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25 Volt LED?

Discussion in 'Electronic Basics' started by Holden Bonwit, Aug 25, 2004.

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  1. I was curious if anyone knew of a vendor who makes/sells high voltage
    LEDs. I have a 25 volt power supply, and for various reasons don't
    want to step down the voltage to ~12 volts with a voltage regulator.

    If it helps, or makes a difference, I'm looking for an IR led with a
    wavelength of about 830 nanometers, but would like to hear of any
    suggestions that people have for LED vendors.

    Thanks,

    Holden
    hbonwit04 at yahooDOTcom
     
  2. John Larkin

    John Larkin Guest


    All IR led's have about the same inherent voltage drop, 1.2 volts or
    so. Just add a series resistor.

    John
     
  3. w_tom

    w_tom Guest

  4. Seriously, even if such a high voltage LED existed, you would need to
    current regulate, since the forward drop will change with temperature.
    Ideally, you need to get an LED with drive electronics built in. 25v units
    are quite common, since trucks use nominally 24v electronics, and many now
    use LED rear lights, and instrument panel lights like this. Many are made
    by SLi http://www.sli-ml.com/led.htm, and Agilent, but these companies
    rarely do IR LEDs, so it does make a large difference that this is the
    frequency band needed.
    You don't say what sort of brightness you require.
    Remember also, that you can series connect several LED's, and minimise the
    resistance that has to be used. So (for instance), you could series
    connect perhaps 9 IR LEDs, and then have perhaps a 25R resistor to limit
    the current (you would need to check the typical forward voltage at the
    rated current, but normally this is about 2.4v, for the AIGaAs/GaAS
    Technology normally used at this frequency). This would be for a 100mA
    rated LED, like the QLD-830-100S.
    Some of the LED manufacturers do offer small switch mode drive modules
    that will cover this voltage range.

    Best Wishes
     
  5. Quantum physics dictates that LED voltage is related to wavelength -- about
    1.8 volts for red, 3.6 volts for blue, 4 volts for ultraviolet (the white
    ones are ultraviolet plus a phosphor).

    All LEDs require current-limiting resistors. There are no 12-volt LEDs
    either; LED lamps that run on 12 volts have the resistors built in.
     
  6. Pooh Bear

    Pooh Bear Guest

    The operating voltage of a single LED is typically around 1.6 Volts. This
    is determined by semiconductor physics.

    You can run a single LED by adding a series resistor which sets the
    current. It wastes most of the power in the resistor though.

    You could run, say, 12 LEDs in series ( operating voltage approx 19.2 V )
    and lose less power in the current limiting resistor.

    'LED lamps' use multiple LEDs like this. A.C. versions also require a
    rectifier to provide DC to the LEDs. LEDs only work with current flowing
    in one direction, so need D.C. current.

    In short, there is no *high voltage LED* ( except several LEDs in series
    ) - you have to work within the constraints of science.

    'Clever circuitry' can be used to optimise efficiency - but you're
    clearly not operating in this region of expertise.


    Graham
     
  7. I got some ultrabright whites the other day. Not much info on 'em,
    though. Typical values only given; no maximums. Vfwd was 3.4V; Ifwd
    was 30mA. Can anyone hazard a guess as to what the *do not exceed*
    figures might be? They tend to draw more current as they get hotter
    and when bunched close together and they do generate quite a bit of
    heat. I'm worried I might exceed the 30mA (typ.) figure by too much
    and blow 'em.
     
  8. Ken Smith

    Ken Smith Guest

    At least some white ones are blue and a phosphor and have a drop of about
    3.5V.
     
  9. Dave_S

    Dave_S Guest

    The formula for calculating the resistor to use for current limiting is:

    R = Supply voltage - Forward voltage drop of LED
    --------------------------------------------
    Desired current

    Example:

    R = 5V - 1.6V = 3.4 = 340 Ohms (330 Ohms will do fine [standard value])
    ---------- ----
    10 MA .010
     
  10. Maybe an X-ray LED will run at 24V. ;-)
    Or a combination of series LEDs and a resistor, or (if efficiency is
    critical) a switched mode regulator. I've seen some app notes for LED
    drivers, but not having done any LED work recently, I can't lay my hands
    on the part numbers as quickly as a good search engine might.
     
  11. w_tom

    w_tom Guest

    Wow! So many responses saying LED is not possible. Any low
    voltage incandescent indicator lamp - any voltage AC or DC -
    is sold as an LED from www.ledtronics.com and from other
    companies. They require the lamp number and whether power
    will be AC or DC. That's it.

    Many decades ago, an L1011 crashed into the Everglades when
    the flight crew, so busy trying to determine if an indicator
    bulb had gone out, accidentally disconnected the automatic
    pilot. (It also resulted in the book Ghost of Flight 401).
    As a result, indicator lamps on aircraft even way back then
    were replaced with LEDs. Did they change the voltage? Of
    course not. They simply changed incandescent indicator lamps
    with LED equivalents. The technology is that old - decades
    old.
     
  12. cpemma

    cpemma Guest

    It's worth getting Kingbright's LED Application Note, it has current/voltage
    curves & max specs for most led chemistries, plus tips on fitting,
    soldering, etc.
    http://www.kingbright-led.com/Data/Spec/ApplicationNote.pdf

    The 3 white types they produce are each 30mA max If, though you can go
    considerably higher by pulsing them. But heat is still the problem.
     
  13. John Miller

    John Miller Guest

    Not the voltage of the aircraft, and probably not the voltage going to the
    socket, but they certainly had to change the voltage going to the diode(s)
    proper (said circuitry would likely be part of the LED assembly, if they
    wanted it to be a "drop-in" replacement).

    --
    John Miller
    Email address: domain, n4vu.com; username, jsm

    A political man can have as his aim the realization of freedom, but he has
    no means to realize it other than through violence.
    -Jean Paul Sartre
     
  14. You should also have thermal resistance data and maximum junction
    temperature. At least most of these figures are similar enough for
    "usual" 5 mm LEDs - the thermal resistance from the junction to 5 mm
    outside the case on the lead that the chip is mounted to is generally
    around 250 degrees C per watt. Temperature rise of the chip above the
    temperature at that point is thermal resistance times the amount of power
    being dissipated in the chip.
    If you want really long life, have the junction temperature not exceed
    85 degrees C. If you can tolerate a shorter life, it's OK to go up to
    around 100, maybe 110 degrees C or so.

    Note that the junctions in a cluster lamp can sometimes get that hot at
    less than 30 mA through each LED.

    - Don Klipstein ()
     
  15. Don

    Don Guest

    Foe real bright LEDs look at "Luxeon Star LED" they are super.
     
  16. w_tom

    w_tom Guest

    The point is that LED assemblies have long been made as a
    replacement for any incandescent indicator lamp. Same even
    for traffic lights whereby LEDs now replace incandescent
    lamps.

    The original poster wants an LED light that can operate at
    25 volts. Do they make same incandescent lamp replacements
    for 830 nm LEDs? I don't know. But back when we were first
    purchasing these incandescent lamp replacements, they were
    willing to do any existing LED color as a replacement for any
    incandescent lamp. Often these assemblies contain four or
    more LEDs meaning they also have good intensity.

    LED replacement have long existed for most every
    incandescent indicator lamp.
     
  17. LEDs are current, not voltage driven. In other words, you need a current
    limiting resistor in series with the LED. You can calculate the
    resistance required by (Vin - Vled)/ current, current is usually 20 mA.
    If the voltage is likely to change, or if you are to lazy to do the
    calculation, use a FET as constant current source, I have posted the
    schematic here a couple of times, use google.

    You can also buy LEDs with that FET integrated on the chip, these work
    on any DC source up to 30 V.
     
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