A relay is an electrical control device which effectively works as an automatic switch, to convert a low electrical input into a high-current output, and therefore facilities the interaction between the input and output circuits. A typical relay consists of two key elements; a coil, which receives an electric signal and converts it to a mechanical action, and secondly, contacts which open and close the electric circuit to receive current.
Relays are used in applications where it is necessary to control a circuit using an independent low-power signal, or in applications where several circuits need to be controlled by one circuit. These are usually straightforward on/off circuits where circuits work on logic operations. Originally, relays were used in long-distance telegraph circuits and telephone exchanges, and also in early computers. Relays are extremely versatile components and can carry out the same function in a highly complex circuit as they do in a very simple circuit. As with most electrical components, relays differ in size and capacity.
How do relays work?
In simple terms, a relay opens and closes a circuit, and it controls that circuit by opening and closing the contacts of another circuit. This uses a ‘logic’ sequence of the contacts being ‘normally open’, known as NO, or ‘normally closed, known as NC.
So when the relay contacts are normally open, there is an open circuit when the relay is not energised. When the relay contacts are normally closed, there is a closed circuit when the relay is not energised. So in both cases, applying electrical current to the contacts of the relay will change their state, and hence the user can build a circuit and position relays to control, or ‘switch’, how the circuit works.
How are relays used?
Relays typically have three separate uses:
- An automatic adjustment of the current
- Safety protection
- Interacting between two circuits to convert the current
So relays are used to switch smaller currents in a controlled circuit and therefore are not normally used to control a device which consumes power, unless these are small motors or solenoids which draw low amps and have very simple uses. However, relays can control larger voltages and amperes because they can have an amplifying effect. This is when a small voltage is applied to the coil of a relay and this can result in a large voltage being switched by the contacts.
As a protection device, a relay works to prevent equipment damage. It does this by detecting abnormalities in the electrical current flow, such as under currents and over currents, and also overloads and reverse currents. Using this type of logic, relays are widely used to switch starting coils, and can also work to control heating elements, pilot lights and audible alarms.
Why are relays important?
Building a relay into a circuit means you don’t need wiring and switches with a high amperage. These can be expensive and also take up valuable space in a circuit. So using relays instead means a manufacturer can have a smaller casing on an electronic item which, therefore, can perhaps contain more functionality in the same space and therefore improve the product for the user.
Please note that this section is for information purposes only. Anyone using equipment referred to in this section must be suitably qualified and/or experienced within the respective field. If in doubt before use, please consult a qualified electrician or engineer & thoroughly read all instruction booklets.