In a very rudimentary sense, a relay is a switch that is tripped by another switch. It is a device controlled by an electric signal that allows electricity to be switched on or off in a circuit. It does this through a set of contacts that are either open, so electricity does not flow, or closed, so electricity does flow. For a device that has been around since 1835, the relay has held up quite well and is used in countless modern systems
Simply put, a relay is either on or off. This allows an operator (for example) to control a high energy circuit using a low current switch that can be operated safely from a distance. In fact, relays are commonly used when it is impossible for a direct connection between the control circuit and the output device, either due to risk to the equipment or to the operator.
A basic relay performs this function using an electromagnet, electrical contacts, an armature (metal core with wire windings that can produce movement) controlled by a magnetic field, and a spring. The spring normally holds the armature in place. When a current is applied the electromagnet attracts the armature. This movement closes a set of contacts and electric current flows into the circuit.
Relays can be either normally open (NO) when the contacts are open and the relay is not energized, or normally closed (NC) when the contacts are closed and the relay is not energized. This means that relays are specified as either NO or NC when no power is applied.
There are two types of relays available today:
- Electromechanical (EMR) - function via the physical movement of internal parts that open or close due to a magnetic force when current is applied. Types of EMRs include general purpose relays, reed relays, latching relays, and machine control relays. EMRs can operate with either AC or DC current across a wide voltage range.
- Solid state (SSR) - do not incorporate moving parts and are totally electronic. Types include zero and peak switching relays, instant ON relays, analog switching relays and optically coupled relays. SSRs operate on DC within a voltage range of 3VDC to 32VDC.
There are pros and cons to consider when using either EMRs or SSRs. EMRs are rugged, low cost and are insensitive to EMI/RFI. They can, however, be affected by magnetic fields, corrosion or oxidation, and they can be noisy.
SSRs offer low power consumption, fast response, long life if used within specs, and silent operation. Digital technology has also allowed diverse relay functionality in a single hardware package. They can, however, have a higher initial cost than EMRs, generate more heat, and can be sensitive to EMI/RFI.
Relays are rated based on how much power they can safely switch. Ratings are either AC or DC and usually given in amperes. The amp level of the relay must be as large as the device under control.
Relays are also classified by configuration, or the number of devices they can control simultaneously. The classifications are as follows:
- SPST - single pole, single throw
- DPDT - double pole, double throw
- 3PDT - three pole, double throw
- SP3T - single pole, three throw
Incorporating relays into a design is relatively easy. The process usually starts with determining how the device will be mounted (panel, DIN rail, plug-in or PCB mount). Then identification of the load voltage rating and current type, load current rating, circuit/switching arrangement, and control voltage and type. SSRs require an additional step of choosing a standard or special application part. SSRs may also require additional components for heat dissipation.
Relays, both EMRs and SSRs, are widely used across consumer and industrial products and systems. They function in heating elements, starter coils, alarms, light dimmers, motors and furnace controls. High power relays are used in elevators, power systems, electric vehicles and other transportation platforms. Lower power SSRs are used in electronic appliances, medical equipment, and security systems.
Designers and purchasing professionals who are working with relays must also keep in mind the environment in which the relay will function. Heat, vibration, humidity, electrical surges, and magnetic fields can all alter the performance characteristics of relays. Getting answers to these issues early in the design stage can save time, effort and money.