Voltage and current of a white LED

Discussion in 'General Electronics Chat' started by hp1729, Dec 13, 2015.

  1. hp1729

    Thread Starter Well-Known Member

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    The data sheet says 3.35 V at 30 mA. That does not mean it must only be used at that voltage and current. Attached is a chart of a white LED at various currents. Somewhere around reading 55 is the "rated" value. The LED lights at 1 mA, or less. At 5 mA it is useful. At 50 mA I suspect it would have a shorter life.
    My LED flashlight dumps the full 4.5 V across LEDs rated at 3.5 V with no current limiting resistor. About once a year I lose an LED. Yes, they would last longer with current limiting. But you bare not limited to what is on the data sheet.
     
    Last edited: Dec 14, 2015
  2. dl324

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    Where is the datasheet that you referred to? If you don't operate within the maximum parameters specified by the manufacturer, lifetime will be shorter; but not predictable.

    What batteries are used to "dump" 4.5V across the LEDs? If their series resistance is high enough, that will limit current in the LEDs. What is the voltage when operated at 4.5V with no current limiting? Is the current below the max DC current specified by the manufacturer?
     
  3. spinnaker

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    I do not think hp is asking a question. Hp is educating us on the use of the white LED. At least that is waht I am getting.
     
  4. Papabravo

    Expert

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    No. A datasheet will give you a typical value of forward voltage and current. All manufactured devices can be expected to cluster around such mean values. When the datasheet specifies a min or a max that is usually the plus or minus 3σ point on a normal distribution. That means that 99% of all their devices will operate at the mean value ± 3σ. What you do or don't do with your devices is of no particular concern to the global community.
     
  5. hp1729

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    Re: Data sheet
    Sorry, specific data sheet is lost among many data sheets. My intention was to mention that specific voltage across the LED varied a lot with current. The LED does not have to run at 30.0 mA, or what ever the data sheet says. Like any other diode you can tweak the current through it to get a desired voltage drop, within limits.

    "Predictable life"? I intended only to imply that a shorter life was predictable.

    Re: batteries
    Three AA's across four white LEDs in parallel. I didn't think specifics were important. I have ne easy way of measuring conditions inside the flashlight. I only suggested that not all designs are good designs. Perfection in design is not required.
     
  6. dl324

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    If you want to operate parts out of spec, that's your prerogative; but encouraging others to do it isn't being helpful. Many novices unintentionally operate devices out of spec with no immediate ill effects, but that doesn't mean there won't be any and it's not something that anyone needs to be told they can do.
     
  7. hp1729

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    Found the data sheet.
     
  8. mcgyvr

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    Hey..look at me.. I've drastically reduced the lifespan of an electronic component by using it in a poorly designed circuit due to my lack of fundamental knowledge. Hooray :p
    50,000 hours from an LED... pfft.. who wants that. :rolleyes:
     
  9. hp1729

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    More than that. If it specifies a wide range, like "3.2 to 4.1" Volts what good does it do to make a calculation down to the third decimal point? Parts don't come in that precision. It rates the LED at 30 mA. I usually set them up at about 5 mA. It lights down to 1 or 2 mA. Resistors come in 5% tolerance. Caps often 20%. Transistor gain ... "20 to 200" at a given collector current?
    Precision only exists in then theories in classrooms. My LED flashlight works fine with no resistors as long as I don't expect it to do more than it is designed for.
     
  10. hp1729

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    :)
    What is in your flashlight?
    IR LEDs in coin detector circuits in slot machines predictably die after about 30,000 hours (5 years) if driven at that suggested 30 mA. Yes they still light but the voltage across them drops and they dim down past a functional level.

    The "lack of fundamental knowledge" comes in when you design it for that 30 mA on the spec sheet. Do you not pay attention to how and why your circuits fail? Engineers who only work with that "theoretical and precise" knowledge keep us technicians employed repairing their mistakes.
     
    Last edited: Dec 14, 2015
  11. mcgyvr

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    The datasheet lists "Absolute Maximum" as 30mA... No "good" Engineer would drive it at that unless they had a darn good reason..
    Typically we would shoot for 20mA or less dependent on the ambient temperature,etc...
    I would never use an LED without current limiting either as then lifespan gets out of "predictable" all together..
    I won't even rely on a batteries internal resistance as that varies too..
     
  12. ronv

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    The problem with designing outside the spec is that the results are hard to predict in advance.
    You know you will have bad LEDs quite often because you have tested it for a while. Other people with better batteries or worse leds or poorer connections may have different results.
    So you may be happy, but if you worked for a company that guaranteed the life their flashlights for 5 years and designed it that way you might not be so happy.
     
  13. hp1729

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    The absolute value is usually up around 100 mA. 30 mA is what the listed ratings are stated for.
     
  14. mcgyvr

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    Didn't think we needed to get into duty cycles with this post..
     
  15. hp1729

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    Thank you. I see your point. Yes, maybe I grabbed the wrong data sheet or just recollected the data wrong. Yes, the 30 mA is Absolute maximum. The 100 mA is Pulsed. I don't know if I have a data sheet for the LED I had at hand. I think I got the part from Electronic Goldmine. They often just give rough specs and don't always supply a good part number or data sheet.
    Thanks for reading the sheet closer than I did before posting it.
     
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