protection circuitry for electronics on a transmission line

Discussion in 'The Projects Forum' started by iceman11`, Apr 24, 2008.

  1. iceman11`

    Thread Starter Active Member

    May 4, 2007
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    0
    hey all,

    i am currently design power electronics protection circuitry for a robot that is to be attached to a transmission line (a power line inspection robot).

    i have come up with a crowbar circuit consisting of a triac, zener TVS diodes, and snubber network which provide the protection. this is then followed by a bridge rectifier some filtering and finally a linear regulator. (see circuit attached)

    the question i have, is, once the crowbar circuit activates, (assuming it is design properly) does the triac switch on and stay on?? that is to say, once the overcurrent/voltage transient is over does the circuit i have designed go back into normal operation or does the triac continue to short out the rest of the circuit???

    also, i wud prefer the fuse to only blow of the additional inductance (L2`) cant handle to absorb the transients...this will be better since the technicians wont have to keep on bringing the robot down from the line to replace the fuse.

    please, some advice would be highly appreciated.

    Thank You
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You have a fuse in the line which will blow if the crowbar triac triggers. I wouldn't worry about the snubber - there's nothing left to snub after the fuse melts.

    Your crowbar trigger won't work. You need to look at the voltage across the lines and arrange it to trigger the triac when the potential difference is too high.
     
  3. iceman11`

    Thread Starter Active Member

    May 4, 2007
    39
    0
    this is wat i have in fact done.....by choosing the correct breakdown voltage rating of the zener TVS diodes, the triac will activate.......

    see the attached link for the circuit that this design is based on : http://www.onsemi.com/pub_link/Collateral/AND8012-D.PDF

    the only problem is that i dont want the fuse to blow if the additional inductance (L2`) can handle absorbing a sufficient portion of the transient(for reasons i presented in my initial argument). also, once the triac is on, does it stay on? or does it switch off (and no longer short the rest of the circuit out) when the next half of the AC input comes along??
     
  4. iceman11`

    Thread Starter Active Member

    May 4, 2007
    39
    0
    since it i sptless if the triac remains on while the transient has past..

    if it does stay on...can i add some sort of "detection" circuitry which will then switch of the triac once the overvoltage/overcurrent has passed??
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The triac can't stay on. Look at the material in out Ebook - http://www.allaboutcircuits.com/vol_3/chpt_7/6.html. I'm a lot more comfortable if designers have some understanding of the devices they are working with. I imagine painful accidents made in ignorance.

    By the way - the purpose of a crowbar circuit is to open the supply line in case of a fault. It should blow the fuse.

    If you are seeking to diminish or eliminate the effects of voltage spikes on the line, you might want to do some research into TVS's - transient voltage suppressors.
     
  6. Digi Dave

    Member

    Apr 25, 2008
    26
    0
    Yes the triac will remain conducting until the voltage between mt1 and mt2 is about 1.2v. the only way to switch it off is either short it out or remove the supply.
     
  7. iceman11`

    Thread Starter Active Member

    May 4, 2007
    39
    0
    im asuming that you say 1.2 V since (0.6 V drop across each thyristor)?

    isnt another way to switch the triac off by using the holding point? so have a resitor scheme that detects if the transient has passed, and if it has, then the resistance scheme chosen will drop the current below the holding current, switching the triac off??

    thanks for your help....
     
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