r/c led switch voltage problems

Discussion in 'The Projects Forum' started by markosillypig, Mar 17, 2009.

  1. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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    hi all
    having a little problem with my switch i have made
    the problem is when i connect the led to the d1 the light will not work remove d1 and hey presto it works ok
    is this because i'm adding another diode to the diode on the circuit ??


    and another little problem i have is the output to the led is is 4.9volts this is ok
    but when i connect the solder the led's on the voltage drops to 2.9 and i have no idea why ??? i would idealy like the output voltage to the led's to be about 3.2 but i can use a current limiting resistor if the voltage is a little higher

    i have tried using a 2n7000 to the circuit and the same thing happens
    so i'm lost at the moment
     
    Last edited: Mar 17, 2009
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    718
    When you say "Remove D1", you mean "Short D1" and it works OK?

    From your schematic, you have the anode of D1 facing the anode of the LED, i.e. D1 is backwards.

    What voltage is it running at? You should have a 120Ohm (roughly) current limiting resistor where D1 is, unless there is something in addition to the LED that you need to block voltage into the IC from.

    The voltage across the LED will drop to the forward voltage of the LED, and draw max current, like a short circuit. That current going through the internal resistance of the IC will lower the voltage to meet the LED, sometimes the IC and/or LED will burn out from overload in the attempt to match current draw for short circuit.
     
  3. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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    hi the sorce voltage is 4.9v
    the led is 3.2 vf
    i removed the diode from the board and just soldered the led to the point where the diode was
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The output limit of the 4013 is 2ma, so for full brightness, you will need to use a driver transistor. You need to limit the the current, typically 20mA for a "Super Bright"/clear lens LED, 4.9-3.2=1.7v 1.7v/0.02= 85 Ohms or the next standard value of 100 Ohms.

    Otherwise, either the LED or IC will fail after a period of time.
     
  5. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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    will a 2n7000 do the job ??
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    A 2N7000 MOSFET needs 10V between gate and source to be fully on.

    When turned on at 5V, it will drop 0.4V. Total dissipation for the TO-92 package is 400mW, which is adequate for 20mA. The current limiting resistor should be 82 Ohms, as the RDSon of the MOSFET will also be limiting current.

    Yes, it will work, but you should still use the current limiting resistor.
     
  7. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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    is there an alternative to what i asked about the 1n7000 that will make the cercuit work better ??
    and do you have any better cuircit's that would be a better one
    than the one i have
     
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