After doing some N scale ditchlight installations with separate SM LEDs for each light, I noticed 4--680 ohm resistors on the narrow neck of DN163K1B and K1C decoderss. Wondering if these were added function resistors for F1-F4, I got a confirmatory email from tech support at Digitrax. So, unlike the instruction manuals, you do not need to add resistors when adding extra lighting to these decoders, they have solder pads next to each resistor. Would save a lot of extra work for those who use these extra functions. I'm not aware that the company has ever advertised or even acknowledged the existence of these resistors and pads.
I have to say this doesn't initially make much sense... Did you just get confirmation that they are resistors, or did you get confirmation that they are intended to obviate the need for resistors in LED (or other function) circuits? Are there separate solder pads for using the built in resistors? ...because if these resistors are added in series with the function output, and you then add another resistor in series with an LED, your LED wouldn't light. Please clarify if you can.
I hope ram53 doesn't mind me answering for him. Yes, there are separate solder pads for the resistors. The resistors are not actually on the function outputs, but connected to '+' common. There are four extra resistors on the decoder, one for each extra function, and each one has one side connected to '+' common and the other side connected to a bare solder pad.
I'm guessing he's talking about the 4 small pads located next to the resistors on the narrow portion toward the front
Seems like additional function outputs with integral 680 ohm resistors. If you add more resistance you should only dim the LED. There isn't much difference between 680 and, say, 1680 ohms in light output (depending on voltage, of course).
Thanks for the clarification Robert, all clear now. You're right, the LED would still light. But there could lot of difference in luminosity. (And I like my lights bright!)
Actually LED's are funny creatures, they have a "range" that they like to function in if you're in that range the LED will work well. For a given LED there is almost no intelligible difference in brightness between one with a 660ohm Current Limiting Resistor and an LED with a 1320ohm Current Limiting Resistor. Even sitting right next to each other it's hard to tell which has the higher limiting resistor.
I want to make this clear so that no one tries using these incorrectly. These resistors are NOT on the function outputs, they are connected to '+' common. If you connect an LED between a function output and '+' common, it will not use these resistors, and if you don't include your own it will blow the LED. To use one of these resistors, you connect between a function output and the pad next to one of these resistor - not the one marked '+' common.
Like this? Offhand, I don't recall the exact symbol and polarity for LEDs, so I just put a generic blob load. in the image.
Actually, I wouldn't mind clarifying the polarity question on digitrax decoders. It's confused the heck out of me in the past. My electronics guru friend tells me that digitrax labels things opposite of the normal convention. Or maybe that's me being further confused.
I've never had a polarity problem. The blue is the common positive voltage for functions, and would connect through a resistor to the anode of a LED, the cathode of which connects to the function (negative) lead. For the decoder illustrated, the + common (blue) solder pad is labeled, but the 4 solder pads above it (each connected to + common via a "681" (680 ohm) resistor) are not. These are the "undocumented feature". If you need more than 680 ohms, you would have to use external resistors. BTW, the diagram for a diode is as follows: Anode ---->|--- cathode The cathode is usually marked with a band, a notch or other identifying feature.
I wonder if what he is talking about is the blue "common" decoder connection. Many people assume '-' when they see "common," but on DCC decoders(all of them, not just Digitrax) the "common" lead is '+'.
For those who think of "common" as ground or the negative of the power supply, consider the positive side of the supply. It is also a "common" (for those with an electronics background, that would be the positive supply rail or "Vcc"). Besides, Digitrax has always labeled the blue lead as "+ common".
Actually, now that I've looked back at the locos I got confused over, I think my confusion was simply over interpreting the tiny markings on some SMD leds. Thought the anode was marked as the cathode, got it backwards. Then got really confused trying to figure out why they didn't work. In any case, it's nice to go over this stuff again, I've only installed a few LEDs on decoders so far.
