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L.E.D. question..again

Discussion in 'Electronic Basics' started by Palmer, Dec 16, 2004.

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  1. Palmer

    Palmer Guest

    I'm going to connect 4 L.E.D.'s to a 12 volt source. Each LED is 2.1 volts
    at 20mA. I know that I need to connect them in series right? So I would have
    the positive wire to the first LED and then the negative side of the LED
    number 1 to the positive side of LED number 2 and so on and then the
    negative source wire to the negative side of LED number four. Oh yea ... a
    180 Ohm resistor between the positive wire and the first LED. Is this
    correct? And if my source is a car...which puts out more than 12 volts....is
    a 200 to 220 Ohm resistor ok to use?
    Thanks for the help...
    My be simple for you but I'm trying to learn
    Screamn1
     
  2. The typical drop of 4 LEDs in series at something near rated current
    is 4*2.1=8.4 volts. A car produces something like 13.8 volts when the
    engine is running, so that means you have 13.8-8.4=5.4 volts to burn
    in the current setting resistor.
    You want to keep the current below 20 ma for long life, so the minimum
    resistance would be 5.4/.02=270 ohms. Twice this resistance will
    produce a just noticeable drop in brightness.
     
  3. With the 180 ohms proposed by Palmer, the current at 13.8 volts can be
    30 mA (more likely a little less since the LED voltage drop will be
    slightly higher than at the usual 20 mA).
    Most LEDs have maximum current rating of 30 mA even if their
    characteristics are specified at 20 mA.
    But at 30 mA, do not expect 100,000 hour life expectancy unless thermal
    conditions are especially favorable - both leads of the LED should have a
    temperature not exceeding about 70 degrees C near the case even if you
    stay within the ratings. Having 100,000 hours of life with light output
    staying above 1/2 the "brand-new" amount often requires keeping the
    junction at a temperature not exceeding 85 degrees C.
    White LEDs have it much worse due to phosphor wear - skimp on current as
    much as you can if you need the kind of life expectancy many expect from
    LEDs.

    - Don Klipstein ()
     
  4. Best to use a 220 ohm resistor, since the car battery voltage can go up
    to more than 14 volts when charging. But doing the calculations with a
    battery voltage of 14.8V, it should be more like 330 ohms.
     
  5. Wow, that's a new term for me, Don. "Phosphor wear".

    I'm still running the four white LEDs in series, 24/7, since july 11,
    which is over 5 months. There are two Nichia NSPW510 white LEDs, which
    have been on since even earlier, and two Hong Kong LEDs, all in series
    and running at 20 mA. So far, the Nichias haven't dimmed much, but the
    HK LEDs are down to less than 1/4 brightness, I can't really say how
    much because I'm only making this by calibrated eyeball. ;-) That's
    over 3600 hours, getting close to 4000 hours by next week. Whatever
    this factor is, it's taking its toll on the HK LEDs. It was much worse
    at 25 to 30 mA. The HK LEDs lasted only a few months, and then they
    were really dim.

    Makes a believer out of me when you talk about the lead temp and such,
    junction temp of 85 deg, etc. These four LEDs are close together, but I
    don't think they have ever been warmer than 40 or so degrees C, so I
    don't think excessively hi should be a factor. I think that Nichia uses
    some kind of phosphor that's more resistant to degradation over time,
    and the HK LEDs are just cheap.

    I'll keep updating as time goes on, and as they dim into nothingness. I
    may decide to scrap the HK LEDs and put new ones on right after new
    year's eve, just to start out the new year with a 'fresh start'. ;-)
     
  6. I would like ot see something more quantitative if you are going to go
    through all that trouble, anyway. If you connect a milliamp meter
    directly across a silicon photo diode, you get a pretty linear
    representation of the total light that falls on the diode. A large
    area diode placed directly in front of the LED captures a goodly
    fraction of its output in a fairly repeatable way. Once you have a
    few months data, you may be able to curve fit it to an exponential and
    state the decay rate as a time constant.

    This photo diode is a few bucks from Digikey:
    http://www.photonicdetectors.com/pdf/pdbc140.pdf
     
  7. Well, in the case of the URL you quoted below, that would'nt work
    because that photodiode is photoconductive, not photovoltaic. I had two
    lightmeters, a Weston and a GE, but even with the neckstrap, they got
    banged up to where they don't work anymore. I poked a hole in the end
    of a cardboard box and mounted a PV cell on the other end, but it wasnt'
    very sensitive because the distance was too great, maybe 8 inches
    (20cm). And as I moved the LED slightly in the hole, it changed
    readings. And I found that the milliammeter wasn't low enough V drop,
    too. So I wasn't satisfied with the stability and calibration of that
    setup.
    As for the HK LEDs, to paraphrase Dr. McCoy on Startrek TOS, "They're
    dead, John." I think that's a good enough estimation. :)
     
  8. That means that it is optimized to give a current output (linear
    indication of intensity) instead of a forward biased voltage output
    (logarithmic indication of intensity). Shorting it with a meter is
    very close to what you get with a reverse bias. Add a 9 volt battery
    for reverse bias, if you like, and see how little it affects the
    result.
    If you like, I could send you a nice big one (10 mm square). I have
    measured the output of LEDs many times with them.
     
  9. I would use a constant current generator. It's easy to build:


    +Ucc

    o o Current source
    | |
    .-. |
    | | |
    | | |
    '-' |
    | |/
    o---------|
    | |>
    | |
    | |
    \| |
    |---------o
    <| |
    | |
    | .-.
    | | |
    | | | R
    | '-'
    | |
    | |
    | |
    === ===
    GND GND
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)


    The current this circuit draw, is about the same as through R, and the
    voltage over R is about 0.6V, so for 20mA, the value of R should be 0.6/0.02
    = 30 ohms.
     
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