Resistors calculations for a simple circuit with leds

Discussion in 'General Electronics Chat' started by Jymmy097, Aug 21, 2016.

  1. Jymmy097

    Thread Starter New Member

    Aug 21, 2016
    Hi everybody,

    I signed up in this site just half an hour ago. I need your help solving a very simple problem I cannot understand (my electronics knowledge is very basic: I learnt what I know now at school in physics).

    I need to build a circuit made up of a battery pack (2x 1.3 V in serie) and a led. The first led was lit up by a current of 250 uA (measured with a digital tester) without any other components. The second one, however, was almost burnt (the light was too strong and it soon became red). So I made some calculations in order to find a good resistor. The first led oppsed a resistance of 2.6/(250*10^-6) = 10400 ohms. So I thought that the second led received too much current (I measured the current that it draws without any resistors and it is about 17mA) so it opposed only 2.6/(17*10^-3) = 152 ohms of resistance. I tried some different approaches:
    1. I thought that it should oppose the same resistance as the first led, so I connected the led in serie with a 10kOhms resistor, but the led did not light up.
    2. Then I read somewhere that the led was built for a 2V current, so I used the first Ohm's law to calculate the resistance it should oppose at 250uA that is 8kOhm, but the led did not light up
    3. I used the famous trial-and-error method (I hate it, but it was necessary to make it work) and I found out that the led works well with a 100/160 ohm resistor, but I do not know why.

    Can you please help me solving this simple problem which I cannot work out by myself even if it seems so easy to use Ohm's laws to do it.

    Thanks in advance.

  2. ericgibbs

    Senior Member

    Jan 29, 2010
  3. bertus


    Apr 5, 2008
  4. hp1729

    Well-Known Member

    Nov 23, 2015
    The chart is a good general rule to start with. Keep in mind that all LEDs of the same color may not be the same voltage and current. "High efficiency" LED may differ. That "rated current" is suggested for full brightness. If you don't need that full brightness you can run that 20 mA LED at 2 mA maybe and get sufficient brightness. Select the resistor as needed.