Relays are electromechanical or electronic switches that open and shut circuits. Relays operate one electrical circuit by opening and shutting contacts in another. When a relay contact is usually open (NO), there is an open contact when the relay is not activated, as seen in relay schematics. When the relay contact is normally closed (NC), there is a closed contact when the relay is not activated.
Digital relays are electro-mechanical devices used for controlling electrical loads such as lights, appliances, and machinery. They differ from analog relays in that the output is either on or off, never continuously variable like an incandescent lamp is. The term "digital" here refers to the method by which the relay controls the power: 0's and 1's. An analog signal may be used to control the relay, but it must be very carefully calibrated to prevent false signals that would cause the relay to turn on or off at random.
Digital relays were first introduced around 1960 and have been improving rapidly since then. Today's digital relays are smaller, faster, and more reliable than their older analog counterparts. They also offer greater functionality including timing, holding, and sequencing capabilities.
In order for a digital relay to control electricity, it must contain some form of semiconductor technology. Semiconductors can be divided into two main categories: conductors and insulators.
When a relay contact is usually closed (NC), the contact is closed even when the relay is not powered. Applying electrical current to the contacts will change their state in either situation. The type of relay used in radio controlled devices operates similar to electromagnets. When no current is applied, there is no magnetic field strength to keep the contact closed. However, when an electrical current is applied, the iron core inside the relay becomes magnetized, which creates a strong magnetic field that keeps the contact closed.
Closed contacts transmit power from the source to the load and prevent any electricity from flowing in the opposite direction. For example, if the coil is powering something such as a light bulb then it makes sense for the contacts on the back of the relay to be closed so that the current can flow through them and into the bulb. If they are open then none of this will happen. Relays come in many different sizes and configurations but they all work based on this same concept: transmission of electrical power from one place to another without any other devices getting in the way.
A relay is used in radio control applications to turn on or off components like lights, motors, and radios. A relay can also be used to switch high voltage signals around using low voltage components.
A relay is essentially a device with contacts that opens and shuts a switch in response to an input signal (voltage or current) delivered to a coil. Relays can be divided into two types: mechanical relays and solid-state relays. Mechanical relays are still used in some low-speed applications such as power on/off switches, but they are being replaced by solid-state relays in most other applications.
Solid-state relays use semiconductor components instead of electromechanical components. They are more reliable than their mechanical counterparts and have lower operating temperatures. Also, SSRs can handle higher voltages than mechanical relays.
SSRs can be subdivided into two main categories: contactors and transformers. Contactors are used for switching high amperage loads such as motors. Transformers are used for switching low-amperage loads such as lights. Transformers use multiple coils to provide more capacity at a lower cost than single-coil devices. They usually have larger cores and more windings than contactors so they can carry more current.
Contactors operate on the same principle as electromagnets; when electricity is applied to a coil, a magnetic field is created which causes a metal arm attached to the coil body to move down and make or break electrical contact. The contactor controls the flow of electricity by opening or closing the circuit it's attached to.
A relay is a switch that is powered by electricity. It has a set of input terminals for a single or many control signals, as well as a set of operational contact terminals. The switch may contain an unlimited number of contacts in various contact types, such as make contacts, break contacts, or combinations of the two. A relay can be controlled either directly or indirectly via another device such as a potentiometer.
Relays were originally invented by George Westinghouse and first sold in 1872. Since then they have become an essential component in modern electrical equipment. For example, relays are used in telephone exchanges to select incoming lines or units of equipment, so that each unit will respond only to its destination. They also find use in large scale industrial control systems, where several processes need to be independently stopped or started.
A relay can be activated either instantly (such as a mechanical switch) or with a time delay (such as an electric switch). This allows the controller to operate devices such as motors or lights without interfering with each other. Some applications require more than one type of activation; for example, a motor might have to be instant on but a solenoid could have a delayed action. This would allow the controller to turn on the motor yet leave the solenoid inactive until it sensed a pressure difference across its coil.
Relays are a sort of remote control electrical switch that may be switched to control a high current load using low current. In this lesson, we will go through the fundamentals of relays. Current flowing through the relay's coil generates a magnetic field, which attracts a lever and switches the switch contacts. The switch contacts can be any type of contact, such as a rocker, a dome, or even a mechanical snap-acting switch.
The most common use for relays is to control appliances and machinery on buildings at large commercial sites. For example, a factory might have several hundred light fixtures controlled by a single relay module. Or, a hospital might have hundreds of bedside lamps controlled by a single relay module. Remotely operating lights is an application that continues to grow in popularity.
Other popular uses for relays include controlling power to appliances such as air conditioners, heaters, and dishwashers; providing safety features in automobiles for example, anti-lock braking system (ABS) sensors; and security applications (for example, access controls).
Relays come in three main types: electromechanical, electronic, and solid-state. Electronic relays offer the best performance for many applications, but they are also the most expensive. Solid-state relays are less expensive than electromechanical or electronic relays, but they do not function if moisture gets into the casing of the relay.
Relays are employed when a low-power signal is required to control a circuit (with perfect electrical isolation between the control and controlled circuits) or when numerous circuits must be controlled by a single signal. Relays can also be used as an effective replacement for switches in applications where the need for mechanical switching operations is evident.
The most common use of relays is probably in telephone exchanges, where they are called "switchboards". They allow a number of wires to be connected or disconnected from different parts of the system simultaneously with only one connection to the network. A second important application is in radio receivers where electromagnetic signals need to be switched on or off at high speed according to the input signal. A third example is when it is necessary to make or break the connection between two circuits on a single device such as a microcontroller pin. A fourth example is when a low-power signal is required to open or close a circuit (such as a motor or solenoid), while maintaining complete separation of the power supply connections. This is useful, for example, when monitoring machinery with a microcontroller; if the controller's power source is cut off by accident, the relay will still keep the machinery operating until it shuts itself down automatically.
In general, any time you need to make or break the connection between two circuits on a single device. " - Wikipedia