Can an LED pass current but not light?

Discussion in 'General Electronics Chat' started by Treeman, Mar 15, 2015.

  1. Treeman

    Thread Starter Member

    May 22, 2014
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    15
    While experimenting with my timer project (currently in projects) I noticed that I could clock a second 4017 through some LED's but the LED's did not light while doing this. Is there an explanation or am I magically charmed as well as nonplussed?!
     
  2. MCU88

    Member

    Mar 12, 2015
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    The LED is an diode whereas current flows in the direction of the arrow on the schematic. Anode to cathode. Not enough current to light them but enough for the CD4017 to see it? I duuno man. Electronics can be enough to send you round the twist I will send you insane if you are post some pics or something for greater clarity.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,654
    632
    It sounds like you are passing the clock signal from the output of one '4017 to the clock input of the second '4017. This can be done without the LED lighting. At room temperature the input resistance of the clock input (for the National CD4017) is 100 nanoamps or less. The LED might be making light, but probably not enough to see.
     
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  4. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Thanks Dick. Its not an issue but as MCU88 says a real head scratcher! All part of the love hate relationship with electronics so far. The more you think you know the less you really know and all that.

    Shall I do my electronics in the fridge or oven from now on?
     
  5. MCU88

    Member

    Mar 12, 2015
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    Cars are more fun to work on. Drop the gearbox and do the clutch. Do the turbocharger. Slam down 6-beers and go for a cruise with the tunes fully cranked. Subwoofer shaking the whole car... BOOM BOOM

    Do your electronics in the oven from now on. Sleep in the fridge though.
     
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  6. Treeman

    Thread Starter Member

    May 22, 2014
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    15
    Sorry MC as an ex mechanic I have to disagree with you there on one point! Trees are more fun to work on - that is true.
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    If you look at the VI characteristic for an LED, or any diode, you will note that it can pass some current before the voltage across the LED reaches the threshold voltage. It is only after passing the threshold voltage that there is enough current to produce light.
     
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  8. Arx&Sparx

    New Member

    Feb 19, 2015
    11
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    From what i can see on the data sheet for the CD4017, most of it's input and output current is in the uA range, max reaching 900 uA or .9mA. Most LEDs operate at 20 mA max, some higher, some lower, but in uA range, you're probably not going to see the light, but the CD4017 is getting its supply.
     
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