resistors

[emayer32]

im putting in a custom license plate light (led) i got the wrong resistor i got a 100k ohm 1/2 w 5% i guess i was supposed to buy a 100 ohm 1/2w 5% will this still work i tried to figure it out on my own but im not real sure what the difference betwen100ohm 100k ohm is

[cali gixxer]

the difference is 99,900 ohms...so yes it will have a significant impact

[emayer32]

i know how to do math that wasnt the question lol maybe i should rephrase the question will the led still work with the 100k ohm will it make the light dimmer will it not work at all?

[cali gixxer]

yes, since the LED will not get the proper current, it will be dim like u said. I cant say for sure if it wont work at all since I dont know how much current the LED is rated for. Since the resistance difference is that much, my guess is that it wont work- but thats just a guess. If I were in your shoes, Id go buy the right one

good luck

[emayer32]

ill do that thanks

[ZXLNT]

100 ohms sounds too small, start with a 1000 ohm resistor. If you use to low of a value you run the risk of buring out the LED thru excess current.

[felizcortez]

It sounds like you are trying to do this yourself. So there are a few things to consider.

First is Ohm's law V=IR or Voltage = Current * Resistance

You need to know what the current rating for the LED is. For instance quite a few normal LED's are 25 milliamps. So in this case I = .025 Amps

Voltage is going to be equal to the supply voltage of 12V (however this fluctuates due to the alternator on the bike. At different speeds the alternator will put out different voltage levels which will cause a change in current on your LED which could potentially affect brightness.

To figure out what resistor you will need to create the desired voltage drop to give you the correct current you can use (Vs-Vr) / I = R
(Vs-Vr) / I = R

Let's say you've got a chain (series wiring) of three 2.2 volt LEDs (so Vr is 6.6) that you want to run at 25 milliamps (so I is 0.025, because there are a thousand milliamps in an amp) from a 12 volt supply (so Vs is 12). Now, the equation works out as

(12-6.6)/0.025=R

and R equals 216 ohms. 216 ohms isn't a standard resistor value, but 220 is a standard value.

Just an example if you want help with picking a resistor feel free to PM me and i can give you a hand.

[emayer32]

i have 1 3.6volt 20ma 1100mcd (white) led 100-ohm 1/2 watt 5% resistor i have the part # from radio shack if that would help anyone to see it

[Choco]

http://www.hebeiltd.com.cn/?p=zz.led...tor.calculator

supply volts: 12
drop across led: 3.6
desired current: 20

resistor: 470Ohm, 1/4W


and just FYI, when i did my 4 LED array in parallel, it used a 100Ohm, 1/2 W resistor....so that resistor will fry the $5.29 LED fairly quick like.

[emayer32]

i guess i should have just asked the question before i bought anything thanks everyone choco ur awesome

[Choco]