Relay basics and difference between Relay types - Electromagnetic, Reed, Solid state
This page covers basics of relay and types of relay viz. electromagnetic (EM) relay, reed relay and solid state relay. It also mentions difference between electromagnetic relay,reed relay and solid state relay type. The advantages and disadvantages of these relay types are also described.
Introduction:
• Relay is electronically operated switch.
• Relays are switches which control one circuit through the other without making
any physical contact. They control high power devices using low power signals.
• There are various types of relay based on their construction and usage viz.
electromagnetic relay, reed relay and solid state relay.
Electromagnetic Relay
• It uses electromagnet to mechanically operate a switch.
• Though this relay types are bulky and slow, they are extremely versatile and cost-effective.
• Electromagentic relays are classified into SPST, SPDT, DPST and DPDT based on number of throws and poles.
• The figure-1 depicts electromagnetic relay.
Reed Relay
• It uses ferromagnetic leads for switching.
• The reeds are thin strips of easily magnetizable and de-magnetizable material.
• They are usually manufactured such that reeds are sealed in a glass tube containing inert gas such as nitrogen.
This help reduce corrosion of the contacts. The figure-2 depicts working of reed relay.
• Refer Reed switch basics >> and advantages/disadvantages of Reed switch >> for more information.
Solid State Relay
• It uses semiconductor devices to perform switching.
• These relays use LEDs, photodiodes, SCRs and thyristors for switching a circuit ON/OFF.
• The figure-3 depicts solid state relay.
• Refer advantages and disadvantages of solid state relay >> for more information.
Difference between Electromagnetic relay, Reed relay, Solid state relay
Following table mentions difference between Electromagnetic relay, Reed relay and Solid state relay.
| Features | Electromagnetic Relay | Reed Relay | Solid State Relay |
|---|---|---|---|
| Working principle | Electromagnet when energized moves a level to make contact. | Leads made of ferromagnetic material attract when they are subjected to magnetic field. | Semiconductor devices are used to switch ON or OFF the circuit. There are no physical moving parts involved in the design of solid state relay. |
| Operating speed | Slow | Fast | Fastest |
| Life time | Less | More | Very high |
| Size | Large | Small | Small |
| Cost | Not too costly | Costliest | High cost |
| Power consumption | High | Less | Less |
| Advantages | • Very Cheap • Versatile (Can be used with any circuit) |
• Current requirement is very less • Smaller and faster compare to electromagnetic relay type |
• Very fast • less current requirement • Highly durable |
| Disadvantages | • Subjected to wear and tear |
• Very costly |
• Bound by specific voltage/current characteristics |
| Applications | • Automobiles • Aviation industries • Agriculture |
• Telephone exchanges • Communication |
• PCB for home appliances • PCB for electrical circuits |
Relays are used for wide variety of applications. Few of these are summarized below.
➤SPST relay is used for smart control of traffic signals (Red, Yellow, Green).
➤SPDT relay can be used to control two different speeds of the fan based on
temperature sensor output.
➤DPST relay can be used for water release based on water content in the soil. Appropriate sensors are used for
this purpose.
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Advantages and Disadvantages of other Sensor Types
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