Resistor sizing for LED's on DCC?

[Anonymous]

I am looking for the formula to determine the size resistor to use with LED's on DCC locomotives ands lighted cars. Need to find ohm value and wattage needed for 20ma LED with consideration of heat generated by the resistor.
I'm sure there is a formula on the wed I just can't seem to find it.

Jack

[MisterBeasley]

I use 1K for the larger headlight LED's, and 470 ohm for the smaller indicator lights and signals.

[cacole]

Ohms law is what you are asking for, but it's much simpler to do as MisterBeasley said -- I too just use 1K for most LEDs and they seem to be bright enough.

Now, if you still want to use Ohms law, it is:

I=E/R
E=IR
R=E/I

where R=Resistance, E=Voltage, and I=Current in Amps. So you must know the output voltage of the decoder, the voltage requirement of the LED, and the current draw of the LED in order to accurately compute the resistor value.

Example: For a DCC decoder with an output of 12 Volts and an LED that requires 1.2 Volts at 15mA, you subtract 1.2 from 12 to determine how much voltage must be dropped -- 10.8 Volts. 10.8 divided by 0.015 equals 720 Ohms. Since an LED draws such low current, a 1/4 Watt resistor would be more than sufficient.

Remember, too, that resistors have a tolerance value. Most cheap resistors are accurate to only +/- 10%, so a 720 Ohm resistor could be anywhere between 648 and 792 Ohms; thus a 1K Ohm resistor is close enough for government work.

[rrinker]

Remember the white and golden white LEDs are usualy closer to 3 volts than 1.2 volts. I use 1K resistors, keeps the current low (high enough to light the LED, but well below the max at which point the LED will be destroyed), and they are PLENTY bright.

--Randy

[Anonymous]

I see how to calulate the Ohm value, but how do I calculate the needed watt rating.
I model in N scale so physical size is important and there is not very much air space between the frame and the plastic shell. So is a 1k ohm 1/8 watt resistor going to be OK or will it get to hot.
Thanks for the help.
Jack

[Seamonster]

One of the formulas for calculating power dissipated (watts) is current squared times resistance. Using 0.015 amps and 720 ohms from a previous post, the answer is 0.162 watts. Using a 1,000 ohm resistor, the answer is 0.225 watts. Since 1/8 watt is 0.125 watts, it would get warm. A 1/4 watt resistor would be best. They're pretty tiny and I doubt they would damage the plastic shell even if they were touching it.

[cacole]

Ohms Law for DC power is:

P=EI
P=I2R
P=E2/R

Sorry, these forums don't have superscript to represent the 2 as meaning "squared"

[Anonymous]

Thank you for the help. I pick up some 1/4 watt resisters and start changing my yellow LED's to White LED's. They sure are brite and look muck better in the N scale locomotives.