Electrical Issue with my 2-8-4?

[jonathan]

Running my 2-8-4 (C&O Kanawha) this morning I noticed a slight hesitation that occurs every once in a while. 

I was able to get down close when the hesitation happened.  The reverse light flickers during the hesitation.  Strange, since the reverse light shouldn't come on when running forward. 

Forward light does not flicker at all.

Hesitation only happens on straight track, not curves.

The loco is DCC, but I have unplugged the decoder and put in the bypass plugs (I'm still DC).  Would restoring the decoder cure my little hiccup?


Thanks a million!

Jonathan

[Atlantic Central]

Sounds like dirty track or wheels, or maybe a loose wire or loose pickup wiper.

The backup light flicker is likely just from the surge as power cuts on and off. the lighting circuit has a capacitor or two which may be sending a little power to the backup light when track power is lost.

Clean the wheels and track, check the tender pickup wipers first.

Reinstalling the decoder will not fix it, likely to make it worse.

Sheldon

[jonathan]

Sounds like an easy fix.  It was the light flicker that had me baffled.

Thanks for clearing it up, Sheldon!

Regards,

Jonathan

[Jim Banner]

The momentary flicker of the back up light is, as Sheldon said, a result of a power surge when contact with the rails is lost.  The mechanism, however, involves the motor, not the capacitors.  When you interrupt the current flowing through a coil, including the coils in the armature of a motor, the coil reacts by producing a voltage spike with polarity opposite to the original applied voltage.  The pulse is caused by the collapse of the magnetic field that the flowing current was producing around the coil prior to the interruption.

In dc mode, the directional lights are controlled by diodes.  The diode for the rear light is normally reverse biased by the forward voltage applied to the motor, so the rear light does not receive any current.  With a current interruption to the motor, the reverse polarity spike momentarily forward biases the rear light diode and it flickers on for a moment.  The effect is much more obvious with LED's than with incandescent lamps.  If the motor has a flywheel that keeps on turning after the current to the motor is interrupted, then the motor will act as a generator and may produce a high enough voltage to keep the rear light on a bit longer.  This generated voltage is that same back EMF that can be used by decoders to determine the motor's speed.  But that is a different story.

Jim 

[jonathan]

Thanks, Jim.

Sounds like I have some locomotives with built-in early warning systems to tell me when it's time to clean track and/or wheels... those with a reverse light anyway.

I can appreciate your enthusiasm for the electrical aspects.  The juice does so many unexpected things... at least for a novice like me.

Regards,

Jonathan