LED & Resistor W size

[serenityFan]

Hi, another electrical question. I am helpless in this department  :(

I am trying to install an LED to a loco running DCC.

I doesn't originally have any lightings at all, so I am planning to install an LED and a resistor (Correct?) and connect it to the white/blue wires on the decoder.

I read that I would need a 1k ohm resistor to limit the voltage. The bigger the resistor, the less bright the LED will be.

Ok, the question is what does the wattage in the resistor do? I think I bought 1k ohm, 5W resistors. Now I just read that you actually need 1 W resistors.

How will this impact the light? Will it work at all?

[CaptOblivious]

UPDATE I've done a little reading, and I see now that I was wrong at first. I've corrected the errors in the post below.

The wattage is a measure of power consumption; in this case, it measures the maximum amount of power that the resistor can tolerate without catastrophic failure. A 5W resistor can handle quite a lot of current, but under those conditions it can get quite hot. It is overkill for your needs, as your little LED will only draw a miniscule amount of power, but it will work just fine. Typically, one only needs maybe a 1/2W resistor for our sorts of projects.

The resistance (measured in Ohms) does this. The decoder supplies 12V. The LED wants (depending on the LED) say, 1.5V. If you ran 12V through the LED, it would burn very bright for just a moment, and then release its magic blue smoke. So you need something else to take up the slack (as it were). A resistor does that. If the resistance is too small, the LED burns out; this is unacceptable. If the resistance is too high, the LED may burn dimmer (which may be OK), or it may not come on at all (which is also unacceptable). The brightness of a bulb varies proportionally to the voltage, but not so for LEDs; unless you have a datasheet with the voltage response curve, the effects of undervoltage are not really predictable.

So, conclusions: 5W:OK, 1000Ohm:maybe.

This calculator will help you figure the resistance and minimum wattage you need:
http://led.linear1.org/1led.wiz

Source voltage = 12V
Diode forward voltage is the voltage on your LED: likely 1.5 or 3V
Diode forward current is the current draw of your LED in milliamps, probably in the low two-digits.
It will tell you exactly what resistor you need, including how to visually identify it! :D

[serenityFan]

Thanks, at the end it is quite anti-climactic as when I got home and I checked my resistor, they are actually 0.5 W instead of 5W. I'm installing some lights in my OO flying scotsman, this loco is now like my guinea pig, I try things out first before I do things on my n-scale stock ...  ;D

[serenityFan]

While trying to add an LED to a loco, I have learned the following:

1. Putting the LED + resistor and testing on the chassis is the easy part, putting the shell back together is the difficult part.
2. The wire that I'm trying to use is too thick. In the end, I can't put the wires through where I need it to go. Need thin wires.

Which leads to the question: what do you call the thin wires, like the ones you get attached to dcc decoders? What diameter size are they?

[CaptOblivious]

Wire (in the US) is measured by gauge (like tracks!): The smaller the number, the larger the wire—it's a measurement of how many wire diameters will fit in an inch. How archaic! House wiring is done with 12 or 10 gauge wire. 10 gauge wire is almost impossible to bend by hand if the lengths are small. Most decoders use 30 gauge wire.

[alpineaustralia]

Capt - did you say that a decoder (say a Dz125 or 143) puts out 12 volts?
Do you know what current it puts out. The Digitrax site seems to indicate 125mA to 200mA for all functions and so I would estimate only about 30 - 50mA.
Is this correct thinking?

[CaptOblivious]

Capt - did you say that a decoder (say a Dz125 or 143) puts out 12 volts?
Do you know what current it puts out. The Digitrax site seems to indicate 125mA to 200mA for all functions and so I would estimate only about 30 - 50mA.
Is this correct thinking?

The function output depends on the track voltage. For N-scale, the NMRA standard is 12V, but some throttles may put out 14 or 16V!! My Digitrax Zephyr uses 12V, and as our Japanese trains are rated to 12V max, you should be sure your throttle is doing the same.

The current rating is just a maxium. The actual current will depend on what's attached. An LED+resistor might draw 20-30mA, well within any decoder's max rating. Many bulbs also draw 20-70mA, but the inrush current can cause spikes of up to 10x the steady-state current draw. That 70mA bulb can draw up to 700mA when it turns on, way beyond the specs of any decoder!