A relay is an electrically operated switch. When its two contacts are disconnected, the relay opens, and it turns on when the two contacts touch. Generally, they are designed to be controlled with low voltages, such as 3.3V and 5V, and they are often utilized to isolate low voltage circuits from a high voltage circuit to control high voltage devices. With thousands of relays on the market suiting a myriad of purposes, how does one go about making a selection? To answer this, this blog will outline different types of relays as well as their advantages and disadvantages.
Depending on the operating principle and structural features, relays are usually categorized into three main types, those of which are electromechanical, solid-state, and reed relays. While there are a few other types, their specific uses are a bit more limited. As such, we will only cover the aforementioned types below.
This type of relay is constructed with electrical, mechanical, and magnetic components, and it consists of a coil that induces a magnetic field when energized. This magnetic field draws in a movable contact, or armature, that closes or opens the contacts. When the coil is de-energized, it loses its magnetic field. Meanwhile, a spring retracts the movable contact to its normal position, which then again, opens or closes the contact.
Electromechanical relays are designed for AC or DC sources based on the application at hand. They work on the same principle as electromagnetic induction but the configuration of the coil may differ among types. Moreover, the DC coil has a freewheeling diode to de-energize, while the AC coil takes advantage of laminated core to prevent current losses. These relay types may also be split into two subtypes, those of which include latching and non-latching types. More than that, they can be further classified by their switching mechanism according to the number of terminals. For instance, there are single throw (ST) and double throw (DT) electromechanical relays.
In terms of advantages, electromechanical relays can withstand large inrush currents. Additionally, they have durable structures that are not susceptible to external electromagnetic environments. Lastly, they are cheap and cost-effective, making them a go-to choice for many operations. With regard to disadvantages, electromechanical relays are slower than other types, and as they come in larger package sizes, they are not ideal for small projects. More than that, they have shorter service lives when compared to other options as a result of mechanical wear.
Also called SSRs, solid-state relays are composed of a circuit with various electronic components that perform the same functions as electromechanical relays. More specifically, SSRs utilize solid-state components to carry out the switching operation without any moving parts. Furthermore, when a small external voltage is applied across its control terminals, an SSR switches ON or OFF. In addition, SSRs utilize semiconductor devices to switch the conduction and disconnection of high voltage loads.
Typically, SSRs consist of an LED driver and a photosensitive MOSFET. As current flows through it, it lights up the LED where the MOSFET detects it, triggering the gate of TRIAC (Triode for alternating current) or SCR (Silicon Controlled Rectifier) to switch the load and turn on the high voltage circuit. SSRs have many advantages, some of which include their fast switching speed, silent operation, increased service life, compact design, and more. Unfortunately, they also have disadvantages, those of which lie in their high cost and large contact resistance.
Reed relays are made up of a switch with magnetic strips, or reeds, that are sealed within a glass tube filled with an inert gas which is guided by the external magnetic or induced field from its solenoid. The magnetic field applied to the coil is wrapped around the tube that controls the reeds’ movements, allowing the switch to happen without the aid of an armature. When voltage is applied to the coil, a transverse magnetic field is generated and the reed is magnetized. It is important to note that if the reed relay has an inductive load, a protection circuit must be provided between the relay and the load.
When looking at the advantages of reed relays, the most beneficial include their low power consumption, compact size, fast switching speed, and many others. Some disadvantages, on the other hand, are reed relays’ low load voltage, low current, and susceptibility to inductive loads.
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