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Momentary switch amp rating question

  1. Apr 6, 2014 #1
    Hi all.

    Hope you can help me out.

    I'm making a battery powered electronic cigarette and I need a momentary switch that is rated above 20amps at 8,4v. There tons of switches out there but the ones I like are rated at 3A/250VAC or 2A/ 36V DC etc, do these switches have a higher amp rating if the voltage is lower or is this the maximum amps no matter the voltage ??
    My next question would be if the switches can take more amps the lower the voltage how do I calculate the amp rating at certain voltage??

    Example switch:

  2. jcsd
  3. Apr 6, 2014 #2
    Switch ratings are not so simple to understand. The ratings are based on several things the manufacturer must take into account. The upper current rating must not A) spot weld the contacts B) cause a pitting or sparking residue oxidizing the contact surface that would prevent future proper operation. There are probably other things as well. So no, switch ratings are not a linear voltage, current relationships. Stick to the manufacturer specs.
  4. Apr 6, 2014 #3


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    Homework Helper

    The safe answer is "no". If there is any inductance in the circuit (and there is always some inductance in any real-world circuit!) the high voltage spike when the switch opens is caused by the current changing suddenly to zero, not by the supply voltage in the circuit. The voltage spike and the spark it generates is what causes damage to the switch.
  5. Apr 6, 2014 #4


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    Staff: Mentor

    Wow, why is the current so high?

    And what kind of safety considerations are there in this circuit/device? Do they need FDA approval to be on the open market? Is there an issue with completely vaporizing the liquid versus inhaling not-totally-vaporized material?
  6. Apr 6, 2014 #5
    Thanks for all the info guys/girls.


    The more wattage the more vapor.

    The one I'm building has no safety at all, 2x 2665 batts rated at 50amps with a 0.42ohm coil = 168 watts, the wattage will drop down to 110watts when the batts start to drain.

    Don't think they need FDA approval there are tons of this stuff on the market. Google mechanical mods they are the ecigs with no protection, no circuity. Basically a flashlight.
  7. Apr 6, 2014 #6
    I don't know anything about what you are doing. But I would issue a word of warning about exposing your throat/lungs to high temp vapors. Just sayin!
  8. Apr 6, 2014 #7


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    The system is probably self-limiting. As soon as the liquid starts to boil, the gas bubbles will reduce the heat transfer rate, so the resistance element will probably over-heat and burn out :smile:

    Seriously though, the OP doesn't seem to have any idea how hot a cigarette-sized object will get, with 150 watts of heat input. I suggest an experiment with a soldering iron. Switch on a 15 watt iron for 50 seconds, then pick up the hot end in your hand. That would be a reasonable simulation of heating something with 150 watts for 5 seconds. But don't sue me if you burn your fingers doing the experiment.
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