I am frustrated in My attempts to install leds in the locos at our club layout. Here is the scenario : We are using the Railcommand system,the recievers in the locos are putting out about 18+ volts to the headlight wires. I purchased some sunny white leds & installed them with the 1/8 watt 820 ohm resisters they reccomended . I used 1 resister on each long lead on the led, they fried fairly quickly. Ok I've been all over the web trying to learn what I need to do & found an online calculater for the resisres. I entered 18 volts, 3.0 volts forward, 20 ma. it showed I needed a 1 watt 1 K resister. isn't this a huge resister, space as usual is tight ? I found some 1/2 watt 1 K resisters online, will these work ? Where does the watts come into the equation in reference to leds ? I know some of You will grumble about this post, but help, I'm in a jam here or I wouldn't be asking. Randy..........
It sounds like the calculator you found is very conservative. Here is the calculator I use.
http://www.opamplabs.com/eirp.htm
With 18 volts and 1k, I found a half watt resistor should work.
The answer I got is, 18 volts .018 amp or 18ma current 1000 ohms 0.324 watts.
Max for many LEDs is 20ma
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Lets look at your old values (18V, 820 ohm):
18V / 820 ohms = 0.02195 or 21.95mA...
Looks like you were 1.95mA over rated current there!
18V/1000 ohms = 0.01800 or 18mA (2mA under rated current... this is better)
18V/1200 ohms = 0.01500 or 15mA (5mA under rated current... this is best!)
The size difference between 820, 1K and 1.2K resistors should be zero... its the Wattage rating that usually makes a size difference.
FYI, wattage ratings on a resistor is merely an indication as to how much heat they can dissipate safely.
Power = (I * I) / R
in this case: (0.015 * 0.015) * 1.2K = 0.27W (so a 1/8W resistor won't cut it, but a 1/2W will)
Note that I didn't take the voltage drop of the LED into the equation. This simplifies the math and makes the resistor ratings more conservative.
If you want, I put together a quickie Basic Electricity tutorial. Link's in my sig. Feed back is always welcome!
Dave Loman
My site: The Rusty Spike
"It's a penny for your thoughts, but you have to put your 2 cents in.... hey, someone's making a penny!"
Based on the numbers you gave, a 1/2 watt resistor will work. 15 volts across resistor times 20ma equals 0.3 watts. Or the current squared equals 400ua times 1K is 0.4 watts. Warning: a 1/2 watt resistor may get warm to hot with this load, depending on air circulation.
Looking at the big picture though, I personally think the 1K resistor is on the small side for 18volts on the rails. 20ma is the max current for the LED, not the recommended. Operating the LED at 15ma will give much longer life, less heat in the resistor, and very little perceptible dimming. I would try starting with higher values - as high as 2.2K - and work my way down through 1.5K until the LED brightness is acceptable. Depending on the LED, the 3 volt forward voltage drop may be higher than actual fact, which would again drive the resistance higher. If you operate the LED in the 10-15ma range, you can sometimes get away with an even smaller 1/4 watt resistor, and the 1/2 watt resistor is definitely good.
my thoughts, your choices
Fred W
Thanks for the replies ,Guys, So If I use 1/2 watt 1200 ohm resisters, the leds should have a lot longer life ? How hot will these resisters get ? I have been wrapping everything in shrink tubing. I really appreciate Your help !
Should be warm to the touch, but I do not believe it should melt anything. If in doubt, step up to a 3/4W or a full 1W resistor as previously mentioned.
Thanks so much,off to Radio shack We go........................
Would this work for a motor as well. I have installed a motor in a athern rotary snowplow but need it to turn slower. I have a bachmann decoder in it but it is still too fast. I am using the bachmann dcc system right now as a test bed until I can afford something better. What size resistor could I or should I use?
Mike
alco's forever!!!!! Majoring in HO scale Minorig in O scale:)
It would work similarly, although with a motor there are other considerations. You would first need to figure out what the running current of the motor is. From there you can size the resistor in both Ohms and Wattage.
You can also take the Voltage regulator approach and simply apply a lower voltage to the motor.
What kind of motor are we talking about? What are its electrical properties?
(Also, this may be off topic so a new thread may be warrented)
Thanks to everyone that replied to My questions, I think I finally got it figured out. Rich , The link You supplied was better than the one I had found for the calculator. Dave , Your link was informative , I had been there a few times before. Again, thanks to all that replied, & God bless. Randy.........