RFRelay Types: Electromagnetic, Reed, and Solid State
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This article covers the basics of relays and explores different types, including electromagnetic (EM) relays, reed relays, and solid-state relays. It highlights the differences between these types, along with their respective advantages and disadvantages.
Introduction
- A relay is an electronically operated switch.
- Relays are switches that control one circuit by opening and closing contacts in another circuit without any physical contact between the circuits themselves. They allow low-power signals to control high-power devices.
- Relays are categorized into various types based on their construction and application, including electromagnetic relays, reed relays, and solid-state relays.
Electromagnetic Relay
- An electromagnetic relay uses an electromagnet to mechanically actuate a switch.
- Although somewhat bulky and slower compared to other types, electromagnetic relays are exceptionally versatile and cost-effective.
- Electromagnetic relays are classified as SPST (Single Pole Single Throw), SPDT (Single Pole Double Throw), DPST (Double Pole Single Throw), and DPDT (Double Pole Double Throw) depending on the number of throws and poles.
- Figure 1 (above) illustrates an electromagnetic relay.
Reed Relay
- A reed relay utilizes ferromagnetic reeds for switching.
- The reeds are thin strips made of a material that is easily magnetized and demagnetized.
- These reeds are typically sealed within a glass tube filled with an inert gas like nitrogen to prevent corrosion of the contacts.
- Figure 2 (above) illustrates the operation of a reed relay.
Solid State Relay
- A solid-state relay employs semiconductor devices to perform switching.
- These relays use components like LEDs, photodiodes, SCRs (Silicon Controlled Rectifiers), and thyristors to switch a circuit ON or OFF.
- Figure 3 (above) depicts a solid-state relay.
Difference Between Electromagnetic Relay, Reed Relay, and Solid-State Relay
The following table highlights the key differences between electromagnetic relays, reed relays, and solid-state relays:
| Features | Electromagnetic Relay | Reed Relay | Solid State Relay |
|---|---|---|---|
| Working principle | Electromagnet, when energized, moves a lever to make contact. | Leads made of ferromagnetic material attract when subjected to a magnetic field. | Semiconductor devices are used to switch ON or OFF the circuit. There are no physical moving parts involved. |
| 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 compared 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 |
Relay Applications
Relays find use in a wide range of applications. A few examples are summarized below:
- SPST relay: Can be used for smart control of traffic signals (Red, Yellow, Green).
- SPDT relay: Can be used to control two different speeds of a fan based on temperature sensor output.
- DPST relay: Can be used for water release based on water content in the soil, using appropriate sensors.
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