need help with a 95 amp solid state relay 800 volts

Discussion in 'The Projects Forum' started by natem600, Jan 29, 2013.

  1. natem600

    Thread Starter New Member

    Jan 29, 2013
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    I am looking for a 90 amp 800 volt solid state relay or circuit with 3 to 20 V Ac/DC control. For safety it must fail open. Any help would be greatly appreciated.
     
  2. strantor

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    Oct 3, 2010
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    I don't know of any device that is guaranteed to only fail open. Except a fuse.
     
  3. thatoneguy

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    Feb 19, 2009
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    I'm not aware of any SSRs build for that amount of power.
    Contactors are typically used in that application.

    You'll need two "steps", an SSR to switch 24V@250mA to activate the contactor coil, then the high power contactor itself.
     
  4. natem600

    Thread Starter New Member

    Jan 29, 2013
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    your right a fuse will fail open but it can not be switched on and off. A contactor or breaker can fuse / weld closed *it has happend too many times allready* making it less than ideal.
     
  5. gerty

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    Google "hockey puck triacs" they use them for switching the primaries on large spot welders.

    The failing open part..you're on your own..
     
  6. thatoneguy

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    Fail Safe is difficult.

    Fail resistant could be putting 3 contactors in a row. Assuming the power through them never exceeds the rated value, the chances of all 3 failing at the same moment are minimal. In the event one fails, replace all 3, as the other two have switched the same power the same number of times. Some form of indicator should be used to alert that one has failed.
     
  7. crutschow

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    I would think two in series would be sufficient for reasonable reliability.
     
  8. thatoneguy

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    Depends on how "Fail Safe" the OP desires. Mechanical contacts can't know when they are stuck together, and what to do if they did.

    With two, there's a very slight chance of them failing at the same time, I picked three due to assuming the system may not be monitored or checked often, one check may miss two that are stuck as long as "It's still working" maintenance. If checked often and with indicators to show open state (neon bulb or similar), then two would suffice.
     
  9. strantor

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    you know what I might have to eat my words. I forgot about mercury relays. I googled it and came up with this:
     
  10. strantor

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    and here's where you can get some. They aren't rated for 800V, but I bet if you give them a call, they can tell you what to do.
     
  11. crutschow

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    OK. I was assuming that, if it was that important, then there would be a real-time monitor circuit that would indicate if one of the contacts had stuck and this would give an immediate warning to the operator (or shut down the system).
     
  12. gerty

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  13. gerty

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  14. thatoneguy

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    Even with 3, I'd guess that somewhere, some time, people would keep using the system until it fried because "It was made to be fail resistant, and it still works". :(

    Never underestimate the laziness/ignorance of end users and techs in a critical system. For example, how many people do you know that continually drive around with the Check Engine light on?
     
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