Introduction to Relays

A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof. Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal.

What are relays?

In a very basic sense, a relay is simply a binary actuator. At any point in time it has one of two steady states – either energized and latched, or de-energized and unlatched. For the uninitiated, that essentially means either ON or OFF. In fact, some of the very first computers were built with a large collection of relays to execute Boolean (logic) gates.


From an engineering and design standpoint, basic relays are elegantly simple. They contain an electromagnet, some electrical contacts, an armature that can be controlled by a magnetic field, and a spring. The spring holds the armature in place. The electromagnet attracts the armature when a current is applied. This set-up allows a relay to control a high current electrical load with a low-current electrical signal. This quality facilitates their use to control power in a wide variety of devices and systems.

When a relay is normally open (NO), there is an open contact and the relay is not energized. When the contacts are normally closed (NC), there is a closed contact when the relay is not energized. Application of electrical current changes their state.

In devices that consume a lot of power, and that would normally require an extensive amount of wire, a relay can be used with a pair of low-power wires to control the ON/OFF electrical flow. In fact, since the circuit powering the coil is physically separated from the circuit powering the device, relays are commonly used where it’s impossible to have a direct connection between the control circuit and the output device, either because of risk to the equipment or the safety of the operator.

Relays are widely used to switch devices like heating elements, starter coils, alarms, pilot lights and motors. High power relays are used in electric vehicles and other transportation platforms. Relays can also control the maximum current and voltage that can go through the armature and contacts, the number of armatures used, the number of contacts used, and whether the contact is normally open or closed.

What Kind of Relays are Available Today?

There are generally two broad types of relays: electro-mechanical relays (EMR) and solid-state relays (SSR). Each of these types of relays have several subsets to fit particular applications.

OMRON G2R Relays
OMRON G2R Relays

EMRs (electro-mechanical relays) work via the physical movement of internal components and are open or closed by a magnetic force when a current is applied. EMR types include the following:

  • General purpose relays are simply electromechanical switches that operate with AC or DC current, usually 12V – 230V, and control currents from 2 amps to 30 amps. Their benefits include low-cost, ease of replacement and wide range of configurations.
  • Machine control relays are heavy duty devices used to control starters and large industrial components. While more expensive than general purpose relays, they are more durable and they can easily be combined with accessories to expand their functionality to include timing, latching control and transient noise suppression.
  • Latching relays have two states. They are set either ON or reset OFF by the input of voltage. The relay maintains this set or re-set condition until it receives the next inverting voltage signal.
  • Reed relays are so-named because they consist of two reeds, hermetically sealed in glass to make them immune to contaminants, humidity or fumes. They are normally small, compact and fast operating with most often a single normally open (NO) contact. In operation, the ends of the contact are closed by a magnetic force. Reed relays, easily driven by low power control circuits, are often used to control industrial components like solenoids and contactors which switch high current/power loads.

SSRs (solid state relays) do not incorporate physical contacts and their switching is totally electronic. They consist of an input circuit, a control circuit and an output circuit. Unlike EMRs (electromechanical relays), where the magnetic coil accomplishes the coupling between the input and output circuits, SSRs (solid state relays) do this via the control circuit, which is activated when a voltage higher than the specified pickup voltage is applied to the control input. The relay is deactivated when the voltage applied is less than the specified dropout voltage. Common SSR voltage ranges are 3VDC to 32VDC, making them useful for electronic circuits.

The types of SSRs (solid state relays) include the following:

  • Zero-switching relays work by turning ON the load when the control voltage is applied and the voltage of the load is close to zero. They turn OFF the load when the control voltage is removed.
  • Instant ON relays turn the load ON as soon as the pickup voltage is applied.
  • Peak switching relays turn the load ON when the control voltage is applied and the voltage of the load is at its peak. They turn the load OFF when the control voltage is removed and the current in the load is close to zero.
  • Analog switching relays have a circuit that controls the amount of output voltage as a function of the input voltage. This allows an infinite number of possible output voltages within the rated range of the relay. These relays turn OFF when the control voltage is removed and the load current is near zero.
  • Optically coupled relays accomplish current switching in response to the ON or OFF state of a light source (usually an LED). They are faster than electromagnetic relays and provide isolation between the control & power circuits.

Relay Ratings and Configuration

Relay ratings address their ability to handle power. Ratings are usually given in terms of amperes and amp levels must be as large as the rating of the target device.

Configuration is dependent on the number of things the relay can control simultaneously. With one input and one output it is a simple on/off switch, or single throw. For two outputs, there are two throws. Pole and throw information is designated like this:

  • Single pole, single throw: SPST
  • Double pole, double throw: DPDT
  • Three pole, double throw: 3PDT
  • Single pole, three throw: SP3T

Relay contacts are either normally open (NO) or normally closed (NC), depending on the state that exists when no power is applied.

Common Relay Applications

As you can see, a relay is a rather simple device that performs a very useful function, leading to a diversity of uses across products and markets. All relays operate in the same basic way, but a range of types exist to match selected applications, mountings and environments.

Modern relays are used in areas ranging from consumer electronics and home appliances to autos and transportation. They are employed in process control systems, from small to large. They function as motor controls and motor protection devices. They help control systems from as small as automatic garage door openers to large electric power systems. They have aerospace, defense and marine applications as well as in emergency standby power systems.

It is difficult to imagine a modern product or system in which power is being directed or controlled that does not use some type of relay.

Changes in the Technology of Relays

The application of semiconductor technology in the relay sector has been the biggest driver of change over the past 10 years, utilizing smart power solutions in SSRs (solid state relays) to reliably deliver high current. Digital technology has also allowed many more relay functions to be incorporated in a single hardware package. Plus, the spread of LED technology has led to improvements in optically coupled small signal solutions.


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