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I'm trying to mod a NERF gun to add LEDs onto it, and I'm trying to wire them in parallel. Unfortunately, when I try to wire them, it doesn't work. I have it wired like this: enter image description here

The only issue I could possibly see is that I soldered the two wires together and then connected the LEDs to them, but I figured that that shouldn't be an issue because I figured that's how parallel works. The first LED lights up fine, but the second doesn't, and I'm not sure what I'm doing wrong. I was wondering if it was because I was doing it with 4.5 volts, I was going to add some resistors later but I just wanted it working first. Any help would be appreciated.

Harry Svensson
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AndreasKralj
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1 Answers1

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This works in theory, however not in practice. LEDs do not all have the exact same forward voltage. In the case of LEDs connected in parallel and without their own individual resistors, the LED with the lowest forward voltage will simply conduct most of the current, lighting brighter than the others. The only way to have consistent brightness across each LED would be to pair them with their own resistors.

tbonanno
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  • All right, thanks for the info! So why do the resistors help in evening out the brightness, then? Do they basically act as an equalizer for the current? – AndreasKralj Jul 24 '17 at 17:20
  • At voltages close to the reported forward voltage of an LED, its own resistance (and actual forward voltage) can vary quite a bit. Common practice is to apply a larger voltage than its forward voltage and add a resistor in series. By doing this, the LEDs actual forward voltage can vary while still being similar brightness to others. – tbonanno Jul 24 '17 at 17:26
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    This is absolutely not true. There are many LED torches where up to 20 or more LEDs are placed in parallel and they work very effectively. It's true that there may be some differences in the current flow through each individual LED, but it they the same LED number/architecture then it will not be very noticeable. – Jack Creasey Jul 24 '17 at 17:45
  • Ok, I think I'm beginning to understand now. Just to clarify, the forward voltage is the voltage that is subtracted from the starting voltage coming from the power supply, right? So then what happens if you have an LED with a voltage drop of 1.8V, and one with 3.3V? Would you have to use separate resistors then for each one to ensure they're close to the desired brightness and find the amount of resistance via Ohm's Law? – AndreasKralj Jul 24 '17 at 17:49
  • @AndreasKralj: if integrating 1.8 and 3.3 leds, yes, very much so; you can't "get away" with ignoring minor manufacturing diffs like you can on an SMD COB array... – dandavis Jul 24 '17 at 17:55
  • If you have different LEDs, you would have to wire them in parallel with their own resistors. You can use a simple resistor calculator to determine the right resistor value for your LEDs. They use the simple formula R = (Vs - Vf)/If. – tbonanno Jul 24 '17 at 17:56
  • Ok, thanks! One more question, does the thickness of wire I use matter? I'm using some old headphones that don't work any more for one part of the wire that are thinner than some telephone line I'm using for another. Does using variable wire thickness pose an issue for this project? – AndreasKralj Jul 24 '17 at 18:39
  • This discussion may help you: https://electronics.stackexchange.com/questions/283219/wiring-multiple-led-flashlights-to-single-power-source ....this includes torches with 24 LEDs per unit ...and multiple of these in parallel. – Jack Creasey Jul 24 '17 at 20:17