Tim, note that, as I mentioned above, there is no problem running between DC supplies, like those traditionally used with HO and N, and with some American Flyer. The rectifiers between the transformers and the track isolate the transformers from each other.
The reason for having two (or more) pickups is that the center rail is not continuous. Specifically, it must be interrupted where it crosses over the outside rail in a turnout or crossing. I think the situation is similar for American Flyer operation, except that, instead of multiple rollers, you have multiple wheels on the trucks that collect the current. In the two-rail case, of course, the rail interruptions occur where the (always outside) rails cross over each other. Aside from those considerations, track and wheels are never perfectly clean, and the connection to the track is going to be broken occasionally. So I think we have the same situation in both cases, that is, brief connections between supplies, or longer ones if the train stops in the right (wrong?) place.
In lighted cars continuity is not so critical, so a single roller often suffices in a cheaper car. But two are frequently used to reduce the flickering of the lights. Williams interestingly uses two pickups, but connected to two separate lamps and not to each other.
With a rectifier, the same currents flow in the wires between the transformer and the rectifier and in the wires between the rectifier and the track. The (bridge) rectifier acts like a DPDT switch, reversing the connections 120 times each second; so the currents are flowing in the opposite directions on each side of the rectifier half the time. But they are the same currents. Therefore, with a perfect rectifier, the transformer’s circuit breaker should trip just the same whether the short circuit is downstream from a rectifier or not.
I think that the difference that you observed may be due to the fairly high resistance of the now-obsolete selenium rectifiers that American Flyer used in their rectifier units. They did claim that it could carry a maximum current of only 6 amperes. Even assuming that this numbe
I simply wire a 6 amp or so circuit breaker in series with my track outputs. If a significant fault current was to flow due to different voltages on my transformers then the breakers open. This almost never happens as even when a fault current flows , the fault current is so brief (unless an engine stalls) that nothing happens. E units do not even trip. The voltage spikes with new equipment as Bob nelsen point out may be my problem. I have not had a problem yet but I run mostly post war.
My post refers to 3 rail ac model trains. I use modern resetable circuit breakers in series on the output when using multiple postwar transformers running different blocks. This gives me a little piece of mind should something stall between blocks. If voltages are the same , no significant current will flow. My trains use mostly mechanical e-units . I may need to get tvs devices to protect against voltage spikes that can occur. If they will protect??
I use PW ZWs. They are Phased. I have many many blocks, including sidings and storage yards. I have 4 mainlines that intersect. Basically an engine can go from any siding/storage yard to any place on the layout. The center pin is removed at the block connection. I use GG and Lionel O27 tubular. The center pin is replaced with a plastic toothpick bought at the local grocery. Power to the sidings is managed with toggles and Atlas 205 slide switches. The mainline blocks used a TMCC BPC controller - which is no more than a group of SPDT switches. I used TVS diodes at each power drop along the layout for spike control. I use Fast-blow and resettable fuses - 8/10 amp - to the various sidings/mainlines. NO PROBLEMS. I recognize and appreciate bob’s point about multiple transformers and always have it in mind as I run the trains - 4 at a time crossing blocks here and there. I use the 2RC and the UTAC relays among other types of relays to manage accessories, lights and signals.
Protection from 120-volt faults is why I recommend grounding the layout common (that is, connecting it to the equipment ground of the branch circuit powering the layout, which is connected to the earth). Any such fault will trip the branch circuit’s breaker, in the same way that electrical appliances with three-wire plugs are protected.
No dry cells: “The UTAC circuit board is powered by an AC transformer between nine and twelve volts…Each of the eight output switches can drive up to two amperes of AC power at up to thirty volts.” and “[2RC i]nputs are powered from a fixed-voltage, AC transformer between eight and twenty volts or from train power.”
A TVS diode protects a circuit from overvoltage, but does not provide any sort of back-feed prevention. A conventional diode could do this if desired, but only on a DC circuit, not with AC.
I didn’t mean to suggest rewiring the transformers’ cords, but just to get the benefit of the equipment ground that wasn’t available when they were made. One easy way to do this is to put a single wire into a 3-wire plug, connected to the round equipment-ground pin, leaving the other blades unconnected. Then connect this single wire to the layout common, that is, the outside rails of 3-rail track or the American-Flyer “base”. Other possibilities are to connect to a grounded handy box, if you have one exposed, or to metal plumbing.
That said, I have in fact replaced the original 18-AWG ungrounded cords on my type-Z and 30B transformers with 16-AWG with ground, not just to get the ground, but also because the original cords are a little light by modern standards, especially when plugged into the 20-ampere circuits that the NEC allows. Years ago, equipment grounds in the building wiring were allowed to be 2 AWG sizes smaller than the current-carrying conductors, but now must be the same size, presumably to assure tripping. The same consideration applies to power cords, which are almost always smaller than the building wiring and can benefit from at least being closer to the size of that wiring.
You’ve got it. The ground wire doesn’t need to be insulated; but, if it is, it should be green.
If the electrons tried to flow into that wire, the accumulation of excess electrons on the earth and the positive charge from the unpaired protons in your layout wiring would produce such a voltage difference that they would be stopped in their tracks by the opposing electric field. The effect is so strong that no significant charge can ever flow. Now, if the layout common becomes connected to the 120-volt side of the power circuit, then electrons can flow freely through the ground wire in one direction and through the power wire in the other direction, with no accumulation of charge. The current that flows would quickly trip your house’s circuit breaker or, even faster, trip the GFI, which would sense the imbalance in the power circuit–current in the black wire, none in the white wire–and disconnect the 120 volts before anyone could get hurt.