What is the LED voltage for lightng from DCC decoders?

[Epimetheus]

Please be patient with me. The question might be phrased entirely wrong. Essentially, I am trying to select LEDs to work under DCC control.


Do DCC decoders deliver any particular voltage to lights? Or do they pass through the track voltage?

• I am changing over all the HO scale motive power to DCC.
• I am changing the headlights and tail lights in HO locomotives from incandescent bulbs to LEDs.
• Normal HO DC locomotives run on zero to 12V.
• DCC runs on 15V AC (OK, not real sine waves but it alternates)
• LEDs to my understanding are rated at operating voltage and current.
• Therefore, I should choose LEDs that can hander the proper voltage,
• and throw in a resistor to limit the current

 

[CSX Robert]

Most decoder outputs are track voltage minus about 1.5 volts (due to the voltage drop of the bridge rectifier). Some decoders do have either a regulated output or a pre-installed resistor on some outputs, so you really need to check the particular decoder's spec sheet to be sure.

 

[Bruette]

I am by no means an expert on DCC, but I do know the motors run off of rectified DC. The decoder has a built in rectifier to power the drive motors.

I don't know what the output to the lights are. They may be covered under NMRA DCC specifications, but I don't know about that.

To be on the safe side check the specifications of your decoders, then you can't go wrong. If you can't find them, contact the decoder manufacturer.

FYI: Track power is complex sine wave AC http://www.dccwiki.com/DCC_Power There are other sources of information available as well.

If you would let us know what you learn, I will be interested to know.

 

[Bruette]

Robert you beat me to the punch

 

[Epimetheus]

I will measure voltage after installation

Most decoder outputs are track voltage minus about 1.5 volts (due to the voltage drop of the bridge rectifier). Some decoders do have either a regulated output or a pre-installed resistor on some outputs, so you really need to check the particular decoder's spec sheet to be sure.

Thank you! Now I know it could be pretty much anything. I will measure the voltage and polarity after installing the decoder.
-Doug

 

[Iron Horseman]

Do DCC decoders deliver any particular voltage to lights? Or do they pass through the track voltage?

Depends on the decoder. Many have built in traces that you can cut or jump to change the voltage outputs to the lights.

I will measure the voltage and polarity after installing the decoder.

The only reason you need to know the voltage is so you can use ohms law and calculate the size of the needed resistor. Diodes are not devices driven by voltage, they only care about the current (hence the resistor). Even calculating the resistor size is often an unnecessary exercise. I have run across very few instances in model railroad applications where a 1000 ohm resistor doesn't work great. The only reason one would want to calculate the exact lowest value needed for a resistor is if you want the LED running at maximum brightness. I like running things well under their max specifications so they last longer. Besides, brighter isn't always better.

Even if the voltage output on the decoder is 1.5V, you need to check to make certain that the decoder manufacturer intended that to be used for an LED.

Things advertised as "12 V LEDs" are really an integrated circuit that has an LED and a resistor on one set of leads. The resistor has a value that has been calculated to limit the current to what the diode can handle at 12V.