Having a LED problem in a simple circuit - Page 1
Forum section: electronics / beginners

author: Carlsp
author_rank: Newbie
timestamp: February 27, 2019, 03:09:55 pm
content: 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.Thanks for any help on this.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 27, 2019, 03:16:49 pm
content: 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.

author: Carlsp
author_rank: Newbie
timestamp: February 27, 2019, 03:42:00 pm
content: Thanks much for the answer. I will put a resistor then for each LED. I really appreciate the help.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: February 27, 2019, 04:21:59 pm
content: 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) / RsIf= (9 V - 1.6 V) / 100 ΩIf= 7.4 V / 100 ΩIf= 0.074 AIf= 74 mASTANDARD-LEDS(If= 10 mA)COLORTYPEUfredGaAsP1.6 VredGaP2.1 VorangeGaAsP1.8 VgreenGaP2.1 VyellowGaP2.2 VblueGaN2.9 VwhiteInGaN3.2 VTypically you will get a good amount of brightness from 20 mA LEDs at 10 mA operating current.

author: Carlsp
author_rank: Newbie
timestamp: February 27, 2019, 05:06:57 pm
content: 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.

author: Audioguru
author_rank: Super Contributor
timestamp: February 27, 2019, 06:30:39 pm
content: A modern very bright green LED uses the same materials as a blue LED and therefore has the same forward voltage as a blue LED.You can connect a red LED in series with a yellow LED and use one series resistor. They will still light up when the 9V battery runs down to 6V.

author: rdl
author_rank: Super Contributor
timestamp: February 28, 2019, 07:26:51 pm
content: Newer LEDs can be very efficient. I bought a green LED last month to use as a power indicator on a PC. I wanted one that was "true green" and not that sickly yellow-green that most are (I call them snot green). True green are usually around 555nm, but I ran across a few down around 520-525nm. The choice was kind of limited since I needed a 5mm size. I bought a Kingbright WP7083ZGD/G, InGaN type, which was kind of pricey at $1 for a single.I have a constant current power supply on a breadboard set for about 6 mA. I plugged the LED into that to test it out. I'm glad I wasn't looking directly into it at the time because it was insanely bright. I adjusted it down to a more reasonable brightness, still brighter than a normal green LED though. I had to add a 20k resistor to do that. Out of curiosity, I checked the current draw - only 0.7 mA !!! Wow. Even down to 300uA it was still plenty bright.https://www.mouser.com/ProductDetail/604-WP7083ZGD-G

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2019, 08:37:46 am
content: Quote from: frozenfrogz on February 27, 2019, 04:21:59 pmCorrection: blue and white LEDs are both InGaN and have the same forward voltage. A white LED is just a blue die with a phosphor: a fluorescent material which converts some of the blue light to the longer visible wavelengths which mix together to make white.There are also lime green and yellow LEDs which are blue LEDs with a phosphor which this time converts nearly all the blue to green or yellow. These are generally in clear packages and can be easily identified by looking into the lens at the LED die, which will be covered in a material the same colour as the LED emits.