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LED resistor

Discussion in 'LEDs and Optoelectronics' started by dcuz, Mar 3, 2013.

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  1. dcuz


    Mar 3, 2013
    Hi all, I am almost sure someone has already inquired about this however I was not able to locate the thread, so my apologies if this turns out to be a re-post.

    I have been trying to wire up an LED light and I have been having problems with choosing the correct resistor.

    My LED operates at 3.7-4.2v, the maximum current I’d like for it to operate at is 850-950mah. My power source is a lithium battery capable of 4.2v, 3500mah.

    1. How can I calculate the resistor I need?

    2. What if I wanted to run two of these batteries in parallel where the current would double to 7000mah, would resistor heat become an issue?

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    mAh is a measure of battery capacity, not current. mA is current.

    So you want 850 to 950 mA through a LED from a 3.7 to 4.2V source. Presumably it is capable of the current you require, from your figures for around 3.5 hours.

    Calculating the resistor depends on the LED colour. I would recommend you get an appropriate LED driver though because the input voltage changing from under 3.5 volts to over 4 volts, the LED current will change by a large amount if you use a simple resistor.

    If you really want to use a resistor, look in the tutorials section for a tutorial on LEDs and go for your life.

    Two batteries in parallel would allow the LED to operate for twice as long.

    The heat in the resistor is determined by the current through it and the voltage across it. Making the battery capable of higher current (or capable of the same current for longer) will not make a difference to the instantaneous dissipation in the resistor unless a single battery was unable to supply the current demanded.

    Placing 2 batteries in series would require a different resistor to be used. That resistor would get hotter, and there would be no (significant) increase in the time the circuit would operate.
  3. dcuz


    Mar 3, 2013
    i have found a calculator but none of the variables even ask for the the source current. i.e 3500 vs 7000 ... this would require a different resistor, would it not?
  4. davenn

    davenn Moderator

    Sep 5, 2009
    no they wont cuz its irrelevent. the current capability of the shource in mAH will just give you an idea of how long the battery will last before it goes flat when there is a particular load on it.

    Steve gave you the relevent info

    Also have a look at this tutorial on this forum for LED driving info....

    Last edited: Mar 3, 2013
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