Capacitors have a voltage rating, but this one is never going to see more than 15V or so (if that), so anything you find will work. The cheapest 0.1uF ceramic cap you can find will be fine.
PAUL:
Since DCC VOTAGE is about 14 volts, and 14 volt capacitors are difficult to find, a 25V - 50 v would suffice. (I’m presuming the cap cancels hi frequency noise spikes.
As has been stated here - only, extra long layouts gain a benefit.
Karl: My reference to 60 cycle AC (and feeders) was a simile to what the average HOUSEHOLD SYSTEM gives us - IE: Distribution.
This then is rectified to DC (or DC with PWM coding added) for running our model trains - unless maybe it’s Marklin or old O gage Lionel.
Close enough? I now I have to go catch a #41 Buss.
Here’s a scope of one side of a DCC signal.
There are a number of previosuly good threads on DCC. One is:
http://www.trains.com/TRC/CS/forums/2/851689/ShowPost.aspx#851689
The actual DCC signal at the rails is a differential pulse signal with a DC component to provide the current necessary to drive the power for the motors. The pulse width specifications are such that the differential DC component is maintained to provide minimal variation of current to the motor. If the differential DC component varied with the pulse train then it would be difficult to maintain constant speed with a consistent throttle setting.
I missed this the first time through…
DCC is a hugely strong signal, in fact I think it would be more correct to say that it is a signal that carries power than to say the power carries the signal.
I can’t find the specs right now, but I recall that the DCC signal oscillates at about 280Hz ( give or take ). Also, the voltage on the bus is a form of high frequency AC not DC. Look at all your decoders - they will have either an smt bridge or four separate diodes to rectify the AC signal to a usable DC voltage for motor and accessory control.
Mark.
Vail and Southwestern RR and Don Gibson,
Thanks for the cap info. I’ll run to Radio Shack tomorrow and see what I can find. FYI: my layout’s point-to-point buss run is about 200’ with my Zephyr connected about 150’-175’ from one end. And yes, I do have signal problems where the loco won’t stop or reverse directions when I want it to. I’ve thought it was my radio throttles, but now I suspect the buss.
Oh, and about DCC signal vs. power, the way I’ve heard it described is that with DCC, the power is the signal…which is one of the reasons why it’s superior to the older analog command control systems which superimposed a signal over the DC voltage.
Paul A. Cutler III
Weather Or No Go New Haven
This one might be a case where termination actually could make a difference. I’d be interested in hearing your results!
Here’s the relevant parts of the encoding method from the NMRA spec.
A: Technique For Encoding Bits
The NMRA baseline digital command control signal consists of a streamof transitions between two equal voltage levels that have opposite polarity(1). Alternate transitions separate one bit from the next. The remaining transitions divide each bit into a first part and a last part. Digital Command Stations shall encode bits within this digital command control streamof transitions by varying the duration of the parts of the bits, or frequency of the transitions.
In a “1” bit, the first and last part of a bit shall have the same duration, and that duration shall nominally be 58 microseconds(2), giving the bit a total duration of 116 microseconds. Digital Command Station components shall transmit “1” bits with the first and last parts each having a duration of between 55 and 61 microseconds. A Digital Decoder must accept bits whose first and last parts have a duration of between 52 and 64 microseconds, as a valid bit with the value of “1”.
In a “0” bit, the duration of the first and last parts of each transition shall nominally be greater than or equal to 100 microseconds. To keep the DC component of the total signal at zero as with the “1” bits, the first and last part of the “0” bit are normally equal to one another. Digital Command Station components shall transmit “0” bits with each part of the bit having a duration of between 95 and 9900
Well, like I said, wire nuts work fine. [;)] [(-D]
I would have said that both rails are going between 0 and 12 Volts, at opposite times.
I don’t think I’d say there was any -12 around. But I think we are saying the same thing, with different words.
Agreed. It’s all in where you view your reference point. I used -12V to make the point easier to understand. You are absolutely correct that the signal goes between 0 and 12 volts, at opposite times.
Electrical tape over the ends of the bus wires should eliminate the possibillity of any shorts. May I suggest that you do not “solder joiners”. With a 4’ x 8’ layout you probably will have no trouble with heat or humidity expansion “buckling”, but if you ever plan to alter your track layout, “soldered joiners” will be a headache. Use soldered wire loops, instead. Then if you need to move tracks or insert insulated joiners, there will be no problem. Four, or five “Feeders”, may be adequate, but soldering feeders to every section of track will provide more uniform power to all sections of your track.(which is especially important, with DCC). Joiners and silver-nickel track are rather poor electrical conductors. Bob
Most defintiely NOT. The DCC signal is NOT superimposed on a DC level - this is how older systems like CTC16 worked which is the major downside of such systems. The DCC signal IS the power source. And it’s not 60Hz. The actual frequency varies depending on the actual data being sent since 0 bits are longer than 1 bits.
–Randy