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Indicator light Circuit Breakers

Discussion in 'Question of the Day' started by ship, Feb 23, 2007.

  1. ship

    ship Senior Team Emeritus Premium Member

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    So I blew up a neon lamp indicator light as part of a circuit breaker last night. Spectacular explosion and spark. I remember there was some detail about this bin full of of these breakers... just couldn't remember what it was.

    Turns out, this after finding a sub-note following a lot of web-surfing with the breaker manufacturer that there needed to be a resistor added to the indicator light. Yea, I saw that note on resistors, but thought such a thing was already in there... Because it was a 250v circuit breaker, the end user was to add a properly sized one so that dependant upon the actual voltage the breaker was seeing, that resistor could be changed dependant upon voltage. Oops, I now remember a detail about these breakers I forgot long ago.

    Simple enough, yet how could a resistor really play a factor in say a 120v line voltage system?

    Neon indicator lamp saw 120v, and instantly exploded. Added the proper resistor and it works just fine (once I replaced the breaker). How is such a thing working in concept?
    Last edited: Feb 23, 2007
  2. n1ist

    n1ist Well-Known Member

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    Malden MA
    The resistor is there to limit the current. Neons exhibit negative resistance; without a series resistor, the current will increase to the obvious point of failure.
    Some neon light assemblies have the resistor built in but others, like your breaker, do not so one can select the appropriate resistor for the applied voltage. Usually they are in the 47K to 100K range for 110v.
  3. fosstech

    fosstech Active Member

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    Tacoma, WA USA
    Kinda like LED's then, a current limiting resistor is required to keep the LED from burning itself up. As the temperature rises, the resistance drops, which increases the temperature more, which drops the resistance more, etc....until it draws so much current it burns up.

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