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[gandy dancer#1]

HI: i saw one of your replys on ho in regards to ammeter and volt meters,and you stated that dcc was a special form of ac. Could you enlighten me please??

[Jerrys HO]

gd#1

This may help you understand.
http://www.brian-lambert.co.uk/DCC.htm#Comparison of the two methods of train control..

Jerry

[jward]

since jim hasn't yet replied, i will attempt to explain why you need a special meter to accurately measure dcc voltage & current. please bear in mind that this is a greatly simplified explanation.

standard dc is like a battery. it will provide pretty much a constant voltage and polarity for as long as the power supply is capable of providing it. on a model railroad, the dc is varied by a rheostat, which is a variable resistor. dc is easily measured with a volt or ammeter.

standard ac voltage such as wall power flips back and forth between positive and negative. the voltage is constantly fluctuating and can be grapped as a sine wave, a sort of parabolic curve. think of a series of rounded mountains and valleys. since this wave is pretty much constant, it can be easily calculated. the effective ac voltage for sine wave ac is much less than the variation between the peaks and valleys of the sine wave and is a sort of average value. ac volt meters are calibrated for this average.


dcc is square wave ac. its positive and negative values are the peak values, and don't vary. they are the same value on each side of the wave. think of turning up a dc power pack to full voltage, then rapidly switching the reverse switch back and forth and you have square wave ac. by varying the length of time in the positive and negative cycles, signals can be sent to decoders in the locomotives, which read the signals and convert the signals into the appropriate polarity and voltage for the motor.

trying to use an ac meter calibrated for sine wave ac will give a wildly inaccurate reading on sine wave ac. thus the need for special meters for dcc.

[gandy dancer#1]

Hi jerry and jward: I was assumming that the power supplyon a dcc setup was dc voltage at the rails,and that the encoder tothe cabs was using the rails as a transmission line. I did not know that in dcc the power on the rails is not dc but actually square wave ac. Thanks to you both for clarifying that.I was under the assumption that the small scale stuff as all still dc, and larger scale was ac,leanr something new everyday

[Doneldon]

I was under the assumption that the small scale stuff as all still dc, and larger scale was ac,leanr something new everyday

gd#1-

You're not far off the mark.

Our locomotives are strictly DC. It's just that the decoders only feed it to them part of the time. It's sort of like this: A DC set-up varies the amplitude of the DC current in the rails and all of the motors (locomotives) on those rails will move at the same nominal speed in the same direction. Add more current and they speed up; dial it down and they slow. With DCC, the square-wave AC is at full power all of the time. Our locomotives' speed and direction are set by signals from the DCC controller, signals which will be responded to by only the decoder(s) to which the DCC control message has been set. Thus, loco #168 gets DC power 50% of the time and is told to run left to right; consequently, it runs at half power/speed to the east. On the other hand, the decoder in loco #1272 is told to go right to left at 80%, so it proceeds west at high speed with a through passenger train. And this is one of the two key features of DCC: We can operate multiple trains at different speeds and in different directions on the same track, just like the real railroads do.

For the record, the other primary feature of DCC is the ability to control lights and sounds in pretty much whatever manner we prefer.
                                                                                                                                                                                                      -- D

[Jim Banner]

Sorry for the delay in getting back to you.  DCC uses square waves, not sine waves on the rails.  If the square wave (one positive peak plus one negative peak = one wave) is short, it is detected as a binary one.  If it is long, it is detected as a binary zero.  The command station uses these ones and zeros to write messages to the decoders, much the way we write messages to one another by email using strings of ones and zeros.

Normally, the positive peaks are the same length as the negative peaks so the average dc voltage on the rails is zero.  But some DCC sets, including the E-Z Command but not the Dynamis, can make the positive peaks longer than the negative peaks (or vise versa) for the zero waves.  This is referred to as 'zero stretching' and is used to create an average dc voltage on the rails.  This dc voltage is what can operate a dc locomotive without a decoder on DCC tracks.

Rather than wade through 6000 words on the subject, go to the link below and have a look at half a dozen pictures of DCC waveforms.  These are photographs of an oscilloscope screen where a light beam moves across the screen from left to right while the instantaneous voltage moves the beam up or down depending on polarity, and at a vertical distance dependent on voltage.

http://members.shaw.ca/sask.rail/dcc/DCC-waveforms/DCC_waveforms.html

The waveforms a decoder uses to drive a locomotive motor can be found at the link Jerry gave above.  Note the difference between those waveforms and the ones discussed here.  The ones discussed here are either shorter OR longer than a certain length to make the digital ONES or ZEROS of a digital message.  The waveforms at Jerry's link can be any width and their average voltage is applied to the motor as an analogue signal.

If you are really interested in the specifics (and are a bit of a masochist to boot) follow this link to the NMRA website.

http://www.nmra.com/standards/DCC/index.html

Jim

[gandy dancer#1]

Greetings Mr. Banner and thanks for your response, itis exactly what i thought was going on , as having a bkgrnd in electronics and engineering myself, but i was seeking confirmation from you after being aware of your knowledge. I think i will pass on reading the suggested materials I may be a little on the perfectionist side, but i have completed my education several yrs ago, and now am officialy retired and all i want to do is play Hope you have a great weekend and i am sure i wil lbe visiting with you from time to time in the future.