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cigarette lighter

Discussion in 'General Electronics Discussion' started by donkey, Jan 6, 2012.

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  1. donkey

    donkey

    1,298
    56
    Feb 26, 2011
    I have an idea for an electric portable ciagarette lighter.
    the idea is using an heating element connected to a switch and battery.. I have struck an issue however and its to do with temperature.
    I was trying to make it safe and then realised this could get quite hot so I was wondering if there was something like on a fuel bowser pump (when it gets full the handles clicks and it doesnt over flow) but for heat?
    or is there a very small circuit that will totally disable the power until you release then reapply pressure to the switch. I am trying to make it small
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    PTC or resettable fuse

    Could a PTC or a resettable fuse (http://en.wikipedia.org/wiki/Resettable_fuse) be a solution?
    You'd have to place it in a distance to the heating element such that the element is allowed to heat up to a certain maximum. If the maximum is reached, the temperature at the location of the PTC or polyfuse should be such that the resistance increases from that point on, reducing the current to the heating element. The required distance will depend on your construction and thermal parameters and will thus need some experimentation.

    A polyfuse can be used because it is basically a PTC. In normal use, the heat for turning of the polyfuse comes from the current through the fuse, baut as long as the operating current of your circuit is lower than the limit current of the fuse, it will not trip. But if the heat coming from your heating element raises the temperature above the critical point, the fuse will trip and shut of the heater.

    Harald
     
    Last edited: Jan 6, 2012
  3. donkey

    donkey

    1,298
    56
    Feb 26, 2011
    would that cut out power until button is released and then alloow it to work again later? I want it so if the switch breaks or something like that i won't burn anything
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    In a way yes and then again no.
    It will allow reactivation some time after the button has been released or automatically if you keep the button pressed. Since the proposed components are thermally activated:

    1) they will automatically deactivate once the heating element has been switched of
    2) will take some time for coolinmg after the heating element has been switched off.

    By the way: if you have an old electric iron lying around, you might consider scavenging the bi-metal switch from that iron. It works on exactly the same principle, though this may conflict with your desire for a rather small component.

    None of these components will be a fail safe for a broken button: if the button is short circuited, the heating element will be alternately turned on and off (compare with the electric iron) until the battery is empty. If you need a fail safe, you'd have to add something like a monoflop so each push of the button activates the heater just once for a defined time. This might, by the way, even be the smallest and safest solution. I guess the requirements on timing accuracy are not very high, so you could use a very simple circuit like this:
    Lighter.gif


    Normally, the MOSFET (enhancement type) would be off - safe mode.
    Pushing the button will charge the capacitor and the MOSFET will conduct.
    After releasing the button, the capacitor will be discharged by the resistor thus turning the MOSFET off.
    The drawback is that the heater will be on as long as you push the button. But then again, you could add the PTC to handle this case.
    Or you could use something similar to this:
    Lighter2.gif
    Here the pushbutton needs to be of the c.o. type. If niot in use, the left capacitor is charged. When the button is pressed, the charge is transferred to the right capacitor, thus turning on the MOSFET. Regardless of the state of the button, the capcitor will slowly discharge and turn off the MOSFET after some time. The heater can be reactivated anytime by pressing the pushbutton again.

    I think, from now on it is becoming more and more complicated, probably not the simple solution you've been looking for.

    Harald
     
    Last edited: Jan 6, 2012
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