Relays- How many types are there?

Relays, much evolved from their first electromechanical design & evolved during all these years, are available into many flavors. Though the main electromechanical design having a coil is still used in very large quantities, other varieties have also been developed as the requirements demand more & more compactness & less and less current consumption.
Relay types can be broadly categorized as follows-


1.AC relays- These relays are operated through use of AC voltage & current. Though they are the earlier versions of Relays, they are still used for their ruggedness. They are relatively bulky but are extensively used for high power controlling situations.
2.DC relays- DC relays as name suggests use DC power to be operated. They are relatively small in size & use less current to operate. They are mainly used in instruments & gadgets. They are also used to operate their AC counterparts this way a small relay can be used to control very high power.
3.Solid State Relays - Due to coil mechanism,   the traditional relays are not able to switch on & off at high speed. But some situations like temperature controlling demand power to be switched at high speed. This kind of requirement has given birth to a kind of relay, all made up of electronic components- no moving parts. This is called Solid State Relay. This is mainly made up using electronic components like Optocouplers & Triacs. They not only provide high degree of isolation between controlling signal & controlled power but also can be operated at high frequency to finely tuned regulation of high power.
4.Automotive Relays- As the electronic technologies are evolving, they are making inroads into development of latest vehicles. More & more companies are now using electronic controllers to control various systems in their vehicles. This has necessitated the development of DC relays having small footprint, capable of switching high current & able to withstand high temperature and vibration. They are mainly used in latest vehicles.
Telecom relays - This kind of DC relays are very small in size, operate with relatively low currents and switches low power. As the requirement of Telecom sector- switching of low level signals, is perfectly met by these kinds of relays, they are referred as Telecom Relays.
5.Reed Relays - Reed relay also have coil as other relay but instead of a spring mechanism, the coil is wounded around a reed switch. Read switch consists of two small contacts sealed in a small glass tube. When current is passed through coil, due to magnetism the contacts are closed. This kind of relay is able to operate at a speed of hundreds of times in a second but can switch only small amount of current (around 500 mA). They are also small in size, widely used in small current applications.
6.Photo relays- This relay is using Light Emitting Diode (LED), to transmit the input signal through transparent barrier, which triggers the Triac as final output. These relays have small output current capability.
We have to select the required relay as per our requirement and the design needs.