce significant power surges during startup. It's important to identify these surges before specifying a relay to prevent damage to the device.
- Coil suppression: Relay cycling can generate high-voltage transients. Coil suppression involves using additional components in the circuit to protect equipment from these transients. However, this may reduce the relay's lifespan. Determine if a specific coil suppression strategy is necessary for any specified application.
- Latching: A latching relay retains its last contact position even after the activation power is removed. This feature may be required for certain applications.
- Noise: Relays can produce electromagnetic interference (EMI) or radio frequency interference (RFI) noise, which can be more pronounced in high-power devices. Determine the sensitivity of a device or system to this noise beforehand.
- Contact bounce: As a relay cycles, its contacts may experience brief open/close cycles, known as contact bounce, which generate electrical pulses. Depending on the application's sensitivity, this bounce may cause undesirable effects, so it is important to determine whether contact bounce will affect an application before specifying a relay.
Figure 3: Example of contact bounce and the rapidly changing voltages. (Image source: CUI Devices)
Conclusion
Relays are trusted, efficient, and reliable devices that provide secure electrical control of systems and devices, all while keeping the operator safely isolated from the operating current. Power relays, whether electromechanical or solid-state, are specially engineered with enhanced features to manage higher voltages and currents.
As designers assess the power switching needs of their product, CUI Devices is here with a diverse array of power relays and signal relays. Whether handling low-level or high-level current switching, CUI Devices has the relay solutions to meet a variety of needs effectively and efficiently.
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