Sizing current limiting resistors for LEDs and other LED questions?

Discussion in 'General Electronics Chat' started by spinnaker, Dec 11, 2009.

  1. spinnaker

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    I understand how to calculate the value for a current limiting resistor for an LED. If I plug in the numbers for some typical LEDs I get somewhere between 100Ω and 150Ω for a current limiting resistor with a 5V supply. But looking at various projects, I have seen anywhere from 220Ω to high as 1KΩ for projects with a 5V supply.

    Why such a huge range of resistors? The projects I have seen simply mention LED so I would think it was typical LED. Is there a rule of thumb for padding the value of the resistor? I do know that you round up to the next highest common value. But any other rules of thumb?

    The LEDs I have selected all have a operating voltage of 2.5 volts and an operating current of 25ma. If I am doing my math correctly (cheating using a resistor calculator), I get 100Ω as my current limiting resistor.

    Did I do my math correctly? Do I need to pad anymore?

    What is the difference between the various types of LEDs, water clear, diffused, transparent etc ? Is there some place that shows this?
     
  2. t06afre

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    Yes your math is correct
     
  3. spinnaker

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    Thanks and I think I just answered one of my next questions. I was wondering why all the voltages are the same for the various colors I have selected.

    I just noticed that either Mouser is posting either the max values or they are just plain wrong.

    I guess you really need to look at the datasheet!
     
  4. spinnaker

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    And my question is still out there

    "What is the difference between the various types of LEDs, water clear, diffused, transparent etc ? Is there some place that shows this"?
     
  5. spinnaker

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    Wow this is really disturbing. Mouser shows this one as a Standard Green but when you look at the datasheet the manufacturer lists is as Super Bright. Mouser does have other Super Bright LEDs. If they are labeled correctly.
     
  6. Audioguru

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    The Kingbright green LED does not operate at 25mA. That is its absolute max current.
    It is not 2.5V. It is somewhere between a minimum of 2.2V to a max of 2.5V when its current is 20mA, not 25mA.

    If your LED is actually 2.2V then a 100 ohm resistor will feed it 28mA from 5V which is too much and it might burn out.

    Leds are dimmer when their current is less because the value of the current-limiting resistor is higher. Some applications need a dimmer LED and some need less current.

    Regardless of what Mouser says, every LED colour has a different voltage.
     
  7. SgtWookie

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    It is not uncommon to find typographical errors on ANY website.
    Vendors such as Mouser, Digikey, Newark, etc. stock hundreds of thousands - even millions of different parts, and it is a genuine chore to keep the specifications accurate. If you find only one error per 100 different parts, they are doing a very good job.

    As far as resistors used; it is perfectly OK to use a higher-value resistor than the specifications/calculations call for. The LED will not be as bright as it could be, but it will have a very long life. The problem comes in when the resistor is not large enough to sufficiently limit the current for the supplied voltage vs LED Vf; the LED will then have a short life.

    To avoid errors like this, you should always consult the datasheet first, and test samples of the LEDs at recommended current levels for their Vf. Vf at a given current flow for a batch of LEDs will be quite consistent for 75% to 85% of them, but the other 15% to 25% may vary significantly; up to 10%.

    "Water clear" simply means that the LED has no coloration when it is off; it looks like a bead of water or glass.
    Diffused means that the emitted light is scattered; rather like the difference between a frosted incandescent bulb and a clear bulb in a flashlight.
     
  8. spinnaker

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    Yep thanks. Figured that out. Look above.
     
  9. spinnaker

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    Thanks Sarge. And I am guessing that if my project has say 4-5 different colors of LEDs there really is no need to have 4-5 different resistors, just so I do not exceed specifications of my highest values.
     
  10. MikeML

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    Another input: If you are just looking for an "indicator" (not illumination), then there is nothing that says you have to operate any LED at near its MAXIMUM allowed current. I have run some red super-bright LEDs on as little as 500uA (battery powered project), and they are clearly visible even in bright sunlight (a requirement). This is where super-brights shine compared to garden-variety Leds (pun intended) :D
     
  11. spinnaker

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    Thanks Mike.

    And after thinking about it, I guess I should have posted above to not exceed the limits of each of your selected LEDs. You don't want to exceed your current limits but you also want to make sure you have enough forward voltage to light you LED too.
     
  12. Paulo540

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    Viewing angle is another spec to keep track of. The lower the angle, the more focused the beam. Since one of the major advantages of LEDs is the ability to concentrate light in one direction, you can get more direct illumination for less power with a narrower angle (like a 40 degree for example).

    However, if you are using an LED as an indicator, then a wider angle would of course be more suitable and intensity is not really a factor anyhow.

    Oh, and a high mcd per mA ratio is important if you're designing lighting arrays.


    One last thing, Don't forget that whites and blues are 3.3-3.6v whereas most everything else is around 2-2.4
     
    Last edited: Dec 11, 2009
  13. SgtWookie

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    You could use the lowest common denominator if you so choose.

    Be aware that the light intensities of your different colors may vary significantly if you do so.

    The human eye is most sensitive to the green color spectrum.

    Something very important to keep in mind is the focus, or viewing angle/area, that is required for the application vs what is specified in the datasheet.

    For instance, if it is intended as a panel indicator for an automotive application, you would likely want a diffused version. The viewing angle of a diffused LED is quite wide, and "softer" to look at than a tightly focused, narrow view angle LED.

    Some super-bright LEDs achieve very high mcd/cd/lumen ratings by focusing the beam in a very narrow pattern; perhaps 10° to 25°. That might be just fine if you wanted a very narrow-beam flashlight, but practically useless for something like an automotive brake light or turn signal; for the latter application you want a rather wide viewing angle.

    It's been mentioned many times here and elsewhere, but well worth repeating:
    Do not stare at super-bright LEDs, as you can quickly cause permanent damage to your vision.
     
  14. spinnaker

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    I'm using them as indicators for a bench power supply. What is a good viewing angle for something like that ? I think most of the ones I have selected are 34%.


    The LEDs will be for the various voltages selected by a button (3.3, 5, 9, 12 etc.) . I'll have an additional variable output.

    I plan to add an LCD display for voltages and current and have the whole thing controlled by a PIC. A bit overkill maybe but I think it will give me experience for my solar panel project.
     
  15. SgtWookie

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    I used diffused LEDs for my bench supply.

    I didn't want anything bright and glaring in my face; I just wanted a visual indication if it had power.
     
  16. spinnaker

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    Thanks.

    I thought the same. But what about the viewing angle. Or do diffused LEDs have a typical viewing angle?
     
  17. SgtWookie

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    Diffused LEDs have a very wide viewing angle. They're diffused. Basically, if you can see any part of the LED's lens, you'll get about the same brightness as if you were staring at it head-on.

    For my bench supply project, I just used some LEDs that were part of a Radio Shack LED assortment that I'd bought years before. Nothing fancy, nothing super-bright, no need for any fancy stuff - just an "I'm Alive" indication.
     
  18. Audioguru

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    The "super bright green" Kingbright LED that you have selected is a cheap old-fashioned (20 years old?) Chinese one. Modern bright green LEDs are made with the same chemicals as bright blue and bright white ones and are 10 times to 20 times brighter and have a higher forward voltage (3.5V).
     
  19. spinnaker

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    I will probably take that one off the list and just use diffused.

    But I picked it because it was cheap. :)
     
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