led question

white88enochianwhite88enochian Regular
edited November 2011 in Tech & Games
got a bunch of leds out of a music making toy i have red blue and light sesor leds (move yourhand over them and they turnsomething on)


i can hook a blue led to a usb cable fine lights up

when i try a red led i have to put a tiny resitoron it for it to work


does red take more power?

Comments

  • SpinsterSpinster Regular
    edited November 2011
    LED light emitting diode, diodes only allow the electricity to flow through the diode one way. make sure your not wiring them in reverse polarity. usually the longer wire is the positive.
  • DfgDfg Admin
    edited November 2011
    You could be wiring the LED wrong or it could be the Red LED might require more power, best thing you can do is read up on LED's type and power requirements.
  • white88enochianwhite88enochian Regular
    edited November 2011
    well all putall blueleds togetherand they burnt out i had one buy its self and it burnt up in about a minute this didnt happen with the 3 i have in the case i modified
  • DaktologistDaktologist Global Moderator
    edited November 2011
    The blue will need more voltage than the red. It's to do with the semiconductor material used. Hence the reason the red LED will burn up while the blue will work.

    Have a look at http://en.wikipedia.org/wiki/LED#Colors_and_materials . It has info on voltages.
  • chippychippy <b style="color:pink;">Global Moderator</b>
    edited November 2011
    A led is a diode, a light emitting diode. A diode when connected the right way round is a dead short. The ballast resistor is there to limit the current passing through the diode. Like a load. Some led's have ballast resistors built in, some don't. If you are burning led's out then there is no ballast resistor in it and hence the current is not limited.
  • DaktologistDaktologist Global Moderator
    edited November 2011
    chippy wrote: »
    A led is a diode, a light emitting diode. A diode when connected the right way round is a dead short. The ballast resistor is there to limit the current passing through the diode. Like a load. Some led's have ballast resistors built in, some don't. If you are burning led's out then there is no ballast resistor in it and hence the current is not limited.

    It depends on the voltage due to the current-voltage curve of a diode. Above a certain voltage a near infinite current can flow but below it, a small current can flow. Its also the reason why some small LED flash lights don't have limiting resistors. Also most LEDs dont have inbuilt resistors, the larger high power ones sometimes do though.

    Diode-IV-Curve.svg
  • chippychippy <b style="color:pink;">Global Moderator</b>
    edited November 2011
    Blue and white led's are usually 4v and the rest 2v. The USB supply the OP is using will be 5v. All leds without a ballast resistor need an external one. As you say, only power led's have them built in, so chances are he will need current limiting resistors on them all. The blue one will probably work for a while, but will in the end burn out.
    You can calculate the resistor value from this formula.

    R=(Vs-Vl)/I

    Where R=the resistance
    Vs is the supply voltage
    Vl is the led voltage
    I is the led current

    So if Vs=5v and Vl=2v (normal v for most led's) and I=10mA (common current for most led's)

    then R=(5v-2v)/0.01A
    so R=300 Ohms

    For the blue led Vs=5, Vl=4, I=20mA (common for blue led's)
    so R=(5v-4v)/0,02A
    so R=50 Ohms

    of course I'm guessing at the current of the led's but this will stop them burning out so quickly.
  • white88enochianwhite88enochian Regular
    edited November 2011
    yeah the red ones didnt burn up the blue ones did and theyburnt faster when i had for connected
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