Author Options:

LED idea brewing! need affirmation on design/wiring? Answered

So this will be my first LED project and after reading just about all there is to read i think i have it down, but b4 i go out and buy everything i want to consult the PRO's and see if it is right and turn this into a instructable. So if you could take a gander at the attached professional scribbled design and give me some feedback id appreciate it. and to clarify im not sure if im going to run this off of 120v or 12v. Thank you!!



Best Answer 6 years ago

Series does not matter on the order ! ;-)


ok thanks i understand now.. but not fully, i guess there's always a why!!! but that's for later. Thanks for your help.


6 years ago

You got it.

I can't do more then wire a cable for a trailer on my car and on the new one
I paid the dealer to use the kit.... :-p

Thanks appreciate the help alot!!

after looking at the diagram i realized the leds are in series and then the "series" of leds are in parallel..am i correct? is that why the resistors are on the negative side?

So this diagram would be correct according to the website?


Not Quite :)
You are wise to seek assurances.


haha i was close, yours is better cause i want to put a button switch in the center :-). Then i would just take your design and wire those in parallel, right?
my formula would still be..
(12v-9.9v) /.02
105 ohm resistor
and i would just use the next highest available resistor.

Hand me a car that needs water pump timing belt whatever but if it needs any electrical work(besides changing a light bulb) no thank you lol. I passed on being a tech at my work cause all the cars now are just one big computer!! Thanks appreciate the assistance

That is exactly what is happening and as iceng says above (below?)the order of resistors and LEDs in series doesn't matter.

If you want to learn about this, replicate all the numbers with your own calculations. You don't need much more than Ohms Law.

Yea i got confused for a sec then realized the order of the comments changed on me...

Yea i do my own calculations then verify them on a checker like that website. just in case. thanks for your help appreciate it.

NEVER run LEDs in parallel !!!

if your power supply is rated higher then your leds right? cause then they would blow up if you put 12v to 3v leds...no matter how many cause each one is getting 12 volts right?

do not run leds in parallel with only 1 resistor. Each parallel led (or string of leds) needs a method to control the current. Voltage doesn't hurt diodes (within reason), current does.

Otherwise your design isn't terrible. Another problem I have with it is you're running SINGLE leds on 12 volts (as you said, they're parallel)...that means you use 3.3 volts and WASTE 8.7 volts...throwing away almost 70% of your energy.

Save you the math, use a wizard like http://led.linear1.org/led.wiz

3 leds
12 volts
20 ma
3.3v each

...yields 120 ohm resistor and 3 leds in series...

Make a bunch of those strands, 3 leds and a resistor...
then put THOSE in parallel on 12 volts.

(run the wizard again with 30 leds instead of 3 and you'll see what I mean)

Would the attached picture be sufficient? or should i do 3 leds instead of 4 in a row?

I was on ledz.com and the site you referenced and was confused on the referenced site it shows the resistors AFTER the leds .......is that correct?

and im not sure what voltage my leds are yet, i have to do a test on them to find out but calculating the power supply which is 3 aaa batteries and the lines on the resistors i think they are around 3.3. I need to get a multimeter to be sure, they light off of a 3v button battery.

Thank you for your help, i wasnt sure if 12v is enough to power 64 leds thats why i mentioned 120v, ive installed an array of sockets and fans when remodeling both my parents house and have been shocked twice by 120v....it doesnt feel very good...def not something to mess with.


From where bwrussell suggested you visit.
21 groups hare to show.
Hope you understand.


I think to simplify things i might just go with 3 leds in parallel instead of 4. im pretty sure it should still have enough light output.

and i really want to learn how to wire up leds and currents and resistors so id rather do hands on and my own math so i understand it rather then just copying pictures and filling in the boxes

*and please please please don't mess with 120v until you thoroughly understand how voltage and current work. Not only will you blow up your leds in the above example on 120v, you can easily hurt yourself.

THIS has an easy to use calculator for building series-parallel LED arrays with dc power supplies.


6 years ago

It's a veryold rule that,

LEDs of a color do not light parallel to one another.

64 led of 20 ma each would draw 1.28 amperes and need a 6.8 ohm resistor of 12 watts...

Better a series of 3 leds and resistor group in parallel with 20 more groups
for a total of 63 leds is an engineering proper solution.