Zero Stretching

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Summary: Zero Stretching is an optional feature which allows operation of a locomotive without a Multifunction Decoder installed.

Short Definition

Method of running a DC powered locomotive on DCC powered rails.

Also known as Zero Bit Stretching in the Digital Command Control specifications. May be informally referred to as Address 00.

It may be possible to power a single analog (non-decoder equipped) model locomotive by itself or in addition to DCC equipped locomotives through a method known as Zero Stretching. Refer to the documentation for the the DCC system in use, as it is an optional feature.

Zero Stretching

Cautions

This feature was created to make DCC appealing during the early days. It enabled migration while minimizing fears relating to the cost and installation work required to upgrade to DCC. Zero Stretching is not part of the NMRA DCC specification and as such, not all Digital Command Control systems support Zero Stretching.
Note
Some locomotive manufacturers do not recommend operating a non-DCC decoder equipped locomotive on a DCC system. Refer to the instructions before trying your new locomotive on a DCC powered track.

Decoderless Locomotives on a DCC Layout

DCCSIG.PNG

To operate a locomotive which is not equipped with a decoder, the DCC waveform on the rails is modified. This modification affects the "zero pulse". The DCC waveform has a DC value of zero (15V - 15V = 0), so when zero stretching or analog mode is not in use, a non-decoder equipped locomotive will not move. The DCC waveform normally consists of symmetrical pulses, two of which equal one cycle. Under these conditions no torque is produced in the motor.

As shown, the Zero Pulse consists of a pulse, with a nominal duration of 100 microSeconds (µS). The complete cycle is 200µS. Under normal conditions the voltage of the pulse is canceled by the second one in the cycle.

How Does it Work?

To implement Zero Stretching, half the zero bit cycle is modified. One of the pulses can be made a lot longer in duration than the other, which will cause current to flow for a longer period of time. By controlling which half of the cycle is longer, the direction of the motor can be determined. The command station will make the necessary changes to the DCC signal to create the desired effect, based on the throttle's input. As locomotive speed increases the command station will create more Address 00 packets

Since the data packet consists of eight bits, a total of 16 pulses will appear on the rails (two per bit). Which is 1.6mS in total. So it is possible for the stretched bit to equal 1.5mS. The speed is determined by the amount of bit stretching and the rate at which Address 00 packets appear. Zero Stretching will not produce as much torque as using pure DC will.

DCC Waveform, illustrating zero bit stretching. Any pulse between 90 and 110µSecs is a zero pulse, two of which equal one cycle.

Important Warnings

Caution: Because of the frequency of current flow changes and the harmonics present, DC motors will heat up much more quickly than they would on an analog power source, and some motor types can be seriously damaged with only a brief encounter with DCC track. Many motors will buzz and hum when presented with this type of power.

In the early days of DCC there was an unfounded fear that zero stretching would cause motors to overheat, destroying them and melting the body. Those fears were never realized.

  1. Do not leave locomotives that are not equipped with a decoder on the track
  2. All direct current locomotives will respond to the signals created by Zero Stretching. Which may have unintended results.

Coreless Motors

Coreless motors and other types of precision 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 address 00's throttle is closed, the zero stretching is at a minimum, and the motor is stopped. The waveform is not high enough in frequency for the low inductance to limit the current flow when there is no back EMF, so the windings look like a short. They lack the iron core to sink the heat generated by excessive current flow, which will kill them very quickly.

Bandwidth

As locomotive speed increases, more bandwidth will be demanded of address 00, which can have an impact on response times when more than 5 DCC equipped locomotives are also in operation. This technique is a bandwidth hog due to the need to constantly send packets addressed to 00.

PSX Circuit Breakers

  • PSX Circuit Breakers are not compatible. The device relies on DCC power for operation, and may not work at some speed settings.
  • Signal corruption: It is possible that some motors may induce spikes and other noise into the system and cause problems.

See Also

DCC Power Digital Packet