What type of resistor should i use for an LED bulb, that is powered by a USB cable?

Question

I am making a light using a LED bulb and a USB cable. i have tried a 47k ,100k and a 10k resistor and all resulted in low lighting. Is the Resistor the problem or is it something else? and if it is the resistor, then what resistance level should i use? i got the directions on how to make the Light from this site ( http://forums.pcworld.co.nz/archive/index.php/t-52676.html ) however i am not completely sure if all the directions are right.

Answer 1

Some pointers for LED's:
When an LED begins to conduct, the voltage increases gradually while the current increases rapidly. Too much current will overheat the LED and eventually destroy it. The light emitted by an LED is directly proportional to current through the LED. The light output from an overheated LED will soon decrease and the LED may be damaged.
The forward voltage and wavelenght of an LED are directly related, so, it is not always possible to substitute a different LED w/o changing the voltage and current.
Some LED's table for voltage and wavelenght:
Wavelenght(nm)------Voltage
565(green)-------------2.2-3.0
590(yellow)-------------2.2-3.0
615(orange)------------1.8-2.7
640(red)------------------1.6-2.0
690(red)------------------2.2-3.0
880(infrared)------------2.2-2.5
900(infrared)------------1.2-1.6
940(infrared)------------1.3-1.7
don't know blue which is also white, but not have much difference to green.
LED's specified operating current limit must be respected for longevity and that operating LED's at their current limit does not significantly produce more brillance than at 60% of their current rating and ensure long operating life for most LED's.
Use this formula to determine the series resistance:
Rs=( +V-Vled)/Iled
where +V= voltage supply
Rs= series resistance
Vled= 1.7 V average
I led= LED current
Max. current= 20 ma(most LED)
Note:
if you're lazy in determining the resistance, then try this values:
LED voltage= 3.2-3.4V
480 ohm at 12 volts DC supply
240 ohms at 6 volts DC supply
200 ohms at 5 volts(USB voltage) DC
current will be around 25 ma with 30 ma maximum

Answer 2

I'm not sure what that "R 1058" resistor is in the directions on that website. Your values are all way too high, anyway.

For a white LED, the forward voltage is going to be about 2.2 volts, and the current (for the bright ones) is going to be several 10's of milliamps (mA). the USB port provides 5 volts at several hundred mA or so (plenty of current for the LED).

5 volts - 2.2 volts = 2.8 volts drop across the resistor.

2.8 volts / 50 mA = 56 Ohms
50 mA squared * 56 Ohms = 0.14 Watts

A 1/4 Watt 56 Ohm resistor should do the trick.

The LED specifications are off the top-of-my-head based on experience. The actuall LED current for the diode you buy may be more, or less, than 50 mA -- and the forward voltage may only be 2 volts instead of 2.2 volts, or it may be higher. Use the formulas I gave you, but the resistor value will still probably be somewhere in between 30 and 70 Ohms

.

Answer 3

You also need to know the voltage the USB cable outputs, wether the USB cable has its own resistance which it definately does. There are lots of angles to look at. Easy was is just take a 9V battery, put a 10k resister in series with it and wire the LED up. Also make sure you are putting the LED the correct way, the longer wire comming off the LED is where the voltage should be entering. It is the positive side of the LED.

Answer 4

The resistor is used to limit the current though the LED. Different LEDs have different current capacities. For Maximum light without shortening the LEDs life, you want maximum current withou exceeding the LEDs rating.

If you know the LEDs current rating, use the following formula to figure the optimum resistance.

V=IR (v= voltage, I= current, R = Resistance)

(The usb circuit will provide 5 volts.)

If you don't know the current rating, you can experiment, but there is the possibility you will damage the LED. I would expect something in the 470ohm - 1kohm range to work well. When you try it, disconnect immediately if the LED looks too bright, or is hot to the touch.

Note: The typical current rating for a usb circuit is 500ma. If you exceed this current, you risk damaging the usb port and computer.

Answer 5

In an digital circuit there's a resister in sequence with an LED. this is been some years considering i grow to be at school so i do no longer remember the formula. If the shop tells you that this is a right away exchange to the 1157 then perhaps there's a resister integrated into the bulb. The broking could have each and every of the snswers for you.