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Why wont my LED circuit work? Answered

First and foremost, thanks for any help, time, effort, info, etc on this website provided by anyone to anyone. I actually have three small questions.... I'm trying to make a simple LED series circuit using 4 red leds rated at 1.85v@20mA. Im using a 9v to power it with a 100r ohm resistor. Doing the math on the resistor calc, I believe I should be using 85 ohm resistor, so for good measure I went with a 100 ohm. First question... - Is 100r ohm resistor the right type? I'm only asking cause after this didnt work, I started wondering what that "r" in the "100r ohm" package ment Second.. - For a series curcuit of this typ...do I need a resistor on each led or just on the positive lead from the 9v? Third... I tried hooking everything up by just laying the contacts across one anouther and nothing happened. Basically from the 9v+ terminal to the 100r ohm resistor to the positive (longer) lead from the 1st LED. Then the expected 1st LED neg to 2nd LED positive, etc, till there was 4 like this. Then the neg lead fronm the last LED went to the neg terminal of the 9v. Nothing...zip So from there I tried just one LED going to a AAA (1.5v) with the same resistor and the same lack of results. Tried different LEDs and batteries with no luck. Any ideas? Seems too simple to be messed up, but it's obvious that I'm not doing something right. Sorry for the ease of this prob. I know that this has been covered fairly extensively on the this site and the net, but I don't understand what I'm not doing right. Any help would be appreciated. thank you



9 years ago

Also make sure your LED's are connected in series correctly polarised: 1) If they are new unused you'll notice one leg is longer than the other, the longer leg is + or known as the anode. 2) One side of the glass envelope has a flat side, and this is - and known as the cathode.


9 years ago

Try putting a resistor on every two LEDs

If you use this wonderful resource: http://led.linear1.org/led.wiz
You get the attached result (82 Ohm, you were good on the 85)
100Ohm should be striped brown - black - brown (82 is grey - red - black)