The difference between solid state relays and mechanical relays(2)

The difference between solid state relays and mechanical relays(2)

12. Cost and market size

The average cost of solid state relays is much more expensive than mechanical relays. In addition, the market size of mechanical relays is much larger than that of solid state relays.

13. Multiple Contacts

Mechanical relays can have multiple contacts, but solid state can only provide limited output.

14. History and inventors

Mechanical relays have a long history. Joseph Henry invented the first mechanical relay in 1835 to improve his version of the telegraph. Solid State Relays were invented by Crydom engineers in 1972.

15. Electromagnetic interference

Produces very little EMI and provides clean switching for almost any load. Magnetic fields have little effect on solid state relays because, unlike mechanical relays, they do not use magnetic coils to switch loads.

16. Applications

Solid state relays are suitable for controlling slightly inductive loads such as heaters, lighting, and small fans. It can also be used for soft turn-on of high-power incandescent lamps. Mechanical relays are used in a wider range. It can be used in any automation and control application.

Solid State Relay Applications

17. Failure Modes

All electronic components have failure modes. Mechanical relays and their moving parts, contact surfaces, and wound coils generally have a higher rate of immediate failure than solid state relays.

18. Contact bounce and arcing

Solid State Relays do not spark or arc, nor do they bounce back electrically or mechanically. It is also immune to contact bounce. The maximum bounce time of a mechanical relay is the time from the first closing or opening of the relay contacts to the last closing or opening during switching to another switch position. Bouncing can cause short-term contact breaks. Bounce is detrimental to contact life and is especially annoying in applications where relays are used for pulse counting. In this case, bouncing can easily lead to false pulse counts because the contacts continue to make and break the circuit during the bouncing. Contact bounce does not occur in semiconductor based solid state relays. There are no contacts to return. Typical applications where bouncing and arcing can be problematic are data acquisition applications. For mechanical relays, some wait time must be set in the application to avoid measurements during contact bounce. Contact bounce is also a problem in applications where voltage rise needs to be calculated, such as meters and counters. Contact bounce can create false spikes that reduce the reliability of the counter.

19. Voltage requirements

Since the solid state relay does not have to energize the coil or open the contacts, the voltage required to open or close the solid state relay is lower. The mechanical relay is controlled by a power supply from 5 to 48 Vdc. In contrast, solid state relays can operate from power supplies as low as 1.5 Vdc.

20. Direct logical operations

Solid state relays can be driven directly by logic circuits. Mechanical relays require coil drive voltages that far exceed the output drive of logic circuits. Therefore, additional components (such as PLCs or slim relays) are required to allow the mechanical relay to interface with the logic circuit.

21. Weight

Solid state relays are much lighter than equivalent mechanical versions; up to 70% depending on power.

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