Track & Wire Capacitance

Conductors, such as track and wire, have 3 electrical properties.  Resistance is one of the 3 and the most understood.  Beside resistance, 1 of the other 2 remaining electrical properties of conductors is capacitance.  

When layouts used plain old DC power, we never has to worry about this physical property.  It was always there, but it never made itself known until DCC came along.  The properties appearance as a potential problem scales with the size of the layout which of course means more track and the corresponding more wiring to connect it all up with.

To understand how this all relates to our layout wiring and track, lets first understand what a capacitor consist of.


What is a Capacitor?

A capacitor is an electrical (electronic) device that passes current thought itself the voltage across the capacitor contains any form AC voltage.  DCC's voltage is a form of an AC voltage and thus a capacitor will allow DCC current to pass through it.  The amount of current that flows though it will vary with the value of the capacitor.   The higher the capacitance value, the higher the current.  

To learn more about DCC's voltage, go here: DC versus DCC 

The electrical symbol for a capacitor is:   

The 2 dimensional symbol literally says it all.  The two vertical lines are actually representative of two conductive surfaces or plates facing each other but electrically isolated from each other.   (The orientation of the symbol does not matter.)

To learn more about what a capacitor physically is and the rules it follows, go here: Capacitance


So how does this capacitor thing related to track and wires?

 If we look at some track you will notice the parallel rails.  If we remove the ties we are left with two rails that are spaced apart with some distance "d" depending on the gauge of the track.  You can also think of these very same rails as two wire running side by side.  Think of the track bus wires running below the rails feeding power to the rails.  Now the rails/wires look like a very long capacitor which electrically we simply draw as a simple capacitor.  

==  = 









What happens when you make the track or wire longer?

Let look at adding more track segments together:
+=

Since each track segment represents 1 capacitor, then each additional track segment represent an additional capacitor.  You can keep adding more track segments up to "n" until you reach the length you need.  Since the rails are all connected together in parallel electrically, this means all the capacitors are wired in parallel.

This also applies to wires.  If you add more wire to extends it length, it too adds more capacitance.  

We can redraw the track diagram or the wires as a schematic diagram showing all the capacitors in a parallel.   Looking at the schematic, it is now easy too visualize all the parallel plates.  Hence it becomes apparent that each capacitor contributes to the total surface area and thus the capacitance is increased.  We can mathematically calculated the total capacitance as single capacitor "Ceq" by adding up all the capacitors.

C_{eq}= C_1 + C_2 + \cdots + C_n

Bottom line is the longer the track or wire gets, the capacitance increases proportionally.


Capacitance per Foot value for various types of tracks:

Typically you can figure out the total capacitance for a given type of track by reducing everything down to a Capacitance/Inch value.