Hi Guys,
I have installed Digitrax DH163D decoders in each of my 4 Athearn 2-8-2 Mikados.
I was running these engines on a temperary setup using a Bachmann ez-command
controller and all was fine. When I purchased a Digitrax Zephyr and tried running the engines,
I noticed the headlights on all 4 locos were hardly noticeable. I switched back to
the ez-command and the lights were fine. Almost forgot, I changed the LEDs in all locos
to grain-of-rice 1.5v 15mA bulbs and replaced the resistor to 680 ohms. Anyone have
any ideas? I programmed the engines with JMRI and a PR3 but that didn't seem to make
a difference. I'm thinking of trying a 560 ohm limiting resistor but shouldn't the lights
be the same brightness no matter what dcc system I use? Thanks in advance.
Jim
Hi Cacole,
I actually measured the voltage at the headlight. I got 1 volt for the Bachmann and 1/2 volt
for the Digitrax. I guess I'll just swap out resistors until I get 1 volt for the Digitrax. I just
figured that there was some kind of standard voltage when it got to the tracks, guessed
wrong! Thanks Cacole, I will test the track voltage to be sure.
-Dan
Builder of Bowser steam! Railimages Site
Hi Dan,
I actually measured the voltage after the limiting resistor and got 1v from the Digitrax
and 1/2v when using the Bachmann controller. You are probably right Dan, the voltage
out of the decoder is probably 12v.
I was under the impression that if using a 1.5v grain-of-rice 15mA bulb that a 680 ohm
resistor in series would work just fine.
You have the resistor value right, and the decoder's light output (blue wire) should measure 12 Volts DC before passing through the resistor. A variance in the brightness in the bulbs between two different DCC systems indicates a difference in power output of the two systems, and is not related to the decoder, bulbs, or resistors. The Amperage output of the two different DCC systems can also be a factor if you're drawing a significant load by having several illuminated passenger cars or other locomotives on the track and are close to overloading the DCC system, but since this is happening on a test track with nothing else running, that's not likely to be the case.
Your DCC system's output measured with an AC voltmeter should be in the range of 14.5 Volts, plus or minus .1 Volt; i.e., 14.4 to 14.6 to be within the NMRA recommended output. Keep in mind that the accuracy of your VOM can have an effect on the reading you're going to get. A $2 Voltmeter from a dollar store is not going to be as accurate as a $50 meter.
The input voltage into the DCC systems can also affect their output voltage. Perhaps you need a different power transformer for your Digitrax system, and make sure that it is within the range recommended by Digitrax. Measure the AC input voltage of both systems and see if there's a significant variation from what they recommend. You could be feeding too much voltage to the Bachmann system or too little to the Digitrax system, and this can affect their output voltages.
Hello Cacole,
I tested the 2 controllers with a Fluke VOM just before the limiting resistor and got 13.8v for the Digitrax and 17.4v for the Bachmann. The resistor gets rather hot on both systems with the lamp lit.
That voltage reading you're getting with the Bachmann is way, way too high, indicating to me that the Bachmann is putting too much voltage onto the track.
If the resistors get hot, you need a higher wattage rating. If you're using 1/8 Watt, change to 1/4 Watt. Ohms law formula for DC power is P=EI, so by that formula you're drawing 10.5 Volts x .015 Amps = 0.1575 Watts, which is more than 1/8 Watt.
OK, what if I got rid of the Bachmann (which I plan to do) why is the bulb so dim with the Digitrax? I have a BLI Big Boy and everything is fine with him. I have 118 volts at the plug, 13.8 volts before the 680 ohm limiting resistor, the blue wire has the resistor in series and the white wire is attached directly to the 1.5 volt 15mA bulb. All four (4) of my Mikes are wired the same and I'm getting the same results. The Digitrax Zephyr is only a week old. If I'm getting 13.8v shouldn't the light be even brighter than with 12v.?
I'm using 1/4w resistors
I have no personal experience with 1.5 Volt 15ma grain of rice bulbs except some replacement bulbs I purchased from Athearn a few years ago. I don't remember what their ma rating was, but they were inherently dimmer than other brands I have used. I prefer Golden Glow LEDs when replacing locomotive headlights, or the 12 Volt 30ma grain of rice bulbs from Micro-Mark, which require no resistors.
I think the bulbs you are using were receiving too much voltage when they were brighter, but they wouldn't last very long before burning out with that much overvoltage. My only suggestion at this point, other than using a different bulb or the golden glow LEDs, would be to experiment with lower value resistors.
This is where a device I found the schematic of several years ago called the Pot Box comes in really handy. See if you can find a small value potentiometer within the range of 1K to 5K, and wire it up to a decoder and one of these bulbs. Start with the pot set at its highest resistance, then gradually turn it down until the bulb lights at a level you think is sufficient. Then measure the pot's resistance with an Ohmmeter and use that value of set resistor, keeping in mind that the actual headlights on 2-8-2 Mikado engines were not very bright and most railroads did not use headlights in the daytime like they do today.
Thanks Cacole,
Great idea about the potentiometer. I think I'll change out the bulbs to 12v 30mA, cheap fix.
Also play around with the pot idea. Thanks again for all your help.