Re: DCC

Flash Gordon
 

Thanks Mark,

I knew someone had to bring up inductance vs capacitance. And you did a fine job.

Ed S

At 04:29 PM 3/1/2014, you wrote:


From a track bus DCC signal point of view.

Wire Inductance is far worse and larger in magnitude per length of wire than wire capacitance. Inductance causes far more DCC signal distortion and at worse blows up decoders with the voltage spike it can created under a short circuit with long wires. All very observable with a scope.

Capacitance between wires of the track bus does not cause much signal distortion and given the DCC signal does NOT have truly fast signal rise and fall times. There is very little "leaking" current flow between the wires.

Placing the two track bus wires next to each other allows one to greatly reduce the inductance at the expense of some increased capacitance. Twisting keeps the two lose track bus wires close to each over the run to maintain them closely together with the side electrical benefit of reducing the "transformer coupling" effect between this track bus wire and other non DCC wires. Twisting is a form of "shielding" the track bus which prevents coupling both ways...in and out.

The only time wire capacitance can become a problem is with the use of current based "Occupancy" or "Block" sensors used for signaling. The occupancy sensors are by design meant to be very sensitive to small levels of current flow so they can detect the resistive wheel sets on freight cars. To much leaking current can create a false occupancy indication. Most people by default reduce the sensitivity of the sensor to eliminate the false occupancy detection. However this may result in the sensor no longer reliably seeing the lone freight cars sitting on the track it is monitoring for occupancy. This is not unlike a "signal to noise" ratio problem electrical engineers deal with in signal processing. The wire capacitance creates "current noise" such that is hard to see the true signal current being created by the resistive wheel set on the car. This why the only time one should keep some wire spacing of the two wires in the given track bus is the portion of track bus that is being monitored by the occupancy sensor. All the wires BEFORE the occupancy detector should be twisted. To maximize the good signal sensitivity and maximize the twisted wiring benefit, the signal occupancy detector needs to be located right at the start of the block it is to monitor. Putting the occupancy sensor back at some central control point near the booster is the worse thing you can do electrically.

http://www.wiringfordcc.com/track_2.htm#a52

Conclusions:

If you are not implementing signaling and installing new wiring, do the twist over the entire track bus run from the booster to then end of the track. The electrical benefits outweigh the electrical consequences.

Best Regards,

Mark Gurries
Electrical Engineer
DCC Website & NMRA DCC Clinics: www.markgurries.com

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