Function output voltage of EZ-Command decoders

[spoorweg]

I am retrofitting an old Model Power/ROCO FA-1 to DCC using an EZ-Command 2 function decoder #44913 and while the instruction sheet indicates the maximum current for the function outputs (headlights) it doesn't indicate what the voltage is. I want to swap out the lamp with an LED. In order to calculate the correct resistor value, I need that info. I checked the NMRA website for the DCC standards and couldn't find anything there that made sense to me.  Does anyone know?  TIA   John.

[Jim Banner]

Strictly speaking, function outputs have no voltage.  They are pull down transistors - switches connected to the decoder's power supply's negative ("ground") side.  Current is supplied by the function "common" terminal which is typically the power supply's positive side.  As this is unregulated, its voltage depends on the output of the DCC booster you are using.  This in turn depends of what scale you are using and what booster you are using and can range from about 10 volts to 22 volts.

Fortunately, the resistance used with an LED can vary over a wide range and still give satisfactory results.  All that is required is a resistance high enough to limit the current to the LED's maximum current at the maximum voltage likely to be encountered and yet low enough to give satisfactory brightness at the lowest voltage likely to be encountered.

Suppose you are using a typical high efficiency warm white LED.  Maximum current is probably around 50 mA but in most applications will give adequate light at 5 mA.  Minimum resistance would be 380 ohms to limit current to a maximum of 50 mA at 22 volts and maximum resistance would be 1400 ohms to give at least 5 mA at 10 volts.  Something in between, say 820 ohms, would  give 8.5 to 23 mA over the range 10 to 22 volts.

Keep in mind the difference in characteristics between incandescent lamps and LEDs.  The light output of LEDs is a linear function of LED current while the light output of a small lamp varies as the ninth power of the voltage ratio.  This simply means that in the above example, the LED will produce less than 3 times as much light at 22 volts as it would at 10 volts.  To the eye, this is barely noticeable.  A small lamp, on the other hand, would produce 1200 times as much light at 22 volts as it would produce at 10 volts.

My Digitrax booster produces 14 volts when set for H0.  This gives 12 volts out of the decoder.  I use 1000 ohm resistors on virtually all LED installations.

[spoorweg]

Great! Thanks... that makes sense. I figured it couldn't be more than what was applied to the track but didn't know for sure how that worked.  I measured the track voltage for the EZ-Command station and it appears to be about 18 volts according to my multi-meter.  The LEDs came with 510 ohm resistors, which would probably work just fine with their current rating and it would still be under the 100 ma rating of the function output.  I could hook two in series I guess to double the resistance value.  That much I remember from electronics shop in high school.  Or I could just go buy some 1 meg ohm resistors!  Now if only I could figure out how to isolate the motor properly.... 

[Jim Banner]

...  I could just go buy some 1 meg ohm resistors!

Make that 1 k (1000 ohm) resistors and you are in business.  1 meg (1,000,000 ohm) resistors would make your lights a little too dim. 

[spoorweg]

aha. indeed 1K ohm... you'd think I'd I could do multiplication and division and know the difference between K and Meg being in the software business for my real job.  Thanks for your correction.  I used 2 of the supplied 510 ohm resistors in series and it works great... as does the decoder. My first DCC conversion is complete today!