Twisting Power Bus Wires

Summary: A question often asked regarding twisting of the DCC Power Bus. Is it necessary? In short, no, there are other ways to achieve the same result.

This topic is fiercely debated on internet forums and DCC-related mailing lists, endlessly and without any real conclusions. This article tries to provide concise reasoning for either.

In Short

The DCC signal is a robust transmission method, and in most cases, interference is almost a non-starter. The primary reason to twist is to reduce the reactive component in the bus impedance.

It is not necessary to twist the track power bus wires together. Track Power Bus wires should be kept close together, which is easily accomplished by twisting them together. Tying them with a cable tie is another option, and nearly accomplishes the same purpose: Reducing Inductance.

Twisting wires doesn't require them to the tightly wound together and should not be very tightly twisted. Generally, a few twists per foot of wire is sufficient.

Garden railroads tend to have longer wire runs and may bundle various wires together in a conduit. A loose twist of the power bus wires is beneficial in this case. Additionally, garden railroads do not get the benefit of shielding from a structure and may be potentially subject to additional interference than an indoor layout would get.

Twisting: Is It Necessary?

There is always a large debate on the twisting of track power bus wires. Keep in mind that DCC power buses are an 'Unbalanced Pair' - one wire (A) is held to ground while the other (B) is energized, then they flip when A carries a signal and B is held to ground. This is happening constantly and at a high rate.

Other debates center around transmission line theory, which fails to identify that the distances are short (except for outdoor railways) and the load is not fixed at the end of the circuit. In this situation, the load is not stationary, but moving along the track. However, for longer runs such as outside for garden railroads, twisting is more important to help with interference.

Pros of Twisting

• Reduces, or eliminates, induced false signals from one wire to another.
• Lowers the possibility of inductance of the bus, which affects the impedance.
• Easier to identify matching bus pairs.
• Prevent 'cross-talk' / interference of throttle network and LCC wires if bus wires are run together.

Cons of Twisting

• A tight twist will create a capacitor, which leakage currents can confuse any detection coils in use.
• Takes a little additional time to run the bus wires.
• If too much twist was added to the wire, might be difficult to solder tracker feeder wires to the bus wires.

DCC Issues

1. Excessive track bus impedance can cause a multifunction decoder's PWM pulses to be superimposed onto the DCC signal, causing DCC waveform distortion.
2. Ringing: Excessive inductance increases ringing in the DCC signal.
3. The amount of inductance in the bus directly relates to the degree of DCC waveform distortion and other problems such as large voltage spikes.
   • Due to the harmonic content of the DCC signal, it has a lot more in common with alternating current than direct current.
4. Many boosters rely on rate of change to detect a short circuit.
   • Excessive impedance impairs the ability of the booster to detect a short by increasing the RL time constant of the bus. This impedes the rate of change, resulting in excessive currents flowing and damaging rolling stock or track work.
5. Increased voltage drop.
6. Distortion of the DCC waveform from the above items can lead to runaway locomotives.
   • Disabling the Power Source Conversion option in CV29 will prevent this by disallowing analog (Direct Current) operation.

Inductance

Main article: Inductance

By managing the inductance, the DCC signal's integrity can be preserved by reducing distortion and ringing on the power bus, with the resulting reduction in impedance allowing more power to be delivered to the vehicles on the track.

For a more full explanation of inductance, and various issues related to it, see: Inductance.