Voltage Symmetry of DCC Boosters

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There are a number of DCC Decoder manufacturers, Lenz being the most recent, who are implementing a signal control system (which is being evaluated by NMRA) that depends on a slight assymetry of the DCC voltage. This assymetry is detected by the decoder and triggers some function like a stop at a red signal. The DCC signal is typically made assymetrical by inserting a number of switchable diodes in a rail section.

This concept obviously will only work if the DCC signal from the booster itself is symmetrical in the first place with fairly tight tolerances. I have found nothing in the NMRA DCC spec that adresses the symmetry of the DCC waveform and the tolerance required. In fact, one of the DCC Decoder suppliers with this feature actually states that it will only work with Lenz boosters. My question, is there a 'voltage symmetry' requirement in the NMRA spec., if not is someting along those lines contemplated?

thanks, Knut

-- Knut Schartmann (knut@kos.net), October 19, 2004


Well, actually there is. If the basic DCC waveform is not symetrical, then there is a DC component, which of course is also how one operates an 'analog' loco on DCC powered track. This normally is accomplished with 'stretched zeroes' or uneven + and - duty cycle times of the 'square' waves. But adding a DC component to the track voltage waveform can be accomplished also by altering the magnitude of either polarity pulse or simply adding DC in series with the AC DCC signal.

Direct addition of DC [the latter method] can produce excessive voltage magnitude that may amage existing equipment.

Polarity sensitive voltage subtraction (diodes in series with the track circuit will do that) if done in moderation will not disturb existing DCC decoders but can be detected at new ones as a DC component being or not being present. If detected by the docoder, it can be useful to cause an automatic function like whistle, or slow- down and stop. [May not be compatible with the stretched zeroes approach of running an analog loco.]

There are several systems out that do that. None are presently 'blessed' by the NMRA.

-- Don Vollrath (dvollrath@magnetek.com), October 21, 2004.

I don't disagree with anything you said, but it doesn't answer the question. I could find no requirement in any of the NMRA DCC specifications that defines the DCC voltage symmetry tolerance that is acceptable. If this is in fact specified, I would appreciate it if you could point me to the NMRA reference document and relevant paragraph.

Thanks, Knut

-- Knut Schartmann (knut@kos.net), October 21, 2004.

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