From “whatever”… So much has been written about why to twist or not to twist, but has anyone experienced problems before or after installing the wiring?
Good point!
Rich
this sounds like a good reason not to twist (but i’ve had this problem with untwisted wires. we added capacitors across the detector inputs)
from Wiring for DCC, under Simple Bus (without explain why)
Caution: If you use twisted pair wiring within a detection block, it may contribute to false occupancy events. It is best to keep the wiring untwisted and keep that wiring as short as possible at the same time. This is best achieved by placing the block detector near the block it is going to detect as apposed at some central wiring panel.
Greg, are you sure that untwisted wires were your problem? Did you simply twist the bus wires and nothing else to make the problem go away?
Rich
we added capacitors to the detectors. our wires weren’t twisted.
Probably resolving most of the actual concerns in this and in ‘that other thread’ re-animated from 2014:
https://www.allaboutcircuits.com/tools/twisted-pair-impedance-calculator/
Twisted-pair adds some conductor length and hence some marginal DC resistance… see threads on this concern. As DCC maintains peak DC track voltage at all times, the resistance is a less-significant consideration than it would be if using pure analog voltage control.
Likewise, the DCC modulation is imposed across the ‘whole’ of the wiring on most layouts, and ‘crosstalk’ between adjacent runs or bundles may not be significant at DCC effective frequency with DCC modulation structure.
Impedance is a transmission characteristic between end-to-end transmission and reception on the pair. Its importance in reception of DCC signal via a multiplicity of random drops (the feeder taps) is an interesting technical discussion, but vast everyday experience would indicate that its real-world relevance to layout builders is comparatively slight.
No one has mentioned what I thought was the conventional guidance, that if your busses are more than 25’ that twisting the wires and snubbers on the ends are recommended to reduce some kind of electrical problem. I’m not sure what the problem is, not being an EE, but there is definitely a recommendation by qualified individuals out there to this effect. Whatever the problem is, it gets worse with the length of the bus, and 25’ is considered the point at which it becomes good practice to do this.
Every wire has inductance. The amount is proportional to the length and the diameter. As the length increases for a given diameter, its inductance increases. The opposite is true for the diameter: as it increases the inductance of a given length decreases.
To make it more interesting, two wires placed side by side have less inductance than a single wire, because their effective surface area is greater, which is the same as using a larger diameter wire.
Twisting them loosely together just helps to keep them close, the same effect can be had with parallel wires and cable ties.
Twisting them tightly has the undersired effect of creating a capacitor, the tighter the twist, the more you get. It isn’t much, but it adds up over the length of the wire.
Why does this matter with DCC?
With the nature of the DCC signal, which is turning on and off constantly, the wire’s inductance creates a reactive component. Inductors oppose any change in current. They store current in the magnetic flux around the wire, and release it when the current decreases. The nature of an inductor is that it’s effect is frequency based, so excessive inductance can distort the signal.
Ask any physicist and he’ll tell you the same thing.