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4371 Views 3 Replies Latest reply: Mar 3, 2010 9:54 PM by Marquis RSS
vh2 Level 1
vh2
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Dec 14, 2009 12:35 PM

Why use relays over semiconductor switches?

Can anybody help on this? For medium voltage applications, say 100V~ 600V, what is the benefit of using an electro-mechanical relay as opposed to electronic switches, either a thyristor/IGBT/MOSFET? Reliability, voltage isolation, safety issues? There are many advantages to using an electronic switch in terms of speed, avoiding spikes, power consumption, propensity to failure, cost, etc..? So would there be any reason to use an electro-mechanical relay over semiconductor based switches? Thanks so much..  BTW, the application does not involve switched mode power supply or anything that needs any speed in switching. I would really appreciate if anyone can help.

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Tags: semiconductor, thyristor, switches, relay, igbt, mosfet
  • Paul Level 2
    Paul
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    1. Dec 14, 2009 7:15 PM (in response to vh2)
    Re: Why use relays over semiconductor switches?

    One reason you might want to use a relay is if the circuit being controlled needs to be electrically isolated rather than just being "off".

    Electronic switches do not completely isolate and will allow a small leakage current to flow when they are "off".

  • M0DCD Level 4
    M0DCD
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    2. Dec 17, 2009 3:05 AM (in response to Paul)
    Re: Why use relays over semiconductor switches?
    Some other reasons to use electromechanical devices are first that the control/drive circuit is physically and electrically isolated from the load. Secondly, and possibly the most common is that these devices invariably fail open. Yes you can get contact welding, but in interlocking applications such as railway signalling this is the mode that is wanted. SSI (Solid State Interlocking) ends up be a mess of duplicated systems and voting control. Relays usually fail because of the contacts wearing out, but semiconductors often fail short circuit. UK regulation seems to shy away from innovation because of these issues - such as it took them 20+ years to accept LEDs as light sources for signalling.
  • Marquis Level 1
    Marquis
    Currently Being Moderated
    3. Mar 3, 2010 9:54 PM (in response to vh2)
    Re: Why use relays over semiconductor switches?

    Consider both relay/contactor and solid state in some designs - you have the best of both worlds.

     

    Switch the relay with no/low current.

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