Loco lighting without a decoder...is it possible?

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CbarM

HO all the way!
Im curious if its possible to have loco lighting in a dummy without a decoder? Ive been using 0402's and wondered if I could wire em to a rail wiper with a 1000K ohm resister?
 

Greg@mnrr

Section Hand
It's possible to run two light gauge wires from the lead loco to the dummy to power the headlights.

Greg
 

Red Oak & Western

Active Member
That's a maybe. First, since you mentioned "a dummy without a decoder", I assume you're talking about a DCC system. So the rail voltage is from 15V to 24V depending on your system and settings. Next, you'll need the datasheet on your LED. The first one I checked runs at 3.00V max with a current draw of 126mA. With 15 volts to the rails, this would require a 96 ohm 1.5 watt resistor. A 1000K (1Meg ohm) or even a 1000 ohm resistor would drop the voltage below the LED's minimum operating voltage.
 

CbarM

HO all the way!
Im using the SM0402's that I solder magnet wire onto. These I believe are 3.2V max and yes, it is an NCE DCC system. So I would be pretty close with a 100 ohm resistor for each?
 
That's a maybe. First, since you mentioned "a dummy without a decoder", I assume you're talking about a DCC system. So the rail voltage is from 15V to 24V depending on your system and settings. Next, you'll need the datasheet on your LED. The first one I checked runs at 3.00V max with a current draw of 126mA. With 15 volts to the rails, this would require a 96 ohm 1.5 watt resistor. A 1000K (1Meg ohm) or even a 1000 ohm resistor would drop the voltage below the LED's minimum operating voltage.

Whoa! For the small LED's we use in locomotive lighting, 126 ma is a LOT of current. Most of them have a max of around 20ma. Also, the current rating of an LED is it's MAX rating, you generally want to run them at less than that. Additionally, the resistor does not drop the voltage, the LED drops the voltage (for the most part independent of the resistor value) and the resistor limits the current. A 1000k resistor would probably be too much, but a 1k would likely be about right (15v - 3v / 1000 ohm = 12 ma); although, with high efficiency LED's many modelers use 10k or even higher.

Im using the SM0402's that I solder magnet wire onto. These I believe are 3.2V max and yes, it is an NCE DCC system. So I would be pretty close with a 100 ohm resistor for each?

100 ohm would likely blow the LED (see above), I'd start with 1k. I would also put a regular rectifier diode in series with the LED to protect it from reverse voltage. Many people skip this step and depending on the LED you may get away with it, but they really are not designed to withstand high reverse voltages (high for an LED being anything above 5 volts) and you might get reduced life of the LED without the reverse voltage protection.
 

Red Oak & Western

Active Member
Whoa! For the small LED's we use in locomotive lighting, 126 ma is a LOT of current. Most of them have a max of around 20ma. Also, the current rating of an LED is it's MAX rating

That's why I said the OP would need the datasheet for the LED he's using. The data sheet I looked at specified 15 to 20 mA at 2.00V to 2.40V for the red, green, and blue LEDs (which is what I expected), but 70 to 180mA at 2.80V to 3.04V for the white LEDs (which I didn't expect). I used 126mA as a mid-point. They're probably the "super bright" LEDs that appear to be coming out all over the place, so you need the data on the specific LED.

Additionally, the resistor does not drop the voltage, the LED drops the voltage.

Wrong. Please review Kirchoff's law.

Added: I just found another datasheet for 0402 white LEDs that specifies a maximum current of 5mA! Which would use a minumum 3000 ohm resistor.
 
Last edited by a moderator:
...rong. Please review Kirchoff's law...

I did not word my previous reply the best that I could, but to clarify, I am not saying there won't be a voltage drop across the resistor, but the resistor value has little effect on the voltage drop across the diode. It's not like an incandescent lamp where you know the current and you adjust the voltage with the resistor, with an LED you know the voltage (the drop across the LED is fairly constant across a wide range) and you adjust the current with the resistor. Increasing the resistance does not reduce the voltage to the LED, e.g. if you have a 3 volt LED and 15 volt supply, the voltage drop across the LED is going to be ~3 volts and the voltage drop across the resistor is going to be ~12 volts, whether you use a 1k or a 10k resistor.
 

Red Oak & Western

Active Member
Without turning this into an electronics discussion, Robert is correct that the resistor is going to have little effect on the voltage drop across the LED. Unlike many devices, LEDs operate over a very narrow voltage range and have very specific current requirements. That is why it is so important to know the parameters of the specific LED, especially now that there are so many variations of the device know as an LED. Back in the day (I love being able to say that), it was a 5mm red LED that ran 2.0V at 20mA, and that was that.

Back to the original post, I would really suggest getting a basic DCC decoder for around $14 to $15. That would allow the lights to be turned on and off as desired, instead of being on whenever the layout is powered up.
 

MikeOwnby

Active Member
I'd agree with Kevin, just for that ability to turn the light on and off and have it act like it's any other locomotive even without a motor. For unpowered units I usually stick in a Digitrax DZ123. Simple, small, and inexpensive, but it gives you actual control over the lighting. Other manufacturers may have a cheaper unit that does basically the same thing.
 

Mark R.

Custom Painter
Something else to consider is light flicker from the LEDs. You think light flicker is bad with bulbs ? .... LEDs are a thousand times worse as they react instantly to even the smallest voltage loss. If you are going to run LEDs in ANY unpowered equipment (engines / passenger cars / cabooses), I seriously recommend install a stay alive cap. You can easily build you own for pennies. Don't need to go high tech with super caps - a single 25 volt rated cap with as high a mfd. as you can physically fit will run a few LEDs for quite a while. A diode and a resistor and you're done with flickering lights.

You should install a bridge rectifier from the rails though to make sure the cap always sees the correct polarity.

My DCC track voltage is 13.8 volts, so my first cap used was rated for 16 volts - which I assumed would be adequate .... it wasn't. It promptly exploded minutes after being powered up. After changing to a 25 volt, it's been working for over three years now.

Mark.
 

tootnkumin

Well-Known Member
Staff member
You'd better point us in the direction of a schematic for these "bits" Mark. I have a good few LED lighted passenger cars.
 

ronp849

New Member
If the track the Loco is on can be isolated from the DCC, you could feed the LEDs through an externally mounted series resistor and control switch from a 5v supply.

Ron
 

crusader27529

New Member
Two things.....use this site to calculate resistor values:

http://ledcalc.com/

Second, most LEDs have a peak inverse voltage in the 5-6 volt range, so just connecting an LED to the DCC track power will quickly destroy the LED regardless of the value of the resistor chosen. The solution is to rectify the track voltage and then apply it to the LED/resistor, with the correct polarity of course.
 




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