I was wondering why dcc uses an ac current over dc. Does alternating current help with the transmission of packets? From my experiences with microcontrollers and networking, both uses dc current. If I understand this right, from my nce experience, the command station converts the dc current to ac and the locomotive rectifies it back into dc. My guess is that the decoder uses pwm to control motor steps, light brightness and effects. I still fail to see why ac is used over dc. Any guesses? This doesn't change anything, just something that I was curious about. Sent from my iPad using Tapatalk
The “DCC AC” (actually DC with the polarity being switch back and forth) is used to provide full power while containing the pulsed DCC signal. Bob
With that being the cace, do you know what hertz they are using? I haven't had the chance to hook it up to my oscilloscope yet "20 MHz range." Sent from my iPad using Tapatalk
Frequencies Copied this back around 2008. from an online DCC site. Not sure what frequencies might be used now with newer decoders. Just simonize the trigger on your Scope using the basic crystal frequency that the Microprocessor uses. I have done that with a Tek475 dual trace Scope. Yes, you can say the DCC signal is AC that is how the full wave bridge rectifier produces a pulsating DC that is filtered by a couple capacitors. A few non techies love to argue that point. Could not be done by pure DC. Digital infomation is needed. ---------------------------------------------------------------------------------------------------------------------------------------------------------- Older decoders used a motor drive (PWM), pulse width modulation frequencies of about 30 to 200 Hz (cycles per second). PWM takes a constant like 12-18 v dc and drops to zero volts quickly and back to the higher voltage to get effective voltages between 0 and 18V. These frequencies were low enough to cause a hum or buzz in the locomotive. Most decoders today use a high frequency drive of 15kHz to as high as 43kHz. These frequencies are above the range of hearing and that is why they are called silent. This type of drive comes with many different names, depending on the manufacturer. One of the problems with the high frequency drive is slow starts. With these high frequency drives the motor tends to act like it is on dc power and is subject to stiction, where the motor does not start smoothly. The combination of back-EMF and the PID motor control can over come this problem. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Rich
The one bit is ~116 microseconds and the zero bit is ~200 microseconds. So one could say the frequency is ~5 to ~8.6 kilohertz. Train track is not a good signal conductor at much higher frequencies. The official details are here: http://www.nmra.org/standards/DCC/standards_rps/S-91-2004-07.pdf. To see a good representation of the waveform, the oscilloscope should be ~86 kilohertz or better. Bob
DCC puts an AC (or bi-polar) signal on the track which carries power as well as data. The advantage of it being bi-polar is that it makes no difference which way the loco is facing on the track, as rectification in the decoder provides the internal DC for the electronics while the data signal is extracted from the pulse width modulation. Thus when crossing a power district or reversing signal gap, the polarity (more properly, phase) can be adjusted on the fly without affecting train operation. BTW, the base frequency of the DCC track signal is roughly 8 KHz, with harmonics up into the 100 KHz range.
Frequencies A follow up on this. Store the link in Favorites for future reference. http://www.tttrains.com/dcc/dccfaq.htm Rich
The 1 bits in the DCC signal can be stretched so that the bi-polar DCC signal speeds almost 100 times longer at one polarity than the other (basically looks like a pulsed DC voltage for non-DCC locomotives). Bob
Zero Bit Stretching Technical info. Hard on the motor when the loco is not moving. The spot where the two motor bushes contact the commutator will get very hot because of the AC going to the motor. I did some measurements with an infra red temperature scanner. With armature turning, not as hot. Better to park the loco on a siding and turn off the power to the siding. http://members.shaw.ca/sask.rail/dcc/DCC-waveforms/DCC_waveforms.html As usual, store the link in Favorites. Another link about the DCC waveforms. https://sites.google.com/site/markgurries/home/technical-discussions/dc-versus-dcc-bacground Rich
Additional technical info. Store the link. This is only for a loco with a decoder. The motor gets the same pulses from the decoder if the decoder is run by DC voltage or the DCC voltage. PWM Motor Drive https://sites.google.com/site/markg...scussions/decoder-motor-drive/pwm-motor-drive Rich
