Do you need resistors for Ulrich Models LED Light with Fiber Optic?

Model Railroader General Discussion (Model Railroader)
1

Hi, just a quick question. Do you need resistors for Ulrich Models LED Light with Fiber Optic? The LED lights i’m talking about can be found here:

http://www.ulrichmodels.biz/servlet/the-608/LED-Light-with-Fiber/Detail

2

You can do one of two things, send an e-mail to the people you bought it from or take a 3 Volt battery I use a garage door opener battery 2032 and test it out. If it lights giving you a good strong beam chances are you need a resistor on the positive side of the light.

Kev

3

Based on prior experience with Ulrich (and generally most of those who sell LEDs), resistors are purchased separately unless it’s specifically noted they’re included. It doesn’t say that on the webpage and there a suggestion down towards the bottom that their resistor board would make a good match with the item you’re looking at.

There are lots of options in how resistors are used and which ones, so that’s up to you is generally the approach taken. It also requires the customer to know just enough to keep themselves out of trouble, of course, which is not a bad thing IMO

That said, I’ve alwasy found Ulrich to be very responsive to anything I need. A quick email should confirm this. They do have other options with resistors. The board is neat because it’s so flexible and basically gives you what you need for multiple lights on a single loco in a neat, easy to handle package.

4

The LED/fibre optic sets you showed will require resisters unless you will be driving them from an LED ready output.

Joe

5

As a general rule, LEDs always need resistors. Here’s why. Deep down inside, a Light Emitting Diode is an ordinary rectifying diode that happens to glow when it conducts. Diodes have no resistance, once forward biased, they switch ON, like a toggle switch. Current thru any circuit is given by Ohm’s law, I = V/R. When R goes to zero, the term V/R blows up, goes to infinity and infinite amounts of current flow.

In the real world, something will blow before infinite current is acheived, usually the LED, which pops at 20 milliamps or so. That’s not much current, a flashlight battery can do 20 milliamps.

So, LEDs need a resistor in series with them. Rule of thumb, 20 milliamps is the utmost LED’s can take. 10 milliamps will make all LEDs glow plenty bright enough for any model railroading purpose. I always pick a resistor to give me 10 milliamps LED current. If the voltage gets a little high, or the resistor is a little low, I have plenty of margin before the melt-a-LED current of 20 milliamps occurs.

They do sell LED’s with built in resistors. They are not common, but they do exist. And, some electronic circuits intended to drive LED’s have the needed resistor built in. They are not common either. You will blow out fewer LED’s if you always put in a resistor. If the LED or the drive has a built in resistor, the LED will still light, but it will be dim. In which case, you can remove the resistor. This case is not very likely, but just to cover all the bases…