Electronic – Deciding On A Resistor For an LED


Let me preface: This is my first post on this exchange, so I will try to provide as much context as possible. If some details are lacking, or I've left this question overcooked, I apologize.

So, I am building a prop for an upcoming convention on the east coast, and part of it involves lighting. I've tried to reduce and streamline the amount of soldering and the like I will have to do, as I am on a budget of time and attempts, so this might seem a bit unconventional: I am making some light-up vacuum tubes with LEDs, and I was going to use the same method outlined here: http://www.instructables.com/id/Glowing-Steampunk-Flash-Drive/

The end result will be the same, however there will be four in total, and they will be powered by a bus-powered USB hub connected to a 16,500mAh battery, which is intended for charging smartphones, tablets, and other devices of the sort.

I wanted to use this warm white surface-mount LED right here: http://www.digikey.com/product-detail/en/lumileds/MXA8-PW35-0000/1416-1094-6-ND/4031757

I believe I can mostly understand the relevance of each field on its little data sheet, but I need to be pretty much certain on a proper resistance.

So the question is, what resistor should I use for it? The Battery has two outputs, both 5 Volts, but one has a 1 Amp output, and the other is 2 Amps. The test current for the LED is 100 mA, and the maximum is 200 mA, with a forward voltage of 3.05 Volts. If I were to use a 22 ohm resistor, would that be proper for achieving ~roughly~ test voltage? Also, would the test voltage be particularly bright? If not, would it be smarter to order two sets of resistors, one for the maximum and one for the test? It has been a long time since I've done even basic electronics.

I just want to make sure I'm understanding this information correctly, and if I'm not, I would greatly appreciate a knowledge smackdown by someone who does. Thanks in advance!

Best Answer

200 mA will kill it in short order. That's the Absolute MAX.

100 mA and 65 mA are the nominal recommended max currents.

Formula for resistor is:

(V Source - V LED Forward Voltage) / Target Forward Current
(Vs - VF) / If

( 5V - 3V ) / 0.1A = 20Ω

22Ω gives slightly less than 100 mA (2V / 22Ω = ~0.09A) will really not be noticeably dimmer, and will help ensure the stated life of the led is met. A target of 65mA might be better. 2V / 0.065A = ~30Ω. As @Asmyldof points out, enclosing the led in a tube, on thin wires, isn't the typical application the manufacturer expects (on a pcb board with adequate landing pads/copper sizing, and certain free air/distance).

Since you have 2 volts across the resistor at 90mA, that's 2 * 0.09 = 0.18 Watts. You will need a 1/4th Watt resistor. So smd resistors are right out unless you go with two 44 ohm 1206 (1/8th Watt) resistors in parallel. So stick to 1/4th or 1/2 Watt through hole resistors.

Keep in mind, the original instructable used a 0805 20mA led. You want a 3535 led, thats normally 5 times bigger light area and 5x more current. It will be particularly bright and blinding in comparison. You will want to play around with one to find out a good brightness. Use a few resistors to test. Between 30Ω and 200Ω before finalizing which one. And the more current used, the faster your usb power pack will drain. At 50mA you will have twice the battery life of 100mA, generally speaking.