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led wiring question Answered

okay im doing to be making an underhood light for my car and i am planning on making a design with the leds. my questions is i have a +5V voltage regulator to get the system down to 5 volts will i need to put resistors in the series of LED's like every one or 3 and if so what resistor should i use. remember im a very green beginner at electronics so please dumb it down


I means Intensity... ;)

Conventional wisdom has it that ALL LEDs should have an inline resistor. It’s not hard to figure the resistance using the formula R=(VS-VL)/I.

Don’t let all those fancy symbols intimidate you:
VS = Source Voltage – What are you hooking it up to? 9v? 12V?
VL = Load Voltage – In this case most LEDs are 2v (except White or Blue: then use 4v). If you are using more than one LED, hook them in series and add 2+2+2+…(You might need to increase your source voltage!)
I = Current (I don’t know why it’s “I” and not “C”) This should be lower than the Maximum Current Rating of whatever LED you’re using.)

Wiring LEDs in parallel doesn’t make sense, since they ARE, after all, diodes. This means that ONLY the one diode with the lowest threshold will conduct ALL of the current.

You CAN however hook resistor/LED combos in parallel until you run out of current. If I wanted say 20 LEDs to run on a 12 volt system, I might make up 10 strings, each comprised of two LEDs and a resistor, then hook all ten strings in parallel.

Remember that LED brightness is related to current, not voltage, so all the LEDs might not shine with the same intensity, because of differences (up to +/-20%) in the resistors’ actual value. But if you have a drawer full of parts (doesn’t everyone?!?), you should be able to find the parts that will come “eyeball” close to matching.

Disclaimer: There are a ton of “specialty” diodes out there that blink or change colors or whistle “Dixie”, so all bets are off on how these will behave. You can probably get away with hooking them in parallel, since they contain their own internal circuitry. Just follow manufacturers’ directions (if you got ‘em) and have fun!


11 years ago

I am assuming U will run 1 or more white LED's. A white led has a voltage drop of appox 3-4V therfore your leds will need to be wired in parallel for them to work. You need to control the current through each led to stop them from burning out. To do this each led should have its own resitor wired in series with the led. Assuming a 5V voltage regulator, a voltage drop across each led of 3.5V and a led current of 0.020amps, the value of each resitor would be (5-3.5) /0.020 which is 75 Ohms.


11 years ago

make magazine says you wont need resistors if you wire the LEDs in groups of threes on pg 134. :)

I find the current trend to omit the resistors really disturbing. People are counting on parameters that are not tightly controlled to come out "just right" to get both reasonable brightness and reasonable lifetime out of their LEDs, and it's NOT that hard to figure out what the resistor value should be, and resistors are NOT that expensive.

what can i get to make a led panel or assembly. im going to make an led light array for the taillight of an old car and i need a board or panel or something to mount and wire the led's on


11 years ago

Here's another LED instructable.

You can energize LEDs with a 3V cell battery w/o resisters. I'm running 6 10mm LED with 2x3v. LEDs are in parallel. The batteries last for a long time.
If you use a power supply you will need to add a resistor. 4.7k per LED should be ok.


11 years ago

Why do you want to have a 5V regulator in there? Unless it's a fancy switching regulator, it won't be any more efficient to go through a regulator and a resistor than it is to go through a bigger resistor...

Anyway, one of the eBay sellers I've dealt with has a nice table (for 20mA LEDs,
which are the most common type): Single LED resistor table
And there's a reasonable LED instructable here:

This site will help.

I really don't know much about LEDs, but you'll likely want to wire up your LEDs in parallel and not series. I hear it's good to have one resistor for each LED.

So wire up the 5V power supply into all of the LEDs, each with its own resistor, and then wire those all back into another wire. Check the LED info for what the forward voltage is as well as the current and plug those, along with the # of LEDs, into the guru section at the link above.

Does that help?