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Confusion about led's forward voltage

Discussion in 'LEDs and Optoelectronics' started by rahulb, Oct 14, 2019.

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  1. rahulb


    Mar 14, 2018

    I have confusion about the forward voltage of leds.

    Do the colored diffused dome type of leds have same forward voltage as the clear dome type?

    e.g. Blue colored diffused dome type led will have same voltage as the clear dome type?

    colored diffused dome type

    Clear dome type


  2. thedoc298


    Oct 4, 2019
    No each is different. Even same color from different manufacture will be different. Most dvm on dioad test will show the fv drop.
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    As @thedoc298 said, each LED is different. It used to be that you could predict the forward voltage by the color of light the LED emitted. This is certainly NOT true for "white light" LEDs, which use a wavelength-converting phosphor to emit light of a different color than the LED illuminating the phosphor. And it gets more complicated from there. So, your best bet is to get a variable current-limiting power supply, capable of at least five volts open-circuit output, and use it to forward bias your LEDs. If you have a lot of them, bin them according to forward voltage at a specific test current (20 mA is usually safe) and label each bin with the color the LED emits. Common "colors" are invisible infra-red; visible red, yellow, green and blue; invisible ultra-violet; and visible white. You cannot go by the color of the case to identify an LED! Measure the forward operating voltage.

    Back in the day, when LEDs were just becoming available for sale, I bought a selection in panel-mount packages with various colors of plastic lenses: red, yellow, green, and blue IIRC. All of the LEDs worked, but they all emitted red light! I don't think the vendor realized this was a limitation. They apparently just thought they could replace incandescent lamps with LEDs and keep the same package. I didn't know any better either. Back then, if you wanted colored panel lights, they had to be incandescent lamps with color filters, NOT red LEDs with color filters.

    To this day I am still partial to incandescent panel lamps, despite their limited longevity and sensitive to vibration. I think they just "look" better than LED lamps. But <sigh> paying customers disagree, so it's been a long time since any of my designs "featured" incandescent lamps. Oh, well, I could still use them for hobby applications if I could afford the light bulbs, which I generally cannot. LEDs with different color outputs are cheap and readily available. I am becoming quite attracted to the itty bitty surface-mount types that don't even look like LEDs until they "light up" with application of power. Way cool that look on a densely populated digital logic board. A real PITA to hand-solder them to a board.
  4. Audioguru


    Sep 24, 2016
    All white LEDs I have measured use a blue LED (2.8V to 3.5V) and a yellowish phosphor.
    hevans1944 likes this.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The plastic (colored or clear) doesn't influence the LED's forward voltage. In an ideal world, If the same LED chip is used, forward voltage is the same regardless of the plastic.
    However, we do not live in an ideal world. Your best reference is the datasheet of the LED used.
    Note that forward voltage can vary widely for the same type of diode depending mostly on temperature and curent, but also on age (yes, LEDs do change with age).
    davenn likes this.
  6. Bluejets


    Oct 5, 2014
    I think you will find the Op is after the following detail and yes, they are different voltages.
    Even if there are some which simply change the plastic colour, however this would be something from the last 10 years or so.

    Attached Files:

  7. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    These are typical forward voltages as @hevans1944 mentioned. They are good for a rough estimation.
    LED technology has advanced considerably over the years using improved materials and processes. Therefore the datasheet of the LED actually used is the best reference.
    hevans1944 likes this.
  8. bertus

    bertus Moderator

    Nov 8, 2019
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