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LED min and max Current and Voltage

Discussion in 'LEDs and Optoelectronics' started by willwatts, Nov 19, 2014.

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  1. willwatts

    willwatts

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    Nov 15, 2014
    What is the min and max current and voltage an LED can operate?
    The current limiting resistor adjust the "brightness" an LED?
    What are some common current limiting resistor values for an LED?
    What are some common forward voltage bias for an LED?

    Each color of an LED has a different forward voltage and forward current ratings
     
  2. willwatts

    willwatts

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    Nov 15, 2014
    how much current will damage an LED?
    how much voltage will damage an LED?
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Please read this.

    Then, if you have any questions google them first.

    If google doesn't answer them, point us at the best explanation you've found and tell us what parts of it you don't understand.
     
  4. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Vmin ~0.75V Vmax > 12V
    Imin = 0A Imax >> 10A

    NO! The current limiting resistor limits the current!

    0 - 500k ohms

    Do you mean Vf (Forward Voltage) of Vd (Voltage Drop) ? .... See #1

    LOL!

    You need to do some reading....LEDs are current driven semiconductors, the Forward Voltage is a function of temperature and current such that even a small ripple in the voltage can cause a large change in current....typical LEDs achieve >50% of their potential "brightness" @ less than 50% of their rated power.....operating an LED @ or near its rated power will dramatically increase the heat output and if not properly cooled will lead to premature failure....As a general rule of thumb you should not operate LEDs in "continuous mode" above ~75% of their rating w/o sufficient cooling.....

    43 posts since Friday? WOW.

    Fish
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    The forward voltage depends on the LED physics and, indirectly, is a function of color. Shorter wavelength LEDs require a larger bandgap, i.e., more energy to force electron-hole pairs to radiatively combine in a forward biased PN junction to emit photons with energy (inversely proportional to wavelength) near the bandgap energy.

    The forward current depends on LED physical construction, mainly related to its ability to dissipate heat. Early LEDs would operate at a few milliamperes, but the latest versions operate at one ampere or more. The higher current LEDs require proper heat sinks of course.

    You should read this article on Wikipedia for background information before asking more questions. One question at a time, please!
     
    Last edited: Nov 19, 2014
  6. davenn

    davenn Moderator

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    OK guys ...
    let him read the Driving LED's tutorial etc as Steve suggested and wait for him
    to come back with specific queries about bits he didn't understand

    Us mods don't want his threads to become dozens of posts of pointless rambling

    regards
    Dave
     
  7. willwatts

    willwatts

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    Nov 15, 2014
    Both, what is the Min and Max forward voltage of an LED?
    and min max voltage drop of an LED?

    What is the min current i can use to turn on the LED? and what is the max current I can supply to an LED?
     
  8. davenn

    davenn Moderator

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    depends on the LED
    have you read the tutorial yet ??
     
  9. willwatts

    willwatts

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    Nov 15, 2014
    yes i'm reading it, but it doesn't tell me clearly what the min and max current or voltage is? or what sets the brightness of an LED

    How do you know how much voltage and current to supply an LED without having the datasheets?
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You first need to determine why that question is half meaningless. (Hint: LED's are current driven).

    As for the second part of that question -- you guess.
     
  11. willwatts

    willwatts

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    Nov 15, 2014
    yes i know that LEDs are current driven, that's why i asked what is the min amount of current for turn on an LED and what is the MAX current before it gets damaged? how much current damages an LED?
     
  12. Gryd3

    Gryd3

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    Jun 25, 2014
    Spec sheet will cover that.
    Something it does not say though is 'Damage' is not immediate. You could be running an LED fine for a couple hours higher than the max spec current. This will result in a decrease in the life expectancy of the LED and there is no magic number/equation (at least available to the public) that will tell you what the expected life will be if you run it higher than spec, or to give you the magic number of the threshold at which it will suddenly die.

    I used to watch green LEDs change color as a kid while I intentionally fried them... they would fade into a yellow then a red color, and surprisingly they would still light up as long as I didn't continue 'till they released the magic smoke.
     
  13. willwatts

    willwatts

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    Nov 15, 2014
    Is it good to use a zener diode to "clamp" the voltage at the forward voltage or forward drop voltage to protect the LED from getting damaged?

    If a Microcontroller is outputting 3.3 volts and the LED's current is 2.2milliamps

    How do I find the current limiting resistor if i don't know the microcontrollers supply voltage

    The LED current limiting resistor is not just ohms law you have to subtract the Supply voltage of the microcontroller minus the forward voltage minus the forward drop across the LED to get the current limiting resistor
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Any amount greater than zero. The rest is answered by Gryd3.
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No

    Not a question

    You measure the supply voltage

    read this again.
     
  16. willwatts

    willwatts

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    Nov 15, 2014
    so pico current, nano currents, micro current can turn on an LED?
     
  17. Gryd3

    Gryd3

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    According to the information you read, you should already know that the LED is not driven by voltage. It is driven by current. A Zener will not limit the current though an LED, but may protect it if it is used in parallel to an existing LED circuit that has a limiting resistor. (Like a surge protector)

    You don't need to know the microcontroller supply voltage. You already know that it outputs 3.3V Is this high enough to power the LED?
    Use the V = I * R formula to determine the proper resistor, and if you are using an unknown power supply, measure it first.

    You took off the forward voltage twice. Ignore the 'supply voltage' from the microcontroller and pretend it's simply a switch that completes a 5V circuit. You can determine your values this way.
     
  18. willwatts

    willwatts

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    Nov 15, 2014
    These are the formulas for an LED

    R = (V - Vf) / I
    I = (V - Vf) / R

    I would have to get Vf from the datasheets and V is the supply voltage VCC of the microcontroller or is V the microcontrollers output voltage?
     
  19. Gryd3

    Gryd3

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    If the microcontroller is being supplied 100V, but is only sending 3.3V to the LED, how much voltage is going to the LED?
    You can answer this yourself I'm sure. (This is not a REAL example)
     
  20. willwatts

    willwatts

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    Nov 15, 2014
    Yes i know this, the zener is to CLAMP the voltage because i don't know what the LED's maxium voltage is that will damage it

    I'm guessing more LEDs can handle up to +25 volts without getting damaged?
     
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