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Seoul P7 LED Torch

Discussion in 'LEDs and Optoelectronics' started by r_spoo, Sep 15, 2011.

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  1. r_spoo


    Sep 15, 2011
    Hi folks,

    Well what can I say, I saw a videos on utube of some guys who have madee their own torches and I have always wondered how electronics work so thought I would try and teach myself.

    I have read many many websites and understand the basics but think I may have jumped th gun a little with this project......but need to finish it so its not a waste of money...then I will take a few steps back!

    Please if anyone can help me I would be soooooo greatful!!

    So the project is I want to make a torch and have the following components;

    Seoul P7 LED with a constant power current power supply of 2800mA. I have the hint sink and most other little bits I need such as the relector etc but....

    For the power supply I have either a 14.8v battery or can buy a 9.6v battery. The website suggests using a resistor (9.6v - 2Ohms with an 18 watt power consumption), but I cant find these online and am sure I am missing something??

    Although I have read the Ohms law several times I am really struggling to put it into practice.......

    I think I understand that a I need to reduce the current to the LED with a resistor but not sure which one to use?

    Any help anyone can give me would be absolutley brilliant! I would even consider paying for some Skype tuition if thats what it takes as i would love to crack it!!
  2. davelectronic


    Dec 13, 2010
    Led torch

    Hi there.
    whats the led's forward working voltage ? there is a big difference between 9.6 volts and 14.8 volts, get the resistor wrong and the voltage to high and its lights out led. Dave. :)
  3. r_spoo


    Sep 15, 2011
    Hi Dave, I know thats what Im worried about, this is the spec from the website, which one is it your after?

    Seoul P7 Emitter
    2.8A multichip Z-LED with ~90lm/W

    Diameter: 12mm
    Viewing Angle: 130°
    Height: 6.54mm
    Emitting Color: white
    Housing Color: clear
    Lumen typ.: 800
    Lumen max.: 900
    Kelvin typ.: 6300
    mA typ.: 2800 mA
    V typ.: 3.60 V
    V max.: 4.20 V
    Watt: 10,08 W
  4. r_spoo


    Sep 15, 2011
    I also want to make it with 3 switches,

    1 to turn off and on
    and then two to reduce the power

    i was thinking of using different resistors on parrallel circuits controlled by the switches, would that work?
  5. davelectronic


    Dec 13, 2010
    LED torch

    Hi again,
    Yes thats a start, ok i dont do 2800 milliamp led's every day, but i will have a go.
    Ok 9.6 the volts you plan to use, or 14.8 volts ? say 9.6 volts - the led's forward working voltage 3.6 volts = 6 volts divided by the led's working current 2800ma or 2.8 amps = 2.14 ohms resistor at 16.82 watts, but the resistor needs to be bigger to absorb the unwanted power say 18 or 20 watts.
    My maths is not my strong point, and i like electronics ha ha, the power of the resistor was the unwanted voltage squared, divided by the resistance, thats my story, please correct me any member if ive stuffed up please. Led dimming mmmm switching in other resistors seems a poor way to do it, can another member come up with an idea, bearing in mind the word torch, not stadium flood light, joking. Dave. :)
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I'm sure I replied to this earlier... Oh well

    That's fine, except for the amount of energy wasted as heat and the fact that the LEDs will get dimmer as the battery discharges. Another issue is if the resistor drops too little voltage, the reduction in Vf with temperature can cause the current to rise to excessive levels even with the resistor.

    So as noted above, a poor solution.

    The options are either a variable constant current source (preferably switchmode) or the PWM control of a (preferably switchmode) constant current source.

    The descriptions sound similar but there are some differences. Practically though the end result is the same -- the average current through the device is both regulated and variable. So the brightness is variable, but constant once set.

    Have a bit of a look here. We're talking high power LEDs here and the detail is scant in that article so far.

    If you get the circuit for a constant current source, you can use the switches to alter the sense resistor value and therefore the current to the LED.

    A switchmode circuit has the advantage of reducing power loss to the minimum and therefore making the batteries last longer.

    There are some modules that you can purchase which already do most of this for you. However I understand you may want to DIY.
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