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Can photorelays be replaced with and used in the same way as mechanical relays? What is the difference, if any?

A mechanical relay consists of mechanical contacts, whereas a photorelay is a semiconductor contact consisting of MOSFETs for output. Because of the above, photorelays have the following benefits compared with mechanical relays:

  • 1.Free from contact wear and thus highly reliable (long life)
  • 2.Small, lightweight and easy to mount
  • 3.High-speed switching (several times faster than mechanical relays)
  • 4.Low power consumption: several mW (compared to approx. 100 mW for mechanical relays)
  • 5.Less bounce noise owing to the absence of mechanical contacts

However, there are disadvantages:

  • a.In contact resistance (ON-state resistance), the difference between metal and semiconductor is too large.
  • b.In the contact open/close capacity (ON-state current), for photorelays, the range that can be covered by one device is very limited.
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