http://www.dccwiki.com/Zero_Stretching
Caution: Because the power is closer to AC, many DC motors heat up much more quickly than they ordinarily would on an analog power source. Some motor types can be seriously damaged with only a brief encounter with the DCC track. Many motors will buzz and hum when presented with this type of power. If possible, do not leave locomotives not equipped with a decoder on the track to reduce the chance of heat damage to the motor. See the note below about coreless motors.
The DCC AC signal or waveform is modified so that the motor starts to look like DC. Modifying the DCC AC waveform can accomplish both speed and directional control of the DC motor and is covered by the NMRA DCC Standards and RPs.
In the Digital Command Control specifications, it is also known as Zero Bit Stretching. It may be informally referred to as Address 00.
In a segment of DCC-powered track, it may be possible to power a single analog (non-decoder-equipped) model locomotive alone or in addition to DCC-equipped locomotives through a method known as Zero Stretching. Refer to the documentation for the DCC system, as it is an optional feature.
- Zero Stretching is not part of the DCC specification. Not all manufacturers support this feature.
Zero Bit Stretching
In this scheme, zero bits on the track can be extended to create a net effect where current appears to the motor to flow in one direction or another. The positive portion of the pulse can be quite different from the negative portion of the zero bit. The direction can be established by making the waveform "more" positive or negative. In operation, the booster applies a pulse to rail A while holding rail B at ground potential, then grounding rail A and switching on rail B for an equal amount of time. This process repeats to create the DCC waveform seen on an oscilloscope.
The DCC waveform has a DC value of zero, so when zero stretching or analog mode is not in use, a non-decoder-equipped locomotive will not move. The DCC waveform is symmetrical. To make zero stretching work, the command station will make the pulses on one rail longer than those applied to the other, causing the motor to turn. A normal DCC signal has symmetrical pulses of equal duration (period), causing the motor armature to oscillate, with little torque produced rapidly. The altered pulse duration applied to rail A or B causes the armature to turn further in one direction, producing the torque needed for moving the locomotive.
DCC Waveform, illustrating zero-bit stretching.
Note: All direct current locomotives will respond to the signals created by Zero Stretching. Which may have unintended results.
As locomotive speed increases, more bandwidth will be demanded of address 00, which can impact response times when more than 5 DCC-equipped locomotives are also in operation. This technique is a bandwidth hog due to the need to send packets addressed to 00 constantly.
- Coreless motors and other low-inductance motors should not be used on a DCC-powered track (unless a DCC decoder is installed). Normally, current flow is limited by the back EMF that a motor generates when it is spinning, but the DCC waveform is full voltage all the time, even when addressing 00's throttle is closed, the zero stretching is at a minimum, and the motor is stopped. The waveform is not high enough frequency for the low inductance to limit the current flow when there is no back EMF, so the windings look like short. They lack the iron core to sink the heat generated by excessive current flow, which will kill them very quickly. Coreless motors are very expensive.
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