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Discussion in 'LEDs and Optoelectronics' started by donkey, Apr 6, 2011.

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  1. Apr 6, 2011 #1
    donkey

    donkey

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    ok simplest question in the world. its a led so do you need to connect a resistor so as not to cook it? they come with power supplies for 1 watt and up but was just trying to figure this out
     
  2. Apr 6, 2011 #2
    shrtrnd

    shrtrnd

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    Typically yes. You use a resistor to limit the amount of current (amperage) running through the LED, so you don't 'cook' it.
     
  3. Apr 6, 2011 #3
    Resqueline

    Resqueline

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    LED's need a controlled current (& have an approximate voltage drop), unlike incandescent "lumps" that need a controlled voltage (& have an approximate current draw).
    The means can be a resistor (&/or a capacitor if running off AC), a linear constant-current regulator, or a switchmode LED driver. The choice depends on conditions.
     
  4. Apr 6, 2011 #4
    donkey

    donkey

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    the problem i have is i know flashing leds for example you don't need a resistor, is the ultra bright cree lights (or one of the many other ultra bright leds) made for direct battery hookup or is it made like the rest and you need a resistor?
    i was hoping someone had used these leds specifically. they come with a power supply for the 1 watt models but what about those below 1 watt?
     
  5. Apr 6, 2011 #5
    donkey

    donkey

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    the device will be a lamp or in house light running from 12volt solar charged battery. it will run for 4-6 hours at night and when my kids do their make-up. the reason for using the ultra brights is
    A) they last for thousands of hours(so they say)
    B) being ultra bright i can put an inline dimmer on it (this will be a variable resistor (potentimeter i think they're called))
    i know with a pot on it i have a resistor BUT pot go from 0 resistance up. if i don't put a resistor in and the kids make it the brightestest they will blow it if they do this. in the case of the makeup mirrors if anyone has teenage daughters you know they will require the light to be on for an hour while they get the make up just right. while on that topic does anyone have a trowel i can give them to apply their makeup lol
     
  6. Apr 6, 2011 #6
    Resqueline

    Resqueline

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    Flashing LED's incorporate integrated electronic circuits that takes care of driving the chip. Efficiency/brightness is not one of their design criteria.
    Be aware that ultra bright is not neccessarily the same as efficient. A high mCd rating can be achieved by simply narrowing the beam.
    You probably want wide-angle LEDs with a relatively high Lumen per Watt rating, but also with a relatively warm & nice tint like 3-4000K (not a bluish 5-7000K).
    If the Lm rating is missing you can calculate it (approximately) from the mCd & angle spec's, with the help of calculators you find on the 'net.
    Cree makes a lot of different LED's, with different current & voltage ratings. You have to be more specific. Conditions = specifications.
    A single white or blue LED chip has a voltage drop of about 3-3.5V so you can put two (or even three) in series with a resistor, running them off 12V.
    If you apply a resistor + pot setup to high power LED's though it'll waste a lot of power, which you don't want with a solar setup. Use a dimmable switchmode LED driver.
    They're made for many different input & output voltages, & output currents.
     
  7. Apr 6, 2011 #7
    donkey

    donkey

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    resqueline you are awesome, inciteful and very descriptive thank you so much
     
  8. Apr 7, 2011 #8
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Donkey, there is also a sticky which describes some of the basics.

    See if any of this helps you.

    The other point is that LEDs are not manufactured to connect directly to batteries. In those cases where it is done (dodgy Chinese torches anyone?) they rely on either/both of internal battery resistance, and short operating periods.
     
  9. Apr 7, 2011 #9
    donkey

    donkey

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    thank you steve i read the one you were talking of and was just inquiring if cree or high light leds were different.... i know for example their price is lol
     
  10. Apr 7, 2011 #10
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    CREE are simply a manufacturer. They are a clever manufacturer, because they have managed to get their name recognised.

    You do identify something that may be considered a "third" class of LED, those with inbuilt circuitry to enable them to run without resistors. These often flash, or follow some colour sequence. I would consider them as something that looks like a LED, but is actually a device containing a LED. As such, they can operate pretty much however their designer intended.

    CREE LEDs pretty much fall into the "high power LED" class that I (somewhat arbitrarily) split LEDs into. There is no magic -- well, no special magic -- involved in driving them as opposed to any other LED in its class.
     
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