Decoder output to locomotive motor

[Isambard]

As I understand it the typical decoder eg. Digitrax DH123 takes the 4 to 9kHz pulse form input from the track and rectifies it via a bridge rectifier to DC (direct current) to drive the motor.

Is the output pure polarity reversible DC or is there a pulse component? What does use of the term "supersonic decoder " mean, such as used by Digitrax for its DH123 i.e."SuperSonic for quiet motor operation"?

[locoi1sa]

 Silent drive decoders put out a high freq. pulses to the motor at different voltages. Not so much a pure DC witch would not pulse on and off at up to 50 thousand times a second. These pulses are what kicks the motor at startup and lessens as speed increases. TCS calls it dither and NCE calls it PWF. Its the pulses of on and off and also voltage that gets the motor spinning.

  Hope this helps you understand how DCC works.
 

[CSX Robert]

The output definitely is not pure DC. It is a square wave pulse width modulated signal. This means that the power is switched fully on and fully off at a set frequency, and the wider the on pulse, the more power that gets to the motor. For example, if the decoder is supplying 50% power to the motor, then the output is actually fully on 50% of the time and fully off 50% of the time. Sometimes you can hear a buzz caused by these pulses. The term "supersonic"(or "silent drive") means that the frequency of these pulses is high enough that we can not here them.

[UpNorth]

Is'nt the decoder output to the motor/lights pure rectified DC and they simply play around with the pulse  width frequency or modulation of full voltage for the motor.

[Vail and Southwestern RR]

Is'nt the decoder output to the motor/lights pure rectified DC and they simply play around with the pulse  width frequency or modulation of full voltage for the motor.

I believe that the truth here is that it depends.  I don't think that the NMRA specs control the output side of the decoder.  Some function outputs are just the rectified DC voltage, some are regulated to a lower voltage, some are current limited.  I believe that most decoders use pulse width modulation of the rectified DC voltage, but I don't think this is a requirement.  In fact there are decoders for AC motors, which obviously don't! 

[richg1998]

I just looked at a Micro-Tsunami powered by a NCE Power Cab using an oscilloscope. I have not attempted to modify the CVs. Even at the slowest speed possible, the pulses are still 0 to 12 volts. The motor receives 0 to 12 volt pulses at 20khz. The pulses get wider as the throttle is increased. Using the plus motor terminal as a reference, the pulses are 0 to 12 volt positive for forward and 0 to 12 volts minus for reverse. The voltge at the motor terminals is 0 volts with zero throttle. 

The voltage to the lights is a steady 12 volts.

Rich