Noob trying to make parallel LED circuit, need help selecting resistors

So I'm making a parallel LED circuit using THREE CREE x-lamp LEDs which typically run at ~ 3.1v @ 350 mA, but they can max out at 1000 mA.

So my logic may be flawed here but this is how I'm thinking (sorry if I seem stupid):

My power source is 12v and 1 amp, which means if I use three equal resistors, each LED will draw .333 mA, close to the typical value.

Calculation of resistor: R = V/I ------ R = (12-3.1) / .333 (3) = 8.9 ohms (I believe I'm supposed to multiple the denominator by amount of lights I have, correct? So I would use a 10 ohm resistor.

Wattage of resistor: P = VI ------- P = 8.9 ohms * 1 amp = 8.9 watts. So I would use a 10 ohm, 10 watt resistor.

Is this correct?

Also, say I had only one LED. would it draw that maximum amount of amps from the power source, so 1 amp?

Planning on controlling 3 LEDs with PWM pins. I will be using a 12 v power source and the LEDs run at 3 volts. I'm new to arduino and electronics in general so I'm not sure how I go about this.

I noticed there is a power jack on the arduino for a transformer. Do I use this or do I strip the transformer and insert the positive and negative wires into their respective pins? Thanks!

Connect the three LEDs in SERIES. Forward voltage of three LEDs is then almost 10volts. Only 2volts across the resistor. 2/ 0.333 = 6ohm. 2 x 0.333 = 0.66watt A 5.6ohm/1watt resistor will do if you are going to add a mosfet for PWM dimming.

Better use a DC/DC converter to convert to ~4.5volt if you have to connect the LEDs in parallel. Leo..

Using resistors to control high power LEDs is useless. This is because the forward voltage drop changes with temperature. You need to use a constant current supply and yes connect them in series.

The input power socket is the best place to apply power (unless running from USB).

What type of LEDs are you using? Low power? If so you can run them from the output pins using a series resistor, about 270 ohms for a standard resistor.


I'm using high power LEDs that run at 350 mA and 3.1 volts. So I can just plug the transformer into the input power socket?

Thanks for the responses. If I connect them in series am I still able to use PWM for each light? I would think I wouldn't be able to. New to this stuff.

If I connect them in series am I still able to use PWM for each light?

If you mean dim them independently, then (obviously) no. You can’t independently dim parallel connections either.

I'm basically trying to copy this video, it looks like it's in parallel.

it looks like it's in parallel.

No it doesn't, they are separate, - that is they are not in parallel and not in series.


The thread title tells us you are confused about powering the LEDs. But the actual question is about how to power the Arduino.

You've also told about 350 mA per LED (module). An Arduino can't supply such current. You need to use a transistor which can handle currents like that. And a resistor that can do that too. The resistor would need to drop the 12 volts so that there will be 3.1 volts left for the LED, while reducing the current to 350 mA. 12 - 3.1 makes 8.9 volts. But you'd also need to keep the voltage drop of the transistor in mind, most of the times that would be 0.7 volts. So 8.2 volts. Now you can apply Ohm's law to those numbers: U=I*R; R=U/I; 8.2/0.35 = 23.43 Ohms. Pick the next standard higher value, which would be 27 Ohms. But you also need to know how much power would be dissipated, and in this case that would be something that really counts. P=U*I so 8.9 * 0.35 = 2.87 Watts, which is a lot and means a physically big resistor. The transistor also needs a current limiting resistor in its base (which connects to the Arduino pin), 270 Ohms should be OK.

You still can't supply this throug the Ardiuno. Still need to power it from a separate source, split its output to the transistor part, and to the Arduino power jack.

So if I wanted to have each light connected to a different pin, and had a 12 V 1 A power source, what would be the best way to go about doing that?

I have MOSFET transistors btw

"So I can just plug the transformer into the input power socket?"

NO. You will need an AC to DC converter. You can't plug the AC output of a transformer into that socket.

(from documentation): "External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack"

I have a 12V DC power supply. Sorry, could you walk me through this a bit?

I connect to the power jack on the arduino board. With three LEDs, each connecting to a different pin, how would I go about wiring this?

If you want to use the led's at high power, they you need to follow the instructions of MAS3.

Do you have the driver transistors, and resistors that MAS3 suggested?

I could help you hook up low power version, but I am not so good with the high power version, so someone else (MAS3) will need to help with that.


Found this picture in an older thread here on the forum:

Forget the values next to the parts as that isn't exactly what you want to build. If you choose to do it like that, use the calculated values. The transistor to be used should also be one that can handle the current and power you want it to switch. The 5 volt power supply should also be read as 12 volt and the resistor should be changed accordingly (as stated already).

That video showed a 12V LED strip. They already have the resistors in them.
You on the other hand have a raw LED, with no resistors. But your LEDs are power LEDs and the ones in the video are normal 20mA devices.

You do not need FETs like that video you need a 300mA constant current driver. If you wire the LEDs in series you only need one constant current driver. If you use a separate output to control each LED then you need three drivers. That option is more expensive for not much gain.

For the sake of being simple for me let's say I want to run ONE 3.1v 1000mA led.

Can I just run a 12 v DC from the wall into the power jack, then connect a 10 ohm 10 watt resistor to a pin, then connect the transistor to the resistor and wire going from the transistor to the positive terminal of the led, then a wire going from the negative terminal to GND on arduino?

Sorry if I'm frustratingly slow, I'm just trying to understand this.

The resistor connected to the arduino pin needs to be much higher ohms, such as 1 or 2k. you can put the 10 ohm resistor on one of the legs of the LED. You need an external power supply to drive a high current LED.

Can someone give me the wiring diagram for power one 3.1v 350 mA LED through arduino with a 12 v DC power supply to the jack for the arduino?