LED's

[Eriediamond]

An earlier thread about loco LED head lights prompts this question. I've been modeling for years, but no experience with LED lighting. Question; are all LED lights or are they called bulbs, all the same voltage rating? For some reason I want to think they are rated at 1.5 volts but is this correct? I'm going to be building my next layout in N scale so I know I'll be getting into LED lighting and this will be completely new to me. Thanks, Ken

[rrinker]

LEDs have a forward voltage drop, which is how much voltage they will drop across them. At less than this voltage, the LED will not light. Typical red and green and yellow LEDs are 1.6-2.3 volts. The nice Golden White ones that make good headlights are more on the order of 3.5 volts.

However, what's far more critical with LEDs is the current. All LEDs have a maximum forward current rating, beyond which they will be destroyed. Technically, without a current limiter (usually a resistor), the LED 'runs away' passing more and more current until the junction is destroyed.

What you need to know is a) the voltage source you are going to supply b) the forward voltage of the LED you are using (Vf) and c) the LED current (If). You don't want to run the LED at the actual max current rating - a slight spike in voltage would then push it over the edge. I like to shoot for about midrange, that gives you plenty of headroom. For example, if the max rating is 20ma, calculate for close to 10ma, the LED will be plenty bright.

Now the math part (sorry, you just CAN'T get away from math - unfortunately your elementary school math teachers WERE right after all). You need to apply Ohm's Law to calculate the required resistence to limit the current. Ohm's Law says Voltage = Current x Resistence (V=IR). We know the V, and we know the I, we need to calculate R. R=V/I. V is the REMAINING voltage: supply volts - LED voltage. Here's an example:

12v power supply, 3.5v LED drop, and we want 10ma current.

R = V/I
R = (12 - 3.5)/.010 (10ma = 0.010 amps)
R = 8.5/.010
R = 850 ohms

Google 'LED Calculator" and there are a million pages with calculators to help with this. Here'sone on Rob Paisley's site: http://home.cogeco.ca/~rpaisley4/LEDcalc.html

Class dismissed!

--Randy

[scubaterry]

Great info thanks for making it easier.
Terry

[Eriediamond]

Thanks Randy. My electrical knowledge is about grade school level or less. I did complete one of those old Heathkit radios back in the early 50's once and it worked [:O][(-D] Thats why I like the straighr DC instead of DCC for my layout. I' one to have trouble with the TV remote. Thanks, Ken

[rrinker]

Ah but see you can treat DCC like little black boxes - in which case it's EASIER than DC - you don't need all those toggle switches and blocks and wires running all over the place, just a nice heavy gauge bus and simple feeder drops from the track. And even to distill down decoder installation - it's really only 4-7 wires, depending on how many lights you have. 2 to the track, 2 to the motor, and 2 for the headlight. There's no need to get caught up in what all those little bits on the circuit board do, or just what the DCC signal looks like.

--Randy