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Newbie: Resisters and LEDs

Discussion in 'LEDs and Optoelectronics' started by arronhunt, Apr 17, 2012.

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  1. arronhunt


    Apr 17, 2012
    Hi all. Newbie question alert.

    I'm trying to power a couple red LEDs on a breadboard. I'm using a regular 9V battery and I don't want to blow the lights (the hardware store is kind of far, don't want to run there repeatedly lol).

    The LEDs say they can handle 2.6v. I bought a pack of 1-kohm resisters, but I think they are too strong (the led isn't lighting). I connected one directly to the battery and it light up for a second or two before blowing. My question is what is the formula for calculating the strength of resisters I need? I'm guessing its something simple using Ohms law.

    Or is there a chart publicly available?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The first thing to consider is that you misunderstand the wiring on a breadboard.

    The second thing to consider is that the LED is polarised and you may have it around the wrong way.

    The third thing is that you may have 1M instead of 1k resistors (so learn your resistor colour code and.or measure them to be sure).

    Then take a look at the sticky thread about LEDs. However the practical upshot of that thread is that a 1k resistor will be OK (it doesn't give the maximum rated current, but there's often no need for that). In fact, I often use 1k resistors with LEDs on breadboards because most LEDs are rated to 20mA and I very rarely have voltages higher than 20V on my breadboard.
  3. KJ6EAD


    Aug 13, 2011
    You need to understand that LEDs are current dependent devices; too little gets you no light and too much gets you light for a short time. You've already experimentally confirmed the latter. Your LEDs might start lighting with just a few mA of current and their upper limit at normal air temperatures might be 20mA or a little higher.

    The forward voltage (Vf) you stated is 2.6V. That's a batch average and it's associated with a current figure, for example Vf=2.6 @ 20mA.

    The math is fairly simple: Vr = Vs (your battery voltage) - Vf, then R = Vr / Iled (your desired LED current). These can be combined into a single formula: R = (Vs - Vf) / Iled.

    Running the numbers for a 9V battery and one of your LEDs, you get a resistor value of 320 Ohms for 20mA, 427 for 15mA and 640 for 10mA. Some or all of these design values may not be available as standard resistors so we usually take the next higher standard value. Those would be 330, 470 and 680 Ohms respectively in the readily available E12 series. If you used your 1k resistor with 1 LED, you should get a 6mA current, probably weakly but noticeably lit.

    I'll leave it to you to calculate values for two LEDs in series.
    Last edited: Apr 18, 2012
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