# Help, I have a problem with my LED circuit.?

I am having problems with my LED project, I have 20 LEDs in parallel (2 branches of 10) with a 9v adapter as the power supply. The LEDs consist of 3 red, 3 blue, 3 green and 11 white LEDs. I used an LED calculator to determine that the resistor should be 18 ohms (I used the lowest voltage of LEDs, the red being 2.4V to do the calculation). It is in parallel so the circuit looks like the image (but with 20 LEDs). So the problem is, all the LED are working fine until I add the red LEDs to the circuit, then only the red ones light up. How do I fix this. I must confess that my knowledge of electronics is rather limited, I basically learned everything I know from hanging around Instructables. So any help would be appreciated, thanks.

sort by: active | newest | oldest
8 years ago
alexander.m has given you the best answer. You don't necessarily have to add one resistor per LED but the different colours should be grouped under separate resistors at least.

L
ChrysN (author)  lemonie8 years ago
Thanks for your help! I grouped the red leds separately with its own resistor and that did the trick, I'm just writing up my instructable now, I should post it in a few days.
8 years ago
OK, I'll look out for that. L
plumber_bob8 years ago
What is the current rating of the power supply, how many amps, can it handle? Also the way it sounds, you have 2 banks of 10 each LEDs in parallel, but are the 2 banks in series, or parallel with respect to the power supply? Anyway this might give you some things to think about.
ChrysN (author)  plumber_bob8 years ago
The banks of 10 leds are parrallel with respect to the power supply. I am not sure what the amperage of the adapter is, it is a 9v "battery eliminator" which has the same contacts as a 9v battery, so I assume that it would run anything that a 9v battery does. The led calculators don't mention the power supply current rating so I didn't realize that it might be a factor, sorry, like I said, my understanding of circuits is rather limited. How would I determine what current rating I would need?
8 years ago
You just need to tell the wizard they linked how much each led needs (usually 20ma). Calculate for single strings (each colour separately).

power supply>>>>
v v
string 1 string 2
v v

each string has its own unique resistor. 10 LEDS in a row will take a minimum of 15-20 volts...you may need shorter strings, especially for blue. Divide your source voltage by the individual voltage of the led to find how many you can fit in a row. i.e. 9 volts with 3 volt leds will require 3 leds to sink the 9 volts, and a small resistor to keep current in check. 2 volt leds will need 4, (to make 8 volts) and a resistor to drop the last volt.
8 years ago
I am very rusty, it's been years since I was in tech school. But each bank of 10 LEDs can be looked at. as a device. Get each bank to work properly. I do know that it can be frustrating because, circuits don.t always seem to follow the rules like they should. That's how 'tweaking' came to be. Each red Led might need a little resistance to limit what current they draw. Potentiometers might help as they are adjustable resistors. You might try a power supply with more current. Maybe a different kind red LED. Just trying to give you some things to consider. Keep your chin up I pulling for ya!
frollard8 years ago
lemonie and alexander have it: Don't put parallel strings of leds with a single resistor - use a resistor for each parallel string. Each led is in and of itself a relative short circuit - so theres no telling which led out of the parallel set will get the most current, heat up, resist less, then get more current, and eventually die.
alexander.m8 years ago
there is a couple of problems with this, the leds have different voltage requirements depending on the color and the circuit you have, with only one resister only works if all the led's are working. To improve your circuit I suggest that you study ohm's law and go to http://led.linear1.org/led.wiz