LED' s resistor formula

Discussion in 'Electronic Basics' started by Lee, Aug 3, 2006.

  1. Lee

    Lee Guest

    I am looking fot the math formula to figure out the voltage drop i need
    and the associated resistor. such as the led uses 3 volts i have 12 i
    need a resistor that will drop 9 volts. also how to figure out the size
    resistor (watts).
    thanks
    Lee
     
    Lee, Aug 3, 2006
    #1
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  2. Lee

    Tom Biasi Guest

    "Lee" <> wrote in message
    news:...
    >I am looking fot the math formula to figure out the voltage drop i need
    > and the associated resistor. such as the led uses 3 volts i have 12 i
    > need a resistor that will drop 9 volts. also how to figure out the size
    > resistor (watts).
    > thanks
    > Lee
    >


    You need the forward voltage drop of the LED.
    The desired operating current.
    The source voltage.

    How much does the resistor drop and what power is it dissipating?
    E=IR, P=EI
    Hint: the resistor only drops what the LED doesn'nt.
    Can you use basic algebra to manipulate these formulas?
    Give it a shot.
    Tom
     
    Tom Biasi, Aug 3, 2006
    #2
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  3. Lee

    Noway2 Guest

    Lee wrote:
    > I am looking fot the math formula to figure out the voltage drop i need
    > and the associated resistor. such as the led uses 3 volts i have 12 i
    > need a resistor that will drop 9 volts. also how to figure out the size
    > resistor (watts).
    > thanks
    > Lee


    First, determine the amount of current you wish to have flow through
    the LED. You will need to get this information from the datasheet.

    Once you have this piece of information, you can apply ohms law to
    determine the needed value of resistor. Take the supply voltage minus
    the voltage drop of the LED at the desired current and divide by the
    desired current. This will give you the needed resitance. Typically,
    I use 10ma for a general LED and using that value as an example with
    your numbers: 12V - 3V = 9V / 10ma = 900 ohms. You can then use this
    figure to pick the a standard resistor that is near this value. Once
    you have the resistor picked, double check what current values this
    will produce and verify that it is within acceptible range.
     
    Noway2, Aug 3, 2006
    #3
  4. Lee

    HKJ Guest

    Lee wrote:
    > I am looking fot the math formula to figure out the voltage drop i need
    > and the associated resistor. such as the led uses 3 volts i have 12 i
    > need a resistor that will drop 9 volts. also how to figure out the size
    > resistor (watts).
    > thanks


    Your could also use this program:
    http://www.miscel.dk/MiscEl/miscelLeds.html


    It has many different LED circuits for both AC and DC and it will
    calculate all components value for your.
     
    HKJ, Aug 3, 2006
    #4
  5. Lee

    DJ Delorie Guest

    R = (Vcc - Vf) / If

    Vcc = supply voltage
    Vf = LED's forward voltage
    If = desired forward current

    Note that "Vf" needs to include Vol of your output logic, transistor,
    whatever - i.e. you need to calculate the actual voltage drop across
    the resistor in your specific case, so you can calculate the ohms
    needed to pass the desired current.
     
    DJ Delorie, Aug 3, 2006
    #5
  6. Lee

    John Fields Guest

    On 3 Aug 2006 07:17:01 -0700, "Lee" <> wrote:

    >I am looking fot the math formula to figure out the voltage drop i need
    >and the associated resistor. such as the led uses 3 volts i have 12 i
    >need a resistor that will drop 9 volts. also how to figure out the size
    >resistor (watts).


    ---
    View in Courier:

    Your circuit is:

    E1 E2
    / /
    Vcc---[Rs]---[LED>]--GND
    It-->


    The current limiting resistor required is:

    E1 - E2
    Rs = ---------
    It

    The resistor's dissipation will be:


    Pd(Rs) = (E1 - E2) * It


    Per your example, and assuming an LED current of 20mA :


    E1 - E2 12V - 3V
    Rs = --------- = ---------- = 450 ohms
    It 0.02A

    The closest standard 5% resistor which will keep the current at less
    than 20mA is 470 ohms, so the new current in the circuit will be:


    E1 - E2 9V
    It = --------- = ------ = 0.019A
    Rs 470R


    and the rsistor will dissipate:


    Pd(Rs) = (E1 - E2) * It = 9V * 0.019A = 0.172 watts,


    so a standard 450 ohm +/- 5%, 1/4 watt carbon film resistor would be
    OK to use.


    --
    John Fields
    Professional Circuit Designer
     
    John Fields, Aug 3, 2006
    #6
  7. On 03 Aug 2006 11:43:20 -0400, DJ Delorie <> wrote:

    >
    >R = (Vcc - Vf) / If
    >
    >Vcc = supply voltage
    >Vf = LED's forward voltage
    >If = desired forward current
    >
    >Note that "Vf" needs to include Vol of your output logic, transistor,
    >whatever - i.e. you need to calculate the actual voltage drop across
    >the resistor in your specific case, so you can calculate the ohms
    >needed to pass the desired current.


    ....however, LEDs will work fine over a fairly wide range of current,
    so you don't need to get _too_ scientific in your calculations.

    The current rating given on a LED data sheet is frequently the maximum
    recommended current, so you should plan on operating the LED at a
    somewhat lower current, unless you really need maximum brightness.


    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
    Peter Bennett, Aug 3, 2006
    #7
  8. Lee

    DJ Delorie Guest

    Peter Bennett <> writes:
    > ...however, LEDs will work fine over a fairly wide range of current,
    > so you don't need to get _too_ scientific in your calculations.


    Yeah, I usually calculate a minimum resistor value, and another for
    5mA, and use whatever's handy between those two.
     
    DJ Delorie, Aug 3, 2006
    #8
  9. Lee

    Bill Bowden Guest

    Lee wrote:
    > I am looking fot the math formula to figure out the voltage drop i need
    > and the associated resistor. such as the led uses 3 volts i have 12 i
    > need a resistor that will drop 9 volts. also how to figure out the size
    > resistor (watts).
    > thanks
    > Lee


    1. Subtract LED voltage from battery voltage. (12-3=9)
    2. Divide result (1) by desired current to obtain resistor value.
    (9/.02 = 450)
    3. Square current and multiply by resistor to obtain resistor power.
    (.02^2 * 450=180mW)
    4. Use calculator to check answers.

    http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

    -Bill
     
    Bill Bowden, Aug 3, 2006
    #9
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