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Again! Current-limiting resistor for LEDs

Discussion in 'Electronic Basics' started by Mr. Man-wai Chang, Jul 22, 2013.

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  1. The supplied voltage is 3V.

    The forward voltage drop over LED is 3V.

    What is the resistance of current-limiting resistor? :)
     
  2. Daniel Pitts

    Daniel Pitts Guest

    You'll likely end up with a fairly dim (or completely dark) LED, if the
    voltage source is exactly the LED voltage drop. That's because you won't
    have any current at all.

    Try the experiments I suggested earlier. Use a variable voltage source
    and a variable resistance, so that you can play around with both values.
     
  3. Tom Biasi

    Tom Biasi Guest

    Please Ian, he would need sharp tools!
     
  4. Daniel Pitts

    Daniel Pitts Guest

    Perhaps, but I think his questions have helped me to devise explanations
    and experiments to validate the assertions we've been making.
     
  5. Then do I need a current-limiting resistor when connecting two 3V LEDs
    in series to a 6V supply voltage? :)

    Yes, sort of going back to my question I posted days ago! :)
    Later... A LED may be dirt cheap, but blowing one up is still money. :)

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  6. Should I worry about the LED shorting itself in this scenario? :)

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  7. It could be fun to ask EE students to write a short paper about this
    simple LED thingy... :)

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  8. whit3rd

    whit3rd Guest

    It's not enough information.

    Your '6V' nominal supply has a tolerance, and so does the '3V' forward
    voltage on the LED. If the LED were a perfect diode, it'd double its forward
    current every 15 mV or so, and 6.15V would, while being
    nominally still '6V', force about a thousand times the intended current
    through the LED.

    Your LED choice, and battery choice, includes internal resistances
    and temperature coefficients, and measured-value tolerances. LED
    direct to battery CAN work, but you have to examine all the vertices
    (i.e. all the extreme values of temperature, voltage, etc.) and not
    just one point which is the 'nominal' set of values.
     
  9. Jasen Betts

    Jasen Betts Guest

    It depends on the 6V and on the LEDs.
    it's a cheap education.
     
  10. I remember: "essay", not "short paper"! ;)

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  11. nice one.

    what's the resistance of your 3 volt power source, and forward current of
    the LED?
     
  12. Sounds interesting... :)

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  13. So if the input voltage is guaranteed to be lower than the forward
    voltage drop of a LED, no current-limiting resistor would be needed?

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  14. How do you prevent a LED from shorting itself?

    Is the value of input voltage related?

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  15. What's wrong about that?

    All those motherboards out there uses fixed voltage to drive indicator
    LEDs, I supposed.
    I think you are asking for too much from simple LED applications.

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  16. I am talking about those simple 3mm/5mm LEDs here, not those hi-power
    LED lamps....

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  17. P E Schoen

    P E Schoen Guest

    "Ian Field" wrote in message
    Maybe you could add negative resistance?

    When you apply less than the forward voltage drop, the device becomes a
    Darkness Emitting Diode, or DED, and it really sucks (light).

    Paul
     
  18. Shaun

    Shaun Guest

    "Mr. Man-wai Chang" wrote in message
    How do you prevent a LED from shorting itself?

    Is the value of input voltage related?


    Why don't you look up basic electronics on the internet (a course) or go to
    a bookstore a buy a book on basic electronics instead of asking all these
    dumb questions.

    Shaun


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  19. I expected short answers here. Don't worry, I always google if I know
    the right set of keywords.

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