| Electronics > Beginners |
| Having a LED problem in a simple circuit |
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| Carlsp:
Hello all, I have just started my new adventure into electronics and my first simple circuit has a problem. I have connected 4 different colored LEDS, 5mm, in parallel on a breadboard. I have made sure that all the positives and negatives are correctly connected. I have a 100 ohm, 1/4 watt resistor connected to the positive side of the LED's. I am using a 9 volt battery for power. The LED's are red, yellow, green and blue (the colored lens kind). Ok, so when I apply my power source all the LED's light up except for the blue one. I have tried placing it in different positions relative to the others and no luck. I replace the blue LED with any other LED and they all work fine. I have also tried different blue LED's and tested them individually to ensure that they work before placing them in the circuit. I also tried using my bench top power supply, set to constant current @ 20mA and have the same results. They all light up except the blue one. What is special about the blue LED that it will not light in the parallel circuit? I am probably missing something simple here, lol. |O Thanks for any help on this. |
| ArthurDent:
Different color LEDs have characteristic voltage drops when lit. If you use a separate resistor in series with each different colored LED they will all light. If you check the voltage across each LED you will find different voltages of between 1.7 and about 3 volts. |
| Carlsp:
Thanks much for the answer. I will put a resistor then for each LED. I really appreciate the help. |
| frozenfrogz:
Be careful though, 100 Ω seems a bit lowish. Depending on the actual diodes used that is... Usually, red LEDs have a forward voltage of 1.6 V (Uf). With 9 V supply (Ub) and a 100 Ω series resistor (Rs), your LED would see 74mA of current (If): If = (Ub - Uf) / Rs If = (9 V - 1.6 V) / 100 Ω If = 7.4 V / 100 Ω If = 0.074 A If = 74 mA STANDARD-LEDS (If = 10 mA)COLORTYPEUfredGaAsP1.6 VredGaP2.1 VorangeGaAsP1.8 VgreenGaP2.1 VyellowGaP2.2 VblueGaN2.9 VwhiteInGaN3.2 V Typically you will get a good amount of brightness from 20 mA LEDs at 10 mA operating current. |
| Carlsp:
Thanks so much for the info, much appreciated. Yes I just did some calculations and see I am going to need around 275 ohm for just the blue LED's as the datasheet said the the forward voltage is about 3.5 volts. I will match a resistor to each LED and try again. This info is really helpful, thanks again. |
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