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I am creating a LED project and I am in need of assistance on where to put my resistors and any issues w/ burnt out LEDs Answered

I  am creating a project  where a motion sensor turns on a row of lights. Now it will turn them on in a particular order. So while learning LED and resistance I came across some questions.

If I place my LEDs in parallel i understand that I can run all off the same Volts, I just add up the Amps on all. I saw that they recommend putting a resistor on each led in case one burns out. 


1) Why can I just use one after all the LEDs?
2) If I were to wire it so all the LEDs have resistors does that mean I have to use higher resistors so they keep dividing from each other?
3) How can I make each light function separately but use the same Voltage?
4) What is the outcome if a led were to burn out in a series and/or parallel?

If I am being unclear let me give you a random unthought out scenario-

Lets say I have 9V Battery
8 LEDs
2.2 Vf at 20mA each
I believe that I cannot run all the LEDs in series because they will use all the voltage available.
So I must run them in parallel.
My math goes: 9v/ .160mA= 56.25 ohms resistor which would be bumped up to whatever is closest. 
Now comes Q2 above. with the scenario above would I use 8 450 ohm resistors in parallel?
Now with that scenario in play what is Q3
I want to power all the LEDs with 9V or whatever but want to run them separately (turn off and on)...

I am sorry for probably confusing everyone who read this but I would appreciate the help... still learning. Take it easy everyone.


You want to use series AND parallel.

First things first -- don't use 9v batteries, virtually ever. They are terrible. Get an AA battery holder for 3 to 6 in series and you'll have 4.5-9 volts. They are cheaper, easier to find, easier to find rechargables, and have SIGNIFICANTLY more storage/cost than 9v batteries.

Whatever you pick, find out the longest series strand you can make on that voltage. Same as your example 9v (6AA in series)...
2.2Vf means you can fit 4 in 9 volts...
4x2.2V = 8.8V
leftover is 9-8.8V = 0.2V to drop in the resistor.
I = 0.020A (20mA)
V = IR
R = V/I
R = .2V / .02A
R = 10 ohms.

Now every strand you put 4 leds in series with a 10 ohm resistor.

You can put LOTS of these 4-led strands in parallel, since each has its own current limiting resistor.

now when in parallel I am adding the 20mA each correct...
ex: 8 LED 20mA
I would do the math
20mA*8=140mA then just do 9v/.140= 64.285... so i will just put the 64 ohm (if there is a size up i would use) fore each LED correct?
That is doing each LED individually

Now how would i go about controlling each light separately (on/off).
If I put a switch in between them controlled by a arduino or something
what would happen if I opened one or several switches? The rest of the LEDs are running on the resistance for 8 LEDs. Would that be a problem?

Forget the total current. Consider the current in a single led across the supply. Now think about the voltage drop across the resistor one end is at 9, the other end has to be connected to the2.2video on the working led.

So there is 6.8 across the resistor. Since it's carrying 20milliamps. It's resistance has to be 6.8/.020 ohms 340 ohms

Im lost sorry. Why forget the total current when I am dealing with multiple LEDs in Parallel. R=VI On the LED for beginners it explains that I need to add the total current going through LEDs when they are in parallel to get the resistance per. Now in the example you gave above seems as if I am only dealing with only one LED in circuit.

The total current is the current drawn from the supply. Since the current in each LED in PARALLEL is independent of the supply, then we consider each parallel connection on its own.

In parallel connection, you work out the resistor for each leg, and add a resistor of the same value to each leg. You do NOT take the resistance you calculate and then divide the value by the number of legs.

In your example, if you had 100 leds in parallel, they would pull 2A from the supply (0.02 x 100), but each would need a 340 Ohm resistor, which allows 0.02 A to flow in each leg.

I know that if I put resistors in series they add.
What happens if I put in parellel. Lets say 2 100 ohm resistors. Anything?

They add like this:

1/R = 1/100 + 1/100 = 2/100 > R= 100/2 = 50.

So why would I need that equation for? The answer too that question doesn't change what I put in front of the resistors right? I would still put 100 ohms in front right.

That's right: stick with 100 Ohms.

I was answering your question, which was unrelated to your LEDs

So why use the equation. Is it only to find the end resistance after the series? What would I use it for? RE=R1+R2. Also how could I continue the circuitry afterward?

Not really.

In series connection you need the total VOLTAGE to calculate the parameters you need.

Total current = a single diode current (say 20mA)
Total voltage = LED voltage (2.2, say) x number of leds. + resistor drop

In parallel you only need to work out the conditions for a single LED, and apply it to all of them.
Total current = Diode current X number of leds.
Total voltage = resistor drop+diode voltage.

Also..if the LEDs were all in series than I cant really put a resistor on each one can I? I would only have 1 or two whatever is needed in the whole circuit right?

That's right. All the LEDs see the same current all the time, so you only need one resistor.

THIS wizard is an excellent tool for figuring out LED arrays. If you take the time to read the info provided and play with different setups then it can teach you a lot about circuits and LEDs.

I have looked through that website.. Good info but doesn't really explain Individuals and burnt out bulbs. But thanks for the tip.

Wait, so you want them to turn on in a particular order, e.g. sequentially? Well in that case you would need to connect each anode of the LED (I think that's the long lead, look it up before you try) to the output of your... device, give each LED a resistor corresponding to its current requirements, and connect all the cathodes to ground. So technically they are all in parallel. Most LED's use 20mA current, so divide the voltage output by 0.02 and round up to find what kind of resistor to use. Good luck and please ask questions if you're confused.

The resistor isn't there "in case one burns out", its to STOP them burning out.

That really doesn't solve any answers. Plus they all die out eventually.

Do it right and they don't die. Unless you count 100000hours as a short life


This is what I looked at when helping me understand LED's. This is the reason I have questions. But thanks for the thought.