Miniature Electromagnetic Relays
(10 574)The function performed by electromagnetic relays is trivial but extremely important. Relays are electromechanically operated switches that use a low level control signal to switch a much higher voltage or current supply. They can also perform the opposite function, e.g., turn on a lamp on the panel, signalling the flow of high current in the controlled circuit.
An electromagnetic relay consists of a coil and contacts and uses an electromagnet to close or open the contacts. In other words: applying a voltage to the coil closes or opens the circuit (or several circuits, depending on the pin configuration).
Electromagnetic relays are known in automation for many decades. Their design still undergoes some modifications, but their advantages and limitations remain the same. The main advantage is the isolation of the control and controlled circuits. This gives electromagnetic relays a certain advantage over alternatives such as electronic circuits based on semiconductors (transistors). This feature is particularly appreciated in the consumer electronics and white goods industry, where the safety of users is important, even in the event of improper handling. In many cases relays also have the advantage of being simple in design and easy to use in electronic/electrical circuits. Replacing relays in your vehicle, control unit, and machinery is not a problem. Many relays are designed for quick assembly and disassembly, use dedicated sockets, and are produced in standard formats, which facilitates the work of mechanics, maintenance technicians, etc. Unfortunately, relays also have their drawbacks - although the control currents are low, they can be considered substantial (tens or hundreds of milliamps) for energy-saving applications. As they are mechanical components, they have a certain degree of failure and are not able to match their semiconductor counterparts when it comes to the switching frequency.
Let's take a look at the basic features of electromagnetic relays that should be kept in mind when selecting the right components for your applications or when looking for a replacement for a damaged component.
Relay Contact Configuration
The first aspect is the configuration of the contacts. The most important abbreviations are: NO (normally open) and NC (normally closed). This means that the relay-controlled circuit can be normally closed or normally open, depending on the needs. Obviously, in the case when the purpose of the relay is to temporarily interrupt the supply - a relay that opens the circuit when triggered shall be used. This results in less wear on the relay, which will not consume energy during normal operation of the device.
Apart from the abbreviations NO and NC, the most common contact configurations are:
SPST (Single Pole Single Throw) - the simplest switch used to make or break the connection between each input contact and its designated output contact.
SPDT (Single Pole Double Throw) – this switch contains one input, which can switch between two outputs, i.e., can connect to and switch between two outputs. When there is no voltage applied to the coil one circuit “receives” current, the other one does not.
DPST (Double Pole Single Throw) is a switch that has 2 inputs and 2 outputs. A single control signal activates two independent circuits. Such a relay can be used for simultaneously controlling, e.g. a load and an indicator light.
DPDT (Double Pole Double Throw) - two input contacts that can be in one of two positions, i.e. connected to one set of output contacts.
TME customers looking for more pin pitch options can use the technical drawings visible when hovering the mouse over the information icon in the "Contact Configuration" field (this is one of the filters visible at the top of the page).
Basic parameters of electromagnetic relays
In addition to the contact configuration, the important parameters of relays mainly concern their electrical characteristics. A distinction should be made here:
Relay version - this specifies the general characteristics of the element (e.g., whether it is used to control signals or is resistant to high inrush currents of inductive loads such as motors). There is also a division into bistable and monostable versions. Bistable relays, as the name suggests, do not change their state after disconnecting the control voltage.
Nominal coil voltage - the optimum voltage that must be applied to the coil for the relay to perform its function. A voltage that is too low will be insufficient to run the circuit, while a voltage that is too high can damage the electromagnet. The distinction between AC and DC operated systems is also important here.
Maximum switching voltage - the highest potential difference that can occur in the controlled circuit. Exceeding this value may result not only in damage to the relay, but also in electrical breakdown and failure of the controller (this in turn poses a risk to the health and life of the operators!). The same applies to the Maximum load capacity of AC contacts.
Mounting, size, and pitch - relays usually have a characteristic pin pitch, which prevents their incorrect mounting. For PCB mounted components, the output layout is usually compatible with typical pitches (2.54 mm, 5.08 mm, etc.). There are also surface-mount (SMT) relays. However, in the case of industrial solutions, we most often encounter base-mounted elements - this makes it easier to replace the relay and minimises the resistance of the connections.
Coil resistance - this parameter allows you to determine the energy consumption to be expected when using the component. It also makes it possible to estimate whether the relay will not overheat.
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