Many applications must switch circuits with high isolation or be able to switch high voltages and high currents with a low-power control signal. In some cases, a semiconductor-based solution is not suitable. Developers then have to choose between electromechanical relays and contactors and understand how to use them correctly.
Electromechanical relays can switch relatively high currents with a control signal of just a few volts. They also offer good voltage isolation between the control signal and the switched power. However, higher currents and very high switching voltages require contactors, which are essentially extremely powerful electromechanical relays. Most developers are familiar with the many different types of relays, ranging from reed to high power relays. However, only a few people outside of the industrial energy industry are familiar with contactors, which are often used to switch high-voltage circuits and very high loads.
This article covers the difference between electromechanical relays and contactors and details the applications for which they are best suited. It presents a selection of different relay and contactor solutions and gives practical tips on using the individual types.
Relays versus contactors
Both relays and contactors are electromechanical components that operate one or more pairs of contacts via an electromagnet. Single pole relays or contactors only have one pair of contacts. There are also two-pole relays and contactors and the number of contacts can get relatively high. Contactors can be designed as normally open or normally closed contacts. Some relays and contactors also have changeover contacts that combine a normally open and normally closed contact.
Relays are suitable for switching low and medium current loads at relatively low voltages and are available in many form factors, including plug-in and board-mount versions that can be soldered onto a circuit board. Contactors are designed for high currents and high voltages.
The selection of a suitable relay or contactor depends heavily on the load to be switched. The following is a compilation of various load types and tips for handling these loads:
Resistive loads do not have a current surge when they are switched on for the first time. The best-known example of an ohmic load is a simple heater. If its specified current consumption is 10 amps, it can be safely switched with a 10-ampere relay. In reality, there are very few purely ohmic loads. Most loads appear as a combination of two or more types of loads.
Lamp loads draw high currents when they are switched on for the first time. The filament of an incandescent lamp has a high-temperature coefficient. When cold, its resistance can be as little as 5 per cent of the filament resistance of a hot lamp. In this case, 20 times more current is drawn than when the lamp was heated up. An incandescent lamp with 75 watts draws a current of a little more than half an ampere in normal operation. However, when switched on, the cold filament draws an inrush current of 13 amps. Although this inrush current only lasts for about a tenth of a second, all relay contacts through which an incandescent lamp load flows must be suitable for this high inrush current.
Motor loads also draw high currents when they are switched on for the first time. A single-phase synchronous motor with 110 volts AC and 1/3 HP usually draws a little over 4 amps. When starting or with a locked rotor, the same motor can draw over 24 amps. Released from its mechanical load, the motor can draw 6 amps in unloaded operation.
Capacitive loads cause high current surges when switched on because a capacitor always tries to maintain a constant voltage for itself. If you apply a voltage to an uncharged capacitor, you practically cause a short circuit for a short time. Such high inrush currents can cause the relay contacts to weld. Typical capacitive loads are the output voltages of DC-power supplies and other filtered power supplies.
Inductive loads offer a smooth start as the load current increases slowly when switched on. When the load is switched off, however, an inductive voltage spike is induced at the contacts of the relay, since an inductance always tries to maintain a constant current for itself. This induced voltage spike can be high enough to cause an arc at the contacts of the relay, which slowly melts the contacts and can cause pitting on the contact surfaces with every switching process. This explains why snubber diodes are built into the coils of some relays to prevent arcing. Examples of high inductive loads are solenoid drives, electrically operated valves and relays.
Relays in detail
Important specifications of relays are the coil voltage, operation with AC or DC coil, rated current and configuration of the contacts (normally open, normally closed, multi-pole), the number of contacts and the actuation/release times. It is important to avoid switching currents that are too low for a relay to operate reliably. For the correct operation of the contacts of a relay, it is a bit important that a prescribed minimum current is switched. This current removes contaminants that may have built upon the contact surfaces of the relay.
The lower limit for the current with which a relay can be reliably switched depends on various factors such as the material and geometry of the contacts and the mechanical sliding properties of the contact surfaces. All of these factors influence the minimum switching current of a relay. With relays with gold-plated contacts and with fork contacts, currents of only 10 milliamperes can be switched reliably.
Normal reed relays and those with mercury-wetted contacts are suitable for switching applications with low levels. For example, TE Connectivity Potter and Brumfield Relay JWD and JWS reed relays are available with various coil voltages from 5 to 24 volts DC and in various single and double pole configurations.
The reed relay JWD-171-10 from TE Connectivity, for example, has a 24-volt coil with an integrated snubber diode and a normally open contact for switching a maximum current of 500 milliamps at 20 volts. The JWD series reed relays can be mounted on a printed circuit board and have the same footprint as an integrated circuit in a 14-pin DIP package, although they only have eight pins.
Figure 1: TE Connectivity’s JWD Series Reed Relays Potter and Brumfield Relays, housed in a 14-pin DIP package, are available in a wide variety of coil voltages and contact configurations. (Image source: TE Connectivity Potter and Brumfield Relays)
Reed relays are usually not suitable for switching higher loads, for which a larger housing is required in order to be able to accommodate the larger contacts for high currents. The multi-purpose relay G2R-1-DC24 from Omron Electronic Components, for example, is suitable for switching 10 amps at 24 volts. It has a 24 volt DC coil and a single pole double throw (SPDT) contact configuration. This relay is slightly larger than TE Connectivity’s JWD series reed relays, but can also be mounted on a circuit board (Figure 2).
Figure 2: The multi-purpose relay G2R-1-DC24 for circuit board mounting from Omron Electronics, for example, is suitable for switching 10 amps at 24 volts. (Image source: Omron Electronics)
With the G2R-1-SND-DC24 (S), Omron also offers a similar relay that is designed for plug-in plug-in applications. There are several matching additional plugs for this relay design in versions that are suitable for DIN rails, front panels and through-hole boards.
Contactors in detail
A contactor is the equivalent of a relay for heavy-duty industrial applications and is used as a standard in factory and industrial applications. Contactors are somewhat more robust than relays and can usually be easily mounted on a standard DIN rail. Some have additional mounting holes so they can be attached directly to a flat surface. They are designed for switching high loads, such as small motors and stepper motors with several HP, large heating loads and lighting in industrial and residential buildings. Consequently, contactors are designed to accommodate large, high-current wires.
Like relay coils, the contactor coils are available with AC or DC specifications. Contactors that are controlled by a programmable logic controller (PLC) normally have solenoid coils with 24 volts of DC. Coil drives for AC line voltages (including 110, 220 and 240 volts AC) are also common.
As with relays, the electromagnet of a contactor magnetically attracts an actuator or plunger, which then connects to one or more of the contactor’s electrical contact pairs. Unlike relays, contactors have a modular structure, which is why the electromagnet can easily be replaced in order to change the voltage. Since relays are usually not of a modular design, the entire relay usually has to be replaced if the configuration of a relay is to be changed. Thanks to the modular design of contactors, users can also change the arrangement of the actuated contacts.
Contactors usually have multiple sets of contacts. Sometimes contactors only provide high current contacts. However, they can also have a mixture of high and low current contacts that are used for the simultaneous switching of power or signal circuits. The low-current contact is also referred to as the auxiliary contact. The difference between the two contact types is that the high-current contacts are larger than the low-current contacts in order to be able to transfer the higher load currents. A contactor for controlling a three-phase motor, for example, could have three high-current contacts for the motor output and an auxiliary contact for outputting the current operating state of the motor.
For example, the J7KNA-AR-31 24VS contactor from Omron Automation & Safety has a 24 volt DC solenoid and a four-pole on / off switch (Figure 3). The contacts are suitable for 10 amps with a maximum switching voltage of 600 volts AC. Omron’s J7KNA-AR series is modular and allows a wide variety of options to be specified, including coil voltage, contact arrangement (available in 4, 6, and 8-pin versions), and assembly methods.
Figure 3: The J7KNA-AR-31 24VS contactor from Omron Automation and Safety has a 24 volt DC solenoid and a four-pole on / off switch. (Image source: Omron Automation and Safety)
The mechanics of contactors have evolved over time so that it is now possible to mechanically connect several contactors and thereby enable several simultaneous actuation. A mechanical lock can also be implemented, which prevents a contactor from being actuated when an adjacent contactor is being actuated.
Since contractors work with high currents and high voltages, the service life of the contactor can be extended if a contactor with a higher current rating than necessary is used. With larger contacts, the wear due to the actuation is lower due to the more stable construction and the thicker coating.
TE Connectivity Aerospace Defense and Marine’s EV200AAANA is an example of such a high-current contactor. This contactor is suitable for loads of 900 volts and currents of 500 amps or the interruption of a load current of 2000 amps at 320 volts DC via its main current contacts. Several auxiliary contacts are designed for currents of 2 amps at 30 volts DC or 3 amps at 125 volts AC. The contactor EV200AAANA has a magnetic coil with 12 volts of DC. A sealed, non-modular design is used for this contactor (see Figure 4). This contactor is usually used in battery circuits, in emergency power supplies with batteries, in DC power control and as circuit protection.
Special contactors are also available for special applications. For example, many industrial and commercial contactor applications require lighting loads to be switched with very high inrush currents, which can lead to welding of the contacts of a normal contactor. A metal halide lamp, for example, is such a load with a high inrush current. Switching power supplies also represent comparably high capacitive loads that draw high inrush currents. There are special contactors with integrated NTC thermistors that limit the inrush current of the load and thereby prevent the contacts from welding. The same result can be achieved if an NTC thermistor is added to the circuit external to the contactor to limit the inrush current.
Relays and contactors are very efficient components for switching electrical power as long as they have the appropriate specifications; H. the correct coil voltage (including AC and DC) and correctly dimensioned contacts. Relays are available in many forms factors. Contactors, on the other hand, are standardized industrial components that tend to be available in more uniform form factors. The selection depends on the load to be switched, but the type of load (ohmic, capacitive or inductive) must also be taken into account.