How to unlatch an SCR

Discussion in 'The Projects Forum' started by NSmerker, Apr 8, 2010.

  1. NSmerker

    Thread Starter New Member

    Apr 8, 2010
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    I am working on a project that uses an SCR to connect the hot (170Vp 60Hz) leg of an outlet to the neutral and trip the connected circuit breaker. I also have a transmitter circuit connected to the same two prong plug as the SCR. I am using a normally open switch and 9V battery to trigger the gate of the SCR and latch it. When the SCR is latched by closing the switch, and the hot and neutral are shorted, the breaker trips as it should. After this the SCR remains latched, keeping the hot and neutral connected. Is there any suggestions on how to unlatch the SCR? There should be no current through the terminals once the breaker trips.
     
  2. Bychon

    Member

    Mar 12, 2010
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    41
    The definition of SCR says it can't stay on after the breaker trips (there is no current flowing). Either you melted the SCR or you should turn off the switch to the 9 volt battery. ps, I wouldn't be surprised if you melted the SCR.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    SCRs stay conducting until the current stops, even for a millisecond. I agree with Bychon, your SCR is likely toast.

    They come in several flavors too, normal SCRs have their sensitivity reduced by a built in resistor on their gate to anode.

    Have you read the AAC chapter on them?

    http://www.allaboutcircuits.com/vol_3/chpt_7/5.html
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    You're absolutely right. After the breaker has tripped, there will be no current.

    Your SCR is not latched, its toasted or fried or fused. In simple term, it had failed to short-circuited. Placing a direct shut across a mains put tremendous stress on the SCR and it fails.

    If tripping the breaker is what you have wanted to safe guard the transmitter, there are plenty of breakers which incorporate a shunt trip feature where the breaker can be tripped by external signal via providing current to a trip coil mounted inside the breaker.
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The typical 'breaking capacity' rating for a fuse designed to handle a fault on a domestic circuit is 10,000 Amps, even if the fuse is only rated at 5A.

    Remember, until something 'gives' (which takes time), you are shorting out the entire mains supply system from the local substation, including shorting the capacitance formed by the wiring.

    er.... DON'T short the mains...

    As well as being plain dangerous, there is a good chance you will damage other appliances or electronics connected around the house, due to the voltage spikes and current surges caused by a short.

    'Crowbar' switches to short a supply as a last-resort safety feature are not uncommon, but they are used on the low-voltage side of a power supply, not the AC supply side.
     
  6. NSmerker

    Thread Starter New Member

    Apr 8, 2010
    3
    0
    Thanks for the help
     
  7. NSmerker

    Thread Starter New Member

    Apr 8, 2010
    3
    0
    We are currently using a littelfuse s4025l which is rated for 25A rms continuous. The average rating is 16A. Since we are using it on 20A breakers, should we be using an SCR rated for a higher current? Maybe 50A?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    Just to repeat good advice from post #5 -
    Keep in mind that the interruption device only opens after a certain current has been exceeded. A circuit breaker is electromechanical, and operates fairly slowly in electronic terms - in some tens of milliseconds at best. While the overcurrent condition exists, the breaker conducts the full current.

    That is enough time for the crowbar SCR to experience more than enough current to blow it up. You might find that peak current is in multiple hundreds of amps. There is no way to predict how surges like that may affect the several wiring connections that current passes through.

    One suggestion is to go the other way. Use an electronic device called a solid state relay to pass the current. the signal that operates it can turn it off faster than the breaker can trip. The circuit opens up with no surge.
     
  9. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
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    This is about the minimum you could get away with, without it blowing up:
    http://uk.farnell.com/vishay-formerly-international-rectifier/st1230c12k0/scr-thyristor/dp/4590983

    However there are far cheaper, better and safer methods of shutting off a power circuit which would not get you sued as you existing method likely would.

    - You are likely to damage you own equipment, or that of any other property on the same substation, or trip the substation and shut down the whole area.

    Again, DONT!



    Have a look at the AS series part way down this page.
    http://www.schurterinc.com/wwwsc/con_pg17_20.asp

    Note the 'Remote Trip' comment on one column; this means the breaker can be switched off by operating a small pilot coil, without doing anything that could cause damage.
    It's a common requirement, which is why things like this are made - google 'remote trip circuit breaker'.
     
    Last edited: Apr 10, 2010
  10. DickCappels

    Moderator

    Aug 21, 2008
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    Or use a Gate-Turn-Off SCR (GTO SCR). You can do this with most SCRs, but a GTO SCR is designed to turn off with the conduciing area being pinched off nearly uniformly, thereby reducing hot spots that can appear (and destroy the die) when using SCRs that were desiged for other uses.

    A GTO SCR turns on the same as any other, but you can turn it off with a negative pulse to the gate electrode.

    Have a look if you care to:
    http://www.kilowattclassroom.com/Archive/SCRArticle.pdf
    http://www.google.com/patents/about?id=aEQ7AAAAEBAJ&dq=4675798
     
  11. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Gate turn off won't help un-melt his devices..
     
  12. DickCappels

    Moderator

    Aug 21, 2008
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    Yeah, once its melted, its pretty hard to turn off :)
     
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