# What resistor should I use to complete the circuit with a 3V battery and 10 LEDs? Answered

I have connected 5 green LEDs and 5 red LEDs with 2 ply conductive thread and I am unsure as to what resistor I should use to complete the circuit (with a 3V battery).

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## Comments

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It depends on the forward voltage and current of the LEDs, this does vary.

On the assumption the red LEDs require 2.2v and 20mA, you should use a 47ohm resistors (one for each LED in series).

Green LEDs are commonly 2v or 3.2 depending on the LED material.

You can use an on-line LED Resistor calculator like this one to work out what you need if you have your LEDs specs, or just assume they are all 2v and use 56ohm resistors then see what happens (but they might not be as bright as they could be).

In the video he appears to be using a 41ohm resistor, with ten LEDs (assuming a forward voltage of 2.2v each) that allows a current of roughly 2mA per LED (that's pretty low since most LED typically require 20mA for full brightness). The resistor he used is more suited to a single 20mA LED.

Ideally you would use one resitor per LED, but using a single resistor for an LED array is fine if you need to keep components to a minimum. In these cases the resistor value is calculated by using the sum of all the LED's forward current with Ohms law. To drive ten LEDs at full brightness using a single resistor I'd use a 4.7ohm resistor (assuming a 2.2v forward voltage and 20mA per LED).

iceng's reply is also correct, assuming a 5mA current per LED.

The solution is presented in the first image which you need to click to see all of it..

You need to find the voltage across the resistor by subtracting the peak LED voltage from the battery and the simply using ohms Law...

Looking at your video and how dim those LEDs looked.. They were probably drawing 5ma each and ten would pull 50ma or o.05 Amp..

Power of the resistor P = V x I = o.8 Volt x .05 Amp = only 40 milliwatt so a 1/8 Watt 16 ohm resistor will do fine..

BTW running identical LEDs in parallel on one single resistor is the BAD way to light LEDs as can be seen in the next two pictures of a BAD kit of 125 LEDs on a single resistor... Note the variance in intensity because some LEDs have a fractional lower or higher forward voltage in the third digit and the cube has dark spots...