I understand that with DCC systems, motors are driven with a programmable waveform known as Pulse-Width Modulation (PVM) in which the maximum voltage is applied to the motor for some percentage of time. To someone viewing the locomotive, the motor seems to behave using PVM like it does using DC -- the higher you set the throttle, the faster the locomotive goes.
Here's my question. When testing decoders set to factory defaults, should a voltmeter measuring across the gray and orange motor wires read from zero volts to about 12 volts as the throttle moves from zero to 100 percent? The reason I ask is that one brand of decoder I purchased does just that while a second gets to 8 volts at mid-throttle and then stays there as the throttle moves to 100 percent. Both are set to factory defaults. When the second decoder is installed in an n-scale locomotive, the motor only gets to half speed and then goes no higher. Does anyone have any idea of what is going on here? Any help will be greatly appreciated. If more details are needed, please let me know. Thank you.
try forcibly resetting to factory defaults. Sounds like the VMAX (CV5?) is set to limit the max speed.
-Dan
Builder of Bowser steam! Railimages Site
NeO6874 try forcibly resetting to factory defaults. Sounds like the VMAX (CV5?) is set to limit the max speed.
Thanks for the reply. I tried setting V-mid (CV6) and V-max (CV5) at different levels. That did not solve my problem. I also did a decoder reset (CV8 to 008). Over the last two years I have installed six of these decoders in n-scale locomotives with no problem. Last month I purchased two more for two new locomotives. Both had the problem I described. I returned them to the manufacturer and they sent me two new ones -- with the same problem. I sent these back and they again sent me two new ones -- with the same result. I then talked to a tech support person with the manufacturer who said he tested one he had at hand and it drove an HO scale locomotive just fine. He sent me the one he tested -- like the others, it does not put out more than 8 volts to the motor. I am stumped.
Measure your track voltage using the AC range of the VOM. It should read approximately 14.5 volts.
What brand of DCC system are you using? It's voltage output may be low.
You mentioned N scale sound decoders -- are these by any chance MRC? CV 8 is not the reset command for MRC decoders. CV 125=1 to reset MRC, and the documentation I have does not even list CV5 (VMax) or CV6 (VMid) as being ones supported by MRC..
cacole Measure your track voltage using the AC range of the VOM. It should read approximately 14.5 volts. What brand of DCC system are you using? It's voltage output may be low. You mentioned N scale sound decoders -- are these by any chance MRC? CV 8 is not the reset command for MRC decoders. CV 125=1 to reset MRC, and the documentation I have does not even list CV5 (VMax) or CV6 (VMid) as being ones supported by MRC..
The VAC reading for the track is 15.0 volts.
I am using an NCE Power Cab.
The decoders are Digitrax DZ125.
Okay, the problem does not appear to be with your track voltage, NCE Power Cab, or the Digitrax decoder. My guess is that the locomotive's motor is drawing too much current, which doesn't allow the decoder's output to reach it's full output voltage -- but under these conditions the decoder would most likely get very hot and destroy itself.
Did that particular N scale locomotive run okay on DC power before you put the decoder into it?
The Power Cab has low amperage output and could be getting near overload from the current draw of that particular locomotive, but the internal circuit breaker should shut it down if that's the case.
It may be a combination of factors -- heavy current draw of the motor, overload on the decoder, and overload on the Power Cab causing low voltage output.
Try measuring your track voltage with the train running at full throttle and see if the voltage is dropping. If it is, then the Power Cab needs to be beefed up with a booster or heftier transformer, and it would also be a good idea to take a closer look at the locomotive to see why it's drawing so much power.
Turn off the BEMF and try again. CV57=0. I wonder if they are trying new BEMF code across the line - as there is a similar problem with some of the new sound decoders, even worse in that if BEMF is turned on and you hit the horn,t he loco stops. Turn off BEMF and it's fine.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I forgot about the BEMF problem. I installed a TCS M-1 decoder into a steam locomotive and it ran just fine on DCC with CV 29 programmed to disable DC operation. When the original owner sold it to another club member who wanted the decoder reprogrammed so he could take it home and run it on DC, it began to perform very erratically, lurching and periodically stopping. I turned BEMF off and it now runs smoothly on both DC and DCC.
rrinker Turn off the BEMF and try again. CV57=0. I wonder if they are trying new BEMF code across the line - as there is a similar problem with some of the new sound decoders, even worse in that if BEMF is turned on and you hit the horn,t he loco stops. Turn off BEMF and it's fine. --Randy
I set CV57 to 0. Factory default is 6. Now the meter reads 12 volts across the motor wires (instead of 8) at full throttle and the test motor I clipped the decoder to runs noticeably faster.
I'm ready to install the decoder into the original locomotive -- turning off the BEMF seems to have solved my problem. I'll confirm the fix after I get it soldered in and running. Thank you very much Randy.
Bob
I have no problems with TCS and BEMF, which is why I haven't used anything but TCS decoders for the past coupel of years (discounting the Tsunami that came int he Bowser replacement sound chassis for my FT). Not sure what Digitrax is doing with their decoders lately, because otherwise the DZ125 is a nice tiny and fairly low cost option for tight spaces.
I just finished installing the decoder after turning off the BEMF. The locomotive runs just fine now. Thank you again for the great advice.
Edit: Never mind
Nice going Randy!
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!