# Project 1: leds in series vs in parallel **URL:** \ **Category:** Starter Kit\ **Created:** [October 15, 2024, 5:36pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625 "2024-10-15T17:36:41Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![jeromejaglale](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jeromejaglale/32/1098695_2.png) [@jeromejaglale](https://forum.arduino.cc/u/jeromejaglale)\ **Post date:** [October 15, 2024, 5:36pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/1 "2024-10-15T17:36:41Z")
When 3 green leds are in series, they all light up. When they are in parallel (after a 220 ohms resistor), they don't light up. I don't understand why. Any idea? ---
**Author:** ![jeromejaglale](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jeromejaglale/32/1098695_2.png) [@jeromejaglale](https://forum.arduino.cc/u/jeromejaglale)\ **Post date:** [October 15, 2024, 5:48pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/2 "2024-10-15T17:48:42Z")
I just asked ChatGPT and I understood: - in series, each led gets the same current (23mA) - in parallel, that current is divided between the leds, and that's not enough for the leds to light up Solution: put that resistor on each parallel branch, so it limits the current only for that led, or use a smaller resistor. ---
**Author:** ![PaulRB](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/paulrb/32/216330_2.png) [@PaulRB](https://forum.arduino.cc/u/PaulRB)\ **Post date:** [October 15, 2024, 5:57pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/3 "2024-10-15T17:57:55Z")
> [@jeromejaglale](#): > > When they are in parallel (after a 220 ohms resistor), they don't light up. They should light up. If the 3 LEDs are from the same original batch, they should divide the current between them, roughly, but never quite exactly, equally. Each will only be 1/3rd as bright as they would be if only one was connected. If the LEDs are not from the same batch, they may not be very evenly bright. One or two might not light at all. So it's not clear why your parallel LEDs did not light. > [@jeromejaglale](#): > > that current is divided between the leds, and that's not enough for the leds to light up Nonsense. Typical of Chat GPT. ---
**Author:** ![Dave\_Lowther](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/dave_lowther/32/1092885_2.png) [@Dave\_Lowther](https://forum.arduino.cc/u/Dave_Lowther)\ **Post date:** [October 15, 2024, 5:58pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/4 "2024-10-15T17:58:05Z")
> [@jeromejaglale](#): > > When they are in parallel (after a 220 ohms resistor), they don't light up. At least one should light up. Did none of them light up at all, or were they just dim? ---
**Author:** ![camsysca](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/camsysca/32/1015228_2.png) [@camsysca](https://forum.arduino.cc/u/camsysca)\ **Post date:** [October 15, 2024, 6:07pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/5 "2024-10-15T18:07:45Z")
> [@PaulRB](#): > > So it's not clear why your parallel LEDs did not light. I'm betting the poster has series and parallel confused, because if you just swap those two words in the OP, everything would make sense. ---
**Author:** ![PaulRB](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/paulrb/32/216330_2.png) [@PaulRB](https://forum.arduino.cc/u/PaulRB)\ **Post date:** [October 15, 2024, 6:10pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/6 "2024-10-15T18:10:20Z")
> [@jeromejaglale](#): > > When 3 green leds are in series, they all light up. Maybe not. Green LEDs have a "forward voltage" of around 2.2~3.2V, depending on how they were manufactured. If you connect them in series, that's a total of 6.6~9.6V. If you supply only 5V from an Arduino, they should not light, in theory. But perhaps you were supplying 9V or something. EDIT: I think @camsysca figured out the problem! ---
**Author:** ![jeromejaglale](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jeromejaglale/32/1098695_2.png) [@jeromejaglale](https://forum.arduino.cc/u/jeromejaglale)\ **Post date:** [October 15, 2024, 7:33pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/7 "2024-10-15T19:33:12Z")
Thank you all for answering. I tried again, and it looks like I got parallel and series confused, indeed 🤦‍♂️ Sorry. The leds in parallel light up, the leds in series don't. In parallel, as Paul said, the current is divided and each led lights up about 1/3 as bright. But why don't they light up in series then? The current is not divided. But the tension is, that's why? I attached the two circuits for reference. ![IMG_6675](https://europe1.discourse-cdn.com/arduino/original/4X/2/e/8/2e8e79f3b18cc45514e2fec7fc33c243462c3d08.jpeg) ![IMG_6676](https://europe1.discourse-cdn.com/arduino/original/4X/5/d/7/5d73443236ff91d1c77b3cb1f58cac522a4fa4cc.jpeg) ---
**Author:** ![camsysca](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/camsysca/32/1015228_2.png) [@camsysca](https://forum.arduino.cc/u/camsysca)\ **Post date:** [October 15, 2024, 7:40pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/8 "2024-10-15T19:40:01Z")
> [@PaulRB](#): > > Green LEDs have a "forward voltage" of around 2.2~3.2V, depending on how they were manufactured. If you connect them in series, that's a total of 6.6~9.6V. If you supply only 5V As @PaulRB intimated, to have current flow you must achieve each of those voltages "in series". IF you start with 5V, you haven't enough voltage to do so; if any one of the LEDs does not conduct due to that, then you have no current flow through anything in the series circuit. ---
**Author:** ![jeromejaglale](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jeromejaglale/32/1098695_2.png) [@jeromejaglale](https://forum.arduino.cc/u/jeromejaglale)\ **Post date:** [October 15, 2024, 8:14pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/9 "2024-10-15T20:14:33Z")
I see, thank you very much! 🙏 ---
**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\ **Post date:** [April 13, 2025, 8:14pm UTC](https://forum.arduino.cc/t/project-1-leds-in-series-vs-in-parallel/1311625/10 "2025-04-13T20:14:53Z")
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