L.E.D.'S

[BNENGR]

Hello Group,

I'm enjoying our new forum already! Okay, I purchased some L.E.D.'s and Incandescants for loco lighting, building lights and signal lights. I have some 3mm - 3-4 volt with resistors (2 sizes), some 5mm's w/resistors, 3mm multi colored, and 12 & 16 volt incandescants.

I just tried for the heck of it to see how the lights look hooking them up to a DC transformer putting out 18vac with the recommended 470 ohm resistor and the resistor got fairly hot and did so the l.e.d. wires. The l.e.d. itself just got a little warm. Is this normal for them to get hot?

They say they're 3-4 volts. Was that the problem? How do you get 3-4 volts of power for these lights? The 12-16 volt bulbs are no problem. I guess I need help figuring out which l.e.d. to use for the application needed and what size resistor. Is there a chart or something out there? This is somewhat confusing to me. I would like to replace the old crude bulbs in my locos with the "yeloglo" white L.E.D.'s and possibly install ditch lights too.

Thanks,

Paulie

[Alantrains]

Hi Paulie,

For 18volts you need to drop 14.5 volts across the resistor to leave 3.5 volts for the LEDs (White LEDs I assume). LEDs normally need about 20 mA to light up.

Ohms law states  Voltage =Current in amps multiplied by Resistance in Ohms

14.7 volts = 0.020 Amps X  ?Ohms

 which works out to 735 Ohms

So the resistors are not big enough for 18 volts (they work out just right for 12 volts). 

The problem with the 425 ohm resistors is that you have increased the current to something like 14.5 / 425 = 0.035 A (35 mA)

Another equasion is Power = Voltage multiplied by Current

so the power in the resistor would be

14.5 X 0.035 =  0.45 of a Watt (The unit of power)

The power in the LED would be

3.5 X 0.035 =  0.12 of a watt

both of these are  low power but even a 0.5 watt resistor would get very hot while dissipating 0.45 watts. Half, quarter an eighth watt resistors would not last very long (maybe a few hours).

I suggest you add another 425 ohm resistor or change to 1,000 ohms

the power would drop to 0.29watt and the LEDS would still be bright and cool.

Hope you can understand this 

[BlueHillsCPR]

Hi Paulie,

For 18volts you need to drop 14.5 volts across the resistor to leave 3.5 volts for the LEDs (White LEDs I assume). LEDs normally need about 20 mA to light up.

Ohms law states  Voltage =Current in amps multiplied by Resistance in Ohms

14.7 volts = 0.020 Amps X  ?Ohms

 which works out to 735 Ohms

So the resistors are not big enough for 18 volts (they work out just right for 12 volts).

Great answer Alan!  Like you I smelled a rat as soon as Paulie mentioned 18V as his supply voltage.  Really good explanation I thought.

This might be a good heads up for others too.  When you are buying LED's that come with a resistor you may want to ask about supply voltage.  It never occured to me that the package would assume 12V as the supply voltage but it seems that is the case. 

Here are a couple of online calculators to help with resistor selection for LED's

Single LED calculator

Series-Parallel Array Calculator 

[BNENGR]

Hi Alantrains, Thanks for the info.

How many volts on average do most locos on DCC run at? I need to know this in order to properly install L.E.D.'s in them.

Is it safe to say that if I add a 1K ohm resistor to all my L.E.D.'s for any application it should work okay without any problems?

Anyone else in the group please kick in your 2 cents worth too.

Thanks alot guys,

Paulie

[fwright]

Most LEDs that we use in model railroading have a maximum current rating of 20ma, and will turn on at less than 10ma, with brightness increasing with the current.  So you want to size resistors for less than 20ma through the LED with the highest voltage you are likely to see. 

DCC voltages on the track vary from 18v to 12V; most seem to be about the 14.5 volt area.  Some DCC systems allow you to select track voltage.  If the lighting circuit passes through the decoder front end bridge (pretty much has to for DCC control of lighting), the voltage available is dropped by about 1.4 volts.

1K is a good safe bet in most all DCC locomotive applications for LEDs.  The current will almost always be within the working range of the LED.  Only go with less if you absolutely must have brighter LEDs.  You might need slightly more than 1K if you are using the tiny surface mount LEDs in class and marker lights.

my thoughts, your choices

Fred W 

[Texas Zepher]

How many volts on average do most locos on DCC run at?

Whatever the DCC system gives them.   This is a function of the DCC unit.  I have a LENZ that allows me to choose the voltage.   I have mine set at 13V.  Our club has theirs set at 14.5V.  I believe they came from the factory set at 17.5V. which is way too high in my opinion.

I need to know this in order to properly install L.E.D.'s in them.

Check the manual of your DCC unit.

Is it safe to say that if I add a 1K ohm resistor to all my L.E.D.'s for any application it should work okay without any problems?

Pretty much it is safe to say that.  Personally I use 1200 & 1500 ohms.   But depending on the LED (some are now comming with built in resistors) they might not be at full brightness.  Sometimes this isn't a bad thing.  One doesn't want caboose marker lamps to be glaring like a headlight, nor does one want the indicator lights on a control panel lighting the whole room.

The thing here is that LEDs are not "voltage" governed things but "current" driven. They are diodes first that just happen to have a side effect of emmitting light.   Once a diode gets enough voltage to pass current it will pass all available current until it burns out.  The higher the voltage the more current is crammed through.  That is why the resistor is simply limiting the amount of current.  If a resistor is getting hot then it is too small, and probably passing way more than is needed to light the LED.

[Alantrains]

Hi again Paulie,

I don't have DCC but the others have covered most of that.

If you're running the LEDs off a DCC decoder it depends on whether the decoders outputs are set up for LEDs or 12V bulbs.(It should tell you in the decoder instructions)

From what I understand the outputs of DCC decoders are not the same voltage as the inputs, so 18volts track voltage may not produce 18V for lights, just 12 volts.

If however you are running the LEDs straight off the track voltage you need to use the bigger resistors as BNEGR suggested.

cheers 

[Don 2816]

Paulie,

There is an excellent article about installing LEDs at Tony's Train Exchange. If you search there for LED, the following article will be listed:
http://www.tonystrains.com/technews/install-leds-decoders.htm
It's titled Installing LEDs with DCC Decoders.

I've used 1000 ohm 1/4 and 1/2 watt resistors with excellent results on Digitrax DCC.

Don

[Johnnny_reb]

Hello Group,

I'm enjoying our new forum already! Okay, I purchased some L.E.D.'s and Incandescants for loco lighting, building lights and signal lights. I have some 3mm - 3-4 volt with resistors (2 sizes), some 5mm's w/resistors, 3mm multi colored, and 12 & 16 volt incandescants.

I just tried for the heck of it to see how the lights look hooking them up to a DC transformer putting out 18vac with the recommended 470 ohm resistor and the resistor got fairly hot and did so the l.e.d. wires. The l.e.d. itself just got a little warm. Is this normal for them to get hot?

They say they're 3-4 volts. Was that the problem? How do you get 3-4 volts of power for these lights? The 12-16 volt bulbs are no problem. I guess I need help figuring out which l.e.d. to use for the application needed and what size resistor. Is there a chart or something out there? This is somewhat confusing to me. I would like to replace the old crude bulbs in my locos with the "yeloglo" white L.E.D.'s and possibly install ditch lights too.

Thanks,

Paulie

If I read this right you have the leds hooked up to 18 volts "AC". Leds use "DC" not "AC". Please correct me if I'm wrong.

[gandydancer19]

LED's should be used on DC power (except for bi directional types) as they are not really designed to be rectifiers, although a lot of people do it and don't have problems.  Also, if you get the newer Golden White type LED's you should use a larger value resistor.  They use less current.  I use a 5K resistor with Golden White types.  The manufacturer should give recomendations on resistor value.

Elmer.