Electrical Question

[Anonymous]

I am trying to get some LED's to put on one of my trains. I have already burned one out. [V]How do I figure out what kind of resistors I need to keep the imput power at the right level? I would greatly appreciate any advice anyone can give me.
Thanks,
Scott

[SteelMonsters]

Getting voltage in terms of resistance needed to get a given current

LED's light up and burn out by how much current is passing through them. A LED will need a higher resistance as the voltage increses.

Say you have a LED that needs 20 mA to light up brightly (less current is ok but it will reduce the brightness) If it requires 1.7 volts accrosed the LED to pass 20 mA then:
(I'm using ohms law BTW) V = voltage, I = current in amps, R = resistance

V / I = R

1.7 / .020 = 85 ohms This is the resistance in the LED when it's lit up

Now to find the total resistance that will give us 20 mA off of a higher than 1.7 volt source

R(V) = V / .020 (V is simply a variable of voltage, plug in whatever voltage you supply, I will use 15 volts as an example)

R(15) = 15/.020 = 750 ohms which is total resistance,
now we subtract the LED resistance of 85

750 - 85 = 665 ohms which is the value of resistance needed for full brightness ( a higher value is ok)


If you want to guess and check, you can. Start high, and work your way down untill you find the brightness that you want. If you burn an LED out you know you went too far.

Added note, It's best not use too much current because this will significantly shorten the life of the LED. If for some reason you can't settle with the brightness of the LED try to either find a higher output LED.

[Anonymous]

Steelmoinsters,
Thanks for the info, those equasions will be very helpfull. I will put in the highest voltage that might be applied. What does the last color band on a resistor mean?
Thanks again,
Scott

[Anonymous]

What Steelmonster said is pretty right it all boils back to Ohms Law and I have recently done just such a set of calculations. If you use the these formulai you will be ok but you will have to jiggle them around a bit to suit what you hope to obtain and the info that you have available. If ohms law isn't right nuthins right.

Amps = Volts/ohms

Watts = Volts X Amps.

Make sure you get you decimal places correct.


Rgds Ian

[SteelMonsters]

0 Black
1 Brown
2 Red
3 Orange
4 Yellow
5 Green
6 Blue
7 Violate
8 Grey
9 White

5% Gold
10% Silver
20% None
I remember that by:
Bitter Beer Rots Our Young Guts But Vodka Gernally Wins
Get Some Now

1st band: 1st number
2nd band: 2nd number
3rd band: Multiplyer Add # of 0 behind 1st 2 numbers (gold silver and none dividers)
4th band: Tolerance (+/- 5, 10 or 20%, 5% means that the divation from numbered values is at most 5% off either over or under)


Red Red Red Gold = 2200 +/- 5% ohms
Blue Green Brown Silver = 760 +/- 10% ohms

[Anonymous]

I was looking at fiber optic lighting somewhat like this:
http://www.briansmodelcars.com/tutorials/tutorial.asp?TutorialID=31&CurPage=1

[Anonymous]

Thanks for all the info. That was a very interesting website useful too.
thanks again
Scott

[Anonymous]

If you are color blind just use a ohmmeter to read the resistance. you will find the tolerance is sometimes BIG so confirming the color bands with an ohmmeter anyway may save frying a LED when you get close to full intensity. Besides it is faster than desiphering the color bands.

[chug chug]

New to hobby and would like to know if all power packs run thier acc. on 18 volts ac or 18 volts dc. Also what voltage do LGB switch machines run on. Also where can I find out information about specifications of the varies power packs etc. I am having trouble trying to compar the apples and orange as the makers all have different ways of showing thier spec's .

chug chug

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