Track & Wire Voltage Spikes/Ringing

The electrical noise you would see on the track (require an oscilloscope) contains two types of high frequency voltage noise that decoders can see.

1) Voltage ringing.  This occurs all the time and happens under normal operation conditions all the time when there is any change current flow polarity or amplitude.

2) Voltage spikes.  This occur when the is a ABNORMAL operating conditions involving a momentary short circuits and full current flow from the booster.

The root cause ringing and voltage spike is the inductance in the wiring.  To learn more go here: Track & Wire Inductance

Are there oscilloscope images of these voltage problems?  


All of these problems can be addressed in one of two ways:

1) Existing Wiring: Install a RC Filter on the track bus.  For more information go here: Snubber/RC Filter

2) New Wiring: Twist the track bus wires together.  For more information go here: Twisted Pair Bus Wiring

How strong do voltage spikes get?

The strength of a given voltage spike depends on:

1) A short circuit event occurring such as a derailment or some type that occur around a track switch.

2) High Inductance in the wiring due to long "track bus" wire runs between the Booster and the location of short circuit.  The length of the wire is always twice as long as you think it is.  Example: A 20Ft track bus has two wires run with one going 20Ft out and and other going 20Ft back.  Longer track bus runs are worse that short.  

3) Peak current delivered by the booster.  This is a combination of the booster's current rating and it peak current capability.

Since #2 and #3 are very layout dependent but there is yet another factor involved.  Given the short circuit can occur anywhere on the layout, the actual length of track bus wire involved in carrying the short circuit current will vary too.  In other words a short circuit near the booster involves potentially very little track bus wire while one on the other side of the layout will involve all the Track bus wire.  This is 

A) some layouts have problems and some do not.  Stated another way, all layouts have voltage spikes, but only those with strongest voltage spikes see problems.  

B) the type of decoder failure one see's vary.  Some voltage spike are strong enough to literally blow up the decoder while other less powerful are only strong enough to knock the microprocessor into losing is programming.

How is the voltage spike created?

There is an equation that describes the physics of what is happening.

V = L di/dt.


V = Voltage: This voltage spike is on TOP of the track voltage.

L = Henrys:  Inductance of the wire. (Function of wire length)

di = Amps:  Change in current flow in the wire.

dt = Time:  Change in time in which the current changed.

So where there is a short circuit, there is a BIG change in current flow (di) in very SMALL amount of time (dt).  This ratio gives up a big number that is multiplied by the inductance (L) of the wire that is carrying the short circuit current.  V is the resulting voltage that is added ONTO the existing DCC voltage.