Many applications require switching circuits with high isolation or the ability to switch high voltage and high current using low-power control signals. Sometimes, semiconductor-based solutions cannot meet these requirements. In such cases, designers need to choose between electromechanical relays and contactors and understand how to properly apply these devices.
Electromechanical relays can switch relatively high currents using only a few volts of control signal and provide good voltage isolation between the control signal and the switching power supply. However, for very large current loads and very high switching voltages, contactors are required. Contactors can be considered "steroid-fueled" electromechanical relays, offering significantly enhanced performance. Most design engineers are familiar with various types of relays, from reed relays to heavy-duty relays. Those outside the industrial power sector are less familiar with contactors, which are widely used for switching high-voltage circuits and very high loads.
This article will discuss the differences between relays and contactors, and the best applications for each type of device. It will introduce various relay and contactor solutions and provide practical design tips for using each type of device.
Relays and Contactors
Both relays and contactors are electromechanical devices that use solenoids to actuate one or more pairs of contacts. Single-pole relays or contactors have only one pair of contacts. There are also double-pole relays and contactors, which can have a very large number of contacts. The contacts can be normally open or normally closed. Some relays and contactors also have double-throw contacts that include both normally open and normally closed contacts.
Relays are suitable for switching low- to medium-current loads with relatively low voltage, and they come in a variety of sizes, including plug-in and board-mount types that can be soldered to a PCB. Contactors are designed for high-current and high-voltage loads.
The choice of relay or contactor type largely depends on the type of load being switched. Below is a summary of different load types and tips for handling them:
Resistive loads do not experience inrush current upon initial energization. The most common example of a resistive load is a simple heater. If its rated current consumption is 10A, a 10A relay can be used for safe switching. In the real world, purely resistive loads are very rare. Most loads exhibit a combination of two or more types of loads.
A lamp load consumes a large current when first powered on. Incandescent bulb filaments have a high temperature coefficient. The resistance of a cold bulb filament is often only 5% of that of a hot bulb, therefore it consumes 20 times more current when cold compared to when heated. During normal operation, a 75W incandescent bulb consumes slightly more than 0.5A of current, but when first turned on, the cold filament draws an inrush current of 13A. Although this current surge lasts only about one-tenth of a second, any relay contacts driving the incandescent bulb load must be able to withstand this high inrush current.
Motor loads also consume a significant amount of current when first powered on. A single-phase, 110VAC, 1/3 horsepower synchronous motor typically consumes just over 4A. However, during startup or when the rotor is locked, the motor may consume more than 24A. If the mechanical load is removed from the motor and it runs unloaded, the motor may consume 6A.
Capacitive loads exhibit high current surges upon power-up because the capacitor attempts to maintain a constant voltage across itself. Switching voltage to an uncharged capacitor is analogous to momentarily activating a short circuit. This high current upon power-up can create a welding effect, causing relay contacts to close. Typical capacitive loads include DC power supply outputs and other filtered power supplies.
Inductive loads conduct gradually as the load current slowly increases when energized. However, when the load is turned off, induced voltage spikes are generated on the relay contacts as the inductor attempts to maintain a constant current through itself. Each time the power is disconnected, these induced voltage spikes can be large enough to cause arcing on the relay contacts, leading to melting and pitting of the contact surface and degrading contact performance. This explains why some relays integrate absorber diodes on their coils to prevent arcing. High-inductive load types include solenoid actuators, electric valves, and relays.
Relay Explanation
Key specifications for relays include coil voltage, AC or DC coil operation, contact current ratings and configuration (normally open, normally closed, multi-pole), number of contacts, and actuation/release cycles. It is crucial to avoid insufficient switching current, which could prevent the relay from functioning reliably. The proper operation of relay contacts depends to some extent on the switching of a certain minimum rated current (often called the demagnetizing current), as this burns away any trace contaminants that may have accumulated on the relay contacts.
The lower limit of the current that a relay can reliably switch depends on several factors, such as contact material, contact geometry, and mechanical slippage of the contact surface. The minimum switching current specification of a relay must take all these factors into account. Relays with gold-plated contacts and relays with split (detachable) contacts can reliably switch currents as low as 10mA.
Reed relays and mercury reed relays are suitable for low-level switching applications. For example, TE Connectivity Potter and Brumfield Relays' JWD and JWS reed relays are available with coil voltages from 5 to 24 VDC and in a variety of single-pole and double-pole configurations.
For example, TEConnectivity's JWD-171-10 reed relay has a 24V coil with an integrated absorber diode and a normally open contact, with a rated maximum switching current of 500mA at 20V. The JWD series reed relays are designed to be mounted on a circuit board with the same substrate as a 14-pin DIP integrated circuit, but with only 8 pins (Figure 1).
Figure 1: TE Connectivity Potter and Brumfield Relays' JWD series reed relays have the same base surface as the 14-pin DIP, offering a variety of coil voltages and contact configurations. (Image source: TE Connectivity Potter and Brumfield Relays)
Reed relays are generally not suitable for switching higher loads, which require a larger physical package to accommodate larger, high-current contacts. For example, Omron Electronic Components' G2R-1-DC24 general-purpose relay is rated for a switching current of 10A at 24V. It has a 24V DC coil and a single-pole double-throw contact arrangement. This relay is slightly larger than TE Connectivity's JWD series reed relays, but is still designed to be mounted on a printed circuit board (Figure 2).
Figure 2: The Omron Electronics G2R-1-DC24 general purpose printed circuit board mounted relay has a rated switching current of 10A at 24V. (Image source: Omron Electronics)
Omron also offers another similar relay, the G2R-1-SND-DC24(S), designed for slot-mount applications. These relays are available in a variety of compatible sockets, including DIN rail compatible, panel mount, and through-hole board mount versions.
In-depth understanding of contactors
Contactors are the heavy-duty industrial equivalent of relays and are standard components in factory and industrial applications. Contactors are more robust than relays and are typically designed for easy mounting on standard DIN rails. Some contactors have additional mounting holes for direct bolting to flat surfaces. Contactors are designed to switch high loads, such as fractional horsepower and multi-horsepower multiphase motors, large heating loads, and industrial/commercial lighting. Therefore, contactors can accommodate large, high-current conductors.
Like relay coils, contactor coils are also available in AC or DC specifications. Contactors designed to be driven by a PLC (Programmable Logic Controller) typically have 24V DC solenoid coils, but coil drive ratings for AC line voltages (including 110, 220, and 240VAC) are also common.
Like relays, contactors use a solenoid coil to magnetically attract an actuator or plunger, thus establishing a physical connection between one or more pairs of heavy-duty electrical contacts. Unlike relays, contactors are assembled in a modular fashion, allowing for easy replacement of the solenoid coil to change the voltage. Relays are generally not modularly constructed, and changing a relay configuration typically requires replacing the entire relay. The modular structure of contactors also allows users to modify the actuation contact array.
Contactors typically have multiple sets of contacts. Sometimes a contactor contains only high-current contacts, but it can also use a mix of high-current and low-current contacts for simultaneous switching of power and signal circuits, respectively. Low-current contacts are also called auxiliary contacts. The difference between these two types of contacts is that high-current contacts are physically larger than low-current contacts and can carry higher load currents. A contactor designed to drive a three-phase motor may have three high-current contacts to carry the motor power, and one auxiliary contact to indicate the motor's actuation status.
For example, the Omron Automation & Safety J7KNA-AR-3124VS contactor features a 24V DC solenoid coil and a four-pole single-throw contact arrangement (Figure 3). The contacts are rated for 10A, and the maximum switching voltage is 600VAC. The Omron J7KNA-AR series is a modular device, allowing for the specification of numerous options, including coil voltage, contact arrangement (available in 4, 6, and 8-pole versions), and mounting method.
Figure 3: Omron Automation and Safety's J7KNA-AR-3124VS contactor features a 24V DC solenoid coil and a four-pole single-throw contact arrangement. (Image source: Omron Automation and Safety)
The mechanical structure of contactors is constantly evolving. Now contactors can be mechanically connected to support multiple simultaneous actuations or to provide mechanical interlocking to prevent one contactor from actuating along with an adjacent contactor.
Contactors are designed for high current and high voltage; therefore, contactors with a rated current carrying capacity exceeding the absolute required current can have a longer service life. Larger contacts result in a more robust structure and thicker plating, thus preventing significant pitting corrosion during actuation.
The TE Connectivity Aerospace Defense and Marine EV200AAANA is an example of a high-current contactor. This contactor can switch a 900V load and carry 500A of current, or interrupt a 2000A load current at 320VDC via its main power contacts. A set of auxiliary contacts can carry 2A of current at 30VDC or 3A of current at 125VAC. The EV200AAANA contactor features a 12V DC solenoid coil. As shown in Figure 4, this contactor employs a sealed, non-modular design. Typical applications for this contactor include battery switching and backup, DC power supply control, and circuit protection.
Figure 4: TEConnectivity's EV200AAANA hermetically sealed, non-modular contactor can switch 500A of current using a 12V control input. (Image source: TEConnectivity)
Specialized contactors are also available for specific applications. For example, many industrial and commercial contactor applications require switching lighting loads, involving very high inrush currents that could potentially solder the contacts of ordinary contactors together. Metal halide lighting is one such load with high inrush current requirements. Switching power supplies also present similar high-capacitive loads, consuming significant initial inrush current. Some specialized contactors contain NTC surge protectors that initially limit the current flowing to the load, preventing the inrush current from soldering the contacts together. Alternatively, a high-current NTC surge protector can be added externally to the contactors in the power supply circuit to achieve the same effect.
Summarize
When properly configured, including selecting appropriate coil voltages (AC and DC) and contact sizes, relays and contactors are highly efficient devices for switching power supplies. Relays are available in a variety of sizes, while contactors are more standardized industrial devices with a more uniform form factor. The specific choice depends on the load to be switched, as well as whether the load type is resistive, capacitive, or inductive.
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