Originally Posted by: lmedberry 
Cookee,
Thank you for all of this amazing information.
If you turn the square selenium plates on the cars upside down does it run in the opposite way?
How did the selenium chips on the track work?
Lance
Reversing the selenium plate will not change the direction. What it changes is which cycle of the AC wave is allowed through (or blocked) - both mean much the same thing.
This is explained in the previous attached pages, specifically the "Half Wave Principle"
Maybe easiest if you break a chassis down to the most basic form, a simple DC motor. Fortunately, that is easy. Just remove the Selenium plate. But be sure that the contacts which were separated by the plate are now actually touching.
I'm also assuming the chassis does not have any of the pickups with the built-in selenium plate fitted (you showed a photo of one in an earlier post)
Your DC power source (Battery or Trafo) will have + and - terminals. Connecting these to the pickups will make the motor run one direction. Now reverse those wires, the motor will run the opposite direction.
If you have the power source connected so that the chassis runs forward, there is one other way you can make it tun in reverse without swapping the connection polarity - swap the magnets! As easy as that.
For the purposes of this discussion, it might be easiest to decide to let the magnet placement determine which direction the vehicle will run. And that's all fine and simple, and would work very well.
But it has one limitation, and that's to do with the way FALLER wire their track feeders.
Some feeder tracks have a pair of wires for each line, and the lanes are electrically separated. But, earlier track made the two inside rails of each lane to be common. Why this is important is because of the so-called 'squeeze' or bottle-neck track section. This has one lane avoiding a small moulded 'roadwork' area. It's a novelty piece, but by design the two inside rails are in contact at the closest point. I only mention this because if you have this track section, but want to have totally electrically isolated lanes, you're out of luck. It's one or the other. But if you don't have the squeeze section, it won't matter.
Where I can, I will generally refer to the most recent design system, and note as required if there is anything for older systems that should be taken into account.
Ok, back to the vehicle direction. If you want to run all you cars on DC, then you can simply ditch the selenium plates altogether, one controller per track, DC source power, very simple.
Any vehicle that runs in the wrong direction is probably better to open up, and reverse the magnets. But if you prefer not to do this, you can just keep note of which vehicle/s run backwards and reverse the wiring polarity instead.
On the other hand, if you want to run the cars off AC, then you will need to install the selenium plate. It really depends what you want to do with the setup, either traditional slot-car racing or included into a model railway.
Going back to how AC works, here's a very rough description. The current alternates 50 times a second (for 50Hz countries). Therefore the output from the Trafo on the Brown and Yellow wires is 'positive' half of the time, and negative the other half. This is why a DC motor will just hum or buzz on AC, it's working, but is changing direction 50 times a second. Using the selenium plate prevents one half of the current getting through, so the motor sees it as very much like DC (purists and electrical engineers will jump all over this but it's very simplified).
With the selenium plate installed one way round, the vehicle might respond to the 'positive' half cycle. But by simply flipping the selenium plate over, it will now respond to the 'negative' half-cycle. Do not confuse this with Polarity, they are not the same thing, but each can affect the other.
You asked about the selenium plates in the track? - this is one thing to watch for. In the original design, there is normally a selenium plate in each vehicle, and one in each controller. Later designs did away with them in the controller, especially the hand-throttle types you have so the 'control' selenium plate was incorporated into the track instead.
For the half-wave system to work fully as designed, you need a selenium plate for each track lane, and a corresponding one in each vehicle.
Phew, it's real easy to demonstrate one on one, but a written explanation needs a lot of words, or simple diagrams, such as those I posted previously
Does this make any sense? Have I over-explained, or missed something out?
Please keep asking, there is no such thing as a dumb question. Some people get it right away, others get it when explained in a different way but experimentation with your setup is probably the fastest way to learn, just makes notes if anything you change so you can follow the cause and effect.
