# 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
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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.

Jymmy097

Jan 29, 2010
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