Lighting a led after a PTC trip state

Discussion in 'The Projects Forum' started by bignick270, Aug 26, 2009.

  1. bignick270

    Thread Starter Active Member

    Mar 25, 2009
    36
    0
    What would be a way of turning on a LED if a PTC (resettable fuse) is in a trip state?

    DC voltages available to use are 3.3V and 5V.
     
  2. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    One idea is to note that, when the fuse is on, the voltage
    drop across it will be small but, that when it shuts off, the
    voltage drop becomes the supply voltage. Thus, one can
    arrange to have the light turn on when the voltage across
    the fuse goes high. The specifics of how this could be
    done will depend on what is going through your fuse. (Is
    is A.C. from the grid, some other D.C., or what?)
     
  3. bignick270

    Thread Starter Active Member

    Mar 25, 2009
    36
    0
    5V DC will be going through the fuse @ 500mA normally.
     
  4. Gustav180

    Member

    Aug 25, 2009
    17
    0
    I think you can connect a LED + 150 to 680 ohm i parallell with the PTC.
     
  5. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    > 5V DC will be going through the fuse @ 500mA normally

    That's not bad --- a simple LED-transistor combo should do the trick.

    [​IMG]

    I drew the circuit with the PTC on the + side; if it happens to be
    on the - side, use a PNP instead of and NPN and mount the LED
    the other way around. I calculated the resistance based on a
    1.6 V drop across the LED at 10 mA; you may need a different
    value depending on your LED.

    > I think you can connect a LED + 150 to 680 ohm i parallell
    > with the PTC.

    As long as having 10-20 mA flowing through your load
    when the fuse is shut off is not a problem, then do it
    that way and save yourself a transistor.
     
    Last edited: Aug 26, 2009
  6. bignick270

    Thread Starter Active Member

    Mar 25, 2009
    36
    0
    Thanks for the suggestions guys I will evaluate both of them.
     
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