Strange Neon Lamp?

Discussion in 'General Electronics Chat' started by Solardon, Oct 13, 2006.

  1. Solardon

    Thread Starter New Member

    Oct 5, 2006
    While taking apart an old electric blanket, I found a neon lamp as part of it's circuitry. It obviously was not being used for its light. It was inside a metal shield. All the other components in the circuit were resistors.

    This neon lamp has three electrodes. I was wondering if anyone else has seen this type of neon lamp before. Or, what are these type of neon lamps used for?

    I did some searching around online (including wikipedia) and have not found anything yet pertaining to a three electrode neon lamp. I know that ordinary neon lamps are negative resistance devices. This leads me to believe that this lamp is being used in some way as a protective device.

    Any ideas? :confused:
  2. Prodigal


    Oct 12, 2006
    Strange. Are you sure it's a neon lamp and not a rectifier/diode tube? This would seem to fit as you mention the only other components as being resistors. Would have to be a very old blanket, though :S.
  3. richbrune

    Senior Member

    Oct 28, 2005
    Yes, I think I'ts called a "Flashtube" the breakdown voltage of the gas in the lamp causes it to go "on" , and the lamp's total impedance goes very low as long as the voltage at the lamps terminals is above the rated voltage. This will then bypass some of the circuit for protection, similar to a metal oxide varistor.
  4. beenthere

    Retired Moderator

    Apr 20, 2004

    I don't know the "oficial" name for it, but it's the AC equivalent of the gas thyratron tube. As long as the middle electrode keeps the gas ionized, then current will go to the resistance heater strips in the blanket. Seems to me that it might date back to the 1950's. Not sure I'd trust the insulation.
  5. Solardon

    Thread Starter New Member

    Oct 5, 2006
    I have some info on this device. I hooked up the circuit board to the 120V house voltage. The heater cord got warm. I extracted all of this heater cord from the blanket - a daunting task. I might use it as resistance wire for something else.

    The two outer electrodes are line voltage. With exactly 1/2 line voltage from center electrode to each outer electrode. Very little current runs through any electrode during operation. I wish I could draw up a schematic and post it on this forum.

    I do not think it is a flashtube. It is not a zenon flashtube. This looks similar to a NE-2 neon lamp.

    Anyway, appreciate the feedback. This is an interesting one.

    Some links that may be interesting on this: - on memory switches. You'll have to tilt your head to read this one.

    I will research further on this myself. I have an interest in old electronics anyway. Kind of nostalgic.

    Thank you. :)

  6. EngineerJoe


    Nov 4, 2006
    The strange neon lamp device sounds like a control device that uses a bi-metal strip inside to flex with accumulated heat. Thermostats are made with bi-metal strips that bend with temperature. This happens because one metal will expand further and faster than the other, when heated. The different rate of expansion causes curvature in the strip which is often used as a conductor in a control circuit.

    Another application of a similar device was used to start fluorescent lamps on a magnetic ballast. These "starters" appear inside those small cylindrical cans that attach to old lamp fixtures if you can find them anymore.

    Your device is designed to regulate power to a heating element by switching it on and off at some prescribed rate, controlled by the current limiting resistors that allows the neon gas to achieve various temperatures. This in turn, controls the bi-metal strip bend rate. If you watch it operate, be prepared to watch for quite a while. It is designed to be slow.
  7. mrmeval

    AAC Fanatic!

    Jun 30, 2006