# Running DC loco on DCC, 'Zero Bit Stretching'



## wvgca (Jan 21, 2013)

I was running a DC loco, for basic functional testing, on an old MRC Command 2000 DCC system ..and thought that I would share my opinions on this type of operation ..

First of all, not all DCC controllers will support this type of operation ..As far as I'm led to beleive, Digitrax, Lenz, EasyDCC , older MRC systems will ..but NCE, Dynamis, and newer MRC systems such as the Advance2 do not ...One of the more common ways to select control of a DC loco on a DCC system is to select loco 0 [Zero]

The general DCC waveform can be described as a square waveform AC signal, with both positive and negative swings .. pretty straight forward, with a square spike of 58us for a 'One' bit, and at least 100us for a 'Zero' bit ....

And this is where the fun starts, 'at least 100us' ... and that can be considerably longer, and still remain a valid DCC signal ...

In the average DCC controller the frequency of the DCC signal can vary from around 7Khz up to 10Khz, all within the normal human hearing range .. but normally there is no sound noise present, until you try and run a DC loco ....

When you place a DC loco on a DCC powered track, you will quite likely notice a high pitched whine sound, and this is normal .. the DC loco motor is getting full DCC track power of maybe 13 volts, and it's switching back and forth from positive to negative at seven thousand times per second .. but as both positive and negative voltage swings are equal, it doesn't move ...

This is where the term 'Zero Stretching' comes into play ...if you make the positive portion of the Zero bit longer , sometimes well over 1000us, you create what is in effect a 'DC Offset', and the DC loco goes forward, longer zero bits make the loco speed up ...pretty simple, to go in reverse you just stretch the negative portion of the Zero bits instead..

And the tone of the audible whine changes as well, the longer the Zero Bit stretch, the lower the 'average' DCC frequency becomes , and the tone becomes lower as the DC loco is run at a faster speed ..

In general it's not harmful to run a DC loco on a DCC systems, for short time periods .. however it's also not advisable to leave DC loco on DCC track for long periods of time, as the motor is always receiving positive / negative voltage swings of full track voltage, it will normally heat up after a while, and get noticeably warmer sitting idle on a DCC track, much more so than running at high speed under normal DC power ..

In some cases a DC loco sitting idle for long periods of time may actually damage the motor, older 3 pole and newer 'core less' styles are more susceptible to this than the newer 5 pole [or more] can motors, just as a normal function of their design ..

In some cases running a DC loco on DCC controllers can also degrade DCC operation of other normal DCC loco's running at the same time ..as the 'Zero Stretch' packet to run the DC loco takes up a much larger time period than a normal DCC packet, there is much less available time to send out normal valid DCC packets to other loco's or additional DCC controlled devices such as lights or point controllers ..

This little test was done, basically out of curiosity, with an old MRC Command 2000 system, and a 20mhz dual trace basic scope..
Sorry, no pictures 
'


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## DonR (Oct 18, 2012)

I see you list 'easyDCC', did you mean Bachmann's
EZ DCC system? On it, you press the number 10 address
key to run a DC loco according to the manual. I
have not done so and won't for the reasons you mention.

Don


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## wvgca (Jan 21, 2013)

DonR said:


> I see you list 'easyDCC', did you mean Bachmann's
> EZ DCC system? On it, you press the number 10 address
> key to run a DC loco according to the manual. I
> have not done so and won't for the reasons you mention.
> ...


I found information referring to the EasyDCC system .. however if the Bachmann EZ DCC also has that capability, it's good to know, this means a user with that system can run a DC loco if they desire to ..

just as a note, the DC loco that I tried was an older blue box Athearn, and it took about fifteen minutes of sitting idle under DCC power for it to noticeably warm up the motor, I didn't leave it under idle / stopped power for longer than that .. 
The DC motor actually got warmer stopped than it did running at about 1/3 speed ..


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## Lemonhawk (Sep 24, 2013)

Must still be too wet for harvest? It is nice that your doing all these experiments we always thought about but never have done. Nice to know that DC on a DCC system is not a good idea due to heating! I have always heard that and now we know!


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## wvgca (Jan 21, 2013)

Lemonhawk said:


> Must still be too wet for harvest?


getting closer .. finally, canola berries may be ready to swath within a week maybe, and then wait again for maybe three weeks for it to completely mature and dry down ..

Today I'm pouring a new driveway for the house, 11'7" wide, 88'4" long ..no rebar, just fiberglass mesh toothpicks, got three helpers that better be here before the trucks show up at 11am


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## Lemonhawk (Sep 24, 2013)

Long ago an acquaintance decided to put some plastic lining in his pick up and then go the Ready Mix place and get a small load of concrete. He made it home just in time to save the truck, barely getting the concrete out of the bed (and not where he wanted it) as it became rock hard! Hope all your worker bees show up as more is a whole lot better than few to many.


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## Mark VerMurlen (Aug 15, 2015)

Thanks for the information, wvgca. Very interesting. Reinforces my view that I don't ever want to do this on my layout.

Mark


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## wvgca (Jan 21, 2013)

Mark VerMurlen said:


> Thanks for the information, wvgca. Very interesting. Reinforces my view that I don't ever want to do this on my layout.
> 
> Mark


Please don't take my opinions as an absolute "fact" regarding DC motor heating under DCC power ...the most temperature rise that I noticed was at 'idle', and was warm to the touch, warmer than running under straight DC at around 1/3 speed, but not uncomfortable to the touch, ..

As usual there will probably be variances depending on the supporting DCC systems used, , and as well on each individual loco, however I would not expect there to be huge variation 
..
In my limited experience, I would not completely rule out running DC loco's on DCC, but would be careful to not let them sit idle for extended periods of time..

I believe that there is some benefit to being able to run a newly acquired DC loco for basic functional operation, before investing in the cost and time involved to convert it with a DCC decoder .. I have a small DC 18" radius circle for this, but not everyone does ..


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## Tom17 (Jan 14, 2016)

I have the EasyDCC system and at the bottom of page 47 of the manual there is a footnote that address 0 is not usable and it slows the response of the system.


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## wvgca (Jan 21, 2013)

Tom17 said:


> I have the EasyDCC system and at the bottom of page 47 of the manual there is a footnote that address 0 is not usable and it slows the response of the system.


Thanks for the update ... EasyDCC off the list, EZ DCC on


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## Mark R. (Jan 26, 2013)

Most manufacturers are moving away from this option due to the potential damage to motors. Atlas even includes a low value resistor on the dummy plug of their DC engines which a DCC system will see as a short, preventing you from running the engine in this manner. Apparently they were getting tired of all the returned engines with burnt up motors.

Obviously this would be easy to bypass with a dummy plug that did not have the resistor on it, but I'm pretty sure it would void your warrantee if something happened.

Just because you "can" doesn't mean you should. 

Mark.


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## wvgca (Jan 21, 2013)

Mark R. said:


> Just because you "can" doesn't mean you should.
> 
> Mark.



True, I was bored and curious ... 
There have been other times when fear and/or common sense _should_ have stopped me


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