Model Railroading > Resistors for Ditch Lights

Need some help on finding out what resistor to use 3 Volt Ditch Lights I'm using.
Any Help is greatly appreciated.

Paul B
Canton, GA

You can figure this out with Ohm's Law and the power equation.
First let's look at Ohm's Law:
V=iR or V=i x R  or Volts equals amps times ohms
Voltage equals current (amps) times resistance (ohms).
If you are going to operate a 3 volt lamp that works with 15 milliamps (0.015a) then you need to "lose" 9 volts at the resistor.
9=0.015 x R
This equation can be transformed to R=9/0,015 by dividing both sides of the equation by 0.015.
The answer is R= 600 ohms.
Try something close ot 600 ohms, if it looks too bright you can increase the value of the resistor to maybe 680 to cut down the current.
Now about the power:
Resistors generate heat and are rated with two numbers, ohms and watts.
The power in the resistor is:
P=iV 
Watts equials current x Volts
For our example, P = 0,015 x 9 or 0.135 watts, just a little over 1/8 watt so use a 1/6 or 1/4 watt, 600 volt resistor.
Now check your decoder's current rating.
Circuit boards are rated by how much current they can provide to lighting circuits.
A typical example would be about 100 milliamps (100 ma).
So if you have two ditich lights at 0,015 each on that circuit you are using 0.030 amps or 30 ma, well within the limit of the decoder board.
It sounds intimidating but once you get used to making these little equations work for you then it is all under your control.
Good luck

For a 3 volt 15 miliamp incandescent light bulb I use a 560 ohm 1/4 watt resistor for the 12 volt output on a decoder.
For a 1.5 volt 15 miliamp incandescent light bulb I use a 680 ohm 1/4 watt resistor. I have used this combination for years and have never had a burned out light bulb.



Edited 1 time(s). Last edit at 10/13/20 20:59 by santafesteve.

whatdidhesay?

You can also use a resistor board such as the one from Nate's Light Iron Hobbies.
https://www.nateslightironhobbies.com/store/dcc-led-resistor-board.html

I have been told these are also available from another source on ebay, but I have never been able to find them.

Just went there and ordered two., looks neat. Thanks for the heads up
john

No problem.
SeaboardMan Wrote:
-------------------------------------------------------
> Just went there and ordered two., looks neat.
> Thanks for the heads up
> john

There must be something my mind is missing, but is there a reason for individual resistors for each light function circuit, vs. a single resistor on the common side of the circuit? I could see multiple resistors to disipate higher power required by GOW bulbs, but surely LEDs don't draw enough current to overwhelm a resistor on the common (i.e. blue wire part of the circuit).

Thanks. I'm new to DCC, but have learned a thing or two about spark chasing.

Arved Grass
Fleming Island, FL

Since very little information was provided for this thread, I will make a few recommendations so some of you can save allot of time and frustration as there are many products that have tackled this issue.

1. If you are installing sound and you are a Loksound fan, Loksound has resistors on their board that light up 3V LED's. They have been doing this for years. Easy Peasy and a great product.
http://www.esu.eu/en/products/loksound/loksound-select-direct/

2. If you do not want to buy Loksound, then buy the Decoder Buddy board. An awesome product. Also has all the resisitors on board for LED's and support many functions for folks that want to put lots of lights on an engine. Also has the 21 pin interface so you can use Soundtraxx or TCS products with this board including ESU. Finally a mini board detaches from the main board for shell seperation. This is a game changing product and has been out for some time.

https://nixtrainz.com/

Time to move into modern times folks.

Good luck.  Bruce

>
> 2. If you do not want to buy Loksound, then buy
> the Decoder Buddy board. An awesome product. Also
> has all the resisitors on board for LED's and
> support many functions for folks that want to put
> lots of lights on an engine. Also has the 21 pin
> interface so you can use Soundtraxx or TCS
> products with this board including ESU. Finally a
> mini board detaches from the main board for shell
> seperation. This is a game changing product and
> has been out for some time.
>
> https://nixtrainz.com/
>
> Time to move into modern times folks.
>
> Good luck.  Bruce

Thanks for the tip. I ordered 5 of these just now.  Was not aware!!
Thanks,
-Alex