Stan Trzoniec’s article includes the suggestion of using one ZW output for level track and another for a grade up a hill on the same loop. Some of you could have predicted that I would object to that. The problem is that every time the train makes the transition from one block to the other, its pickups create a short circuit across a part of the ZW’s secondary winding, unprotected by the circuit breaker.
I have long advised against this practice for level track, where there is at least a chance of setting the two blocks to the same voltage. But in this case, the voltages are deliberately set different. I don’t know what his settings, track length, and feeder arrangements are; but here is an example of how things can go wrong:
Suppose that the voltage difference is 5 volts and that the total resistance from the ZW’s A terminal to its D terminal, through the train’s pickup, is 100 milliohms. This could be about 25 feet of center rail or 30 feet of 14 AWG feeder, or a combination. Now 14 AWG would ordinarily be safe for wiring a ZW, but not in this case. The fault current when the pickup crosses the gap is 5 volts divided by 100 milliohms, or 50 amperes.
The ZW may well be able to supply this, since the low voltage of 5 volts means that the power drawn is only 250 watts, not a great overload for the primary winding. But the power dissipated in any 14 AWG feeders will be about 10 times what the wire can stand: Wire is rated according to the current that will not heat it to the point of melting the insulation. The current here is 3 1/3 times the wire’s rating; so the temperature rise is over 10 times what the insulation can tolerate, assuming of course that the layout was wired with 14 AWG.
All of this is guesswork, since we don’t know the particulars of the actual wiring. But the numbers I used are plausible, and the situation could be dangerous if the train ever stalled across t
I have heard this before. While I understand the logic behind this fear I can tell you that this never happens. Most model railroad hobbyists at one time or another park trains on sidings and across different blocks where there is a difference in voltage and there is no damage whatsoever. Lionel enthusiasts have been doing this for centuries now and there is just no credible evidence of this practice being a problem. Recently on another forum we had a few operators trial parking various cars and engines across different blocks set at different voltage levels. In no case was any damage or overheating reported. The current flow was negligible. Maybe there is an EE out there who can explain why reverse current flow does not occurs .
A low-enough voltage difference, a high enough track or wiring resistance, and a short enough duration all can make the likelihood of damage arbitrarily low. Many modern “transformers” will not have the problem at all. You will have to give the specifics of the setup for anyone to explain a particular result. As I pointed out, these details were not given in the article; so I made up some plausible ones. The danger is that someone will take the “tip” to be unconditionally safe and try it on his own layout, with bad results.
Lionel once effectively admitted that the circuit-breaker arrangement that their postwar transformers shared was flawed, in the fine print of the service manual for the KW: http://pictures.olsenstoy.com/cd/transfmr/pskw2.pdf
I have an interesting exhibit. It is the actual transformer out of a type-Z that I bought for parts. That transformer lacks a whistle control but is otherwise very similar electrically to the ZW shown in the article. There is a section of the exposed secondary winding that is badly burned. It is in the middle of the winding, with undamaged turns beyond it at either end. I think it is pretty clear that the fault current that must have burned it had to flow through two of the rollers stationed at either end of the burned turns. Otherwise the same current would have flowed throughout the secondary winding, and it would all be burned. So at some time there was a short circuit between two of the output terminals; and a current flowed, long enough and heavy enough, to severely damage the transformer; but the circuit breaker did not trip. I don’t know how to post the pictures here; but I will happily e-mail them to anyone who would be willing to do that for me.
(It’s not a “reverse” current, but a fault current.)
I follow your advice religiously. I am on the other side of young with no academic electrical knowledge. Some argue that your advice is somewhat theoretical and, therefore, not applicable in the “real world.” I favor your approach. The practical aspect is, I think, undeniable and I have not seen an answer to the following basic question. Please excuse me you have previously offered such advice. I have searched and have not been able to find it.
Is there a “convenient” way to safely move a locomotive, on track, from one power source to another as desired? Of course “convenient” is a relative term. I suppose I’m looking for a solution to a recurring issue that the unknowing can apply
I, also, have a BS in math and an MSEE with 50 years of experience. Bob Nelson and I are in perfect agreement about the damage fault currents can do. I suggest everyone follow his advice and avoid smoking your layouts.
I have heard of passenger cars with two rollers burning the wiring because one roller was powered by one transformer output and the other roller was powered by another transformer output.
I have owned Lionel trains since 1950 and have many years of experience with them.
While this is good information to have, in all my years of running 3 rail trains I have never had a problem running trains from one power output to another. As long as they are in phase of course.
Is there any way to get the benefit of applying different voltages to the level and inclined track without the risk of excessively high fault current? Perhaps a resistor in series with the feed to the level track block?
That’s how to do it, George. But, even better than a resistor, whose voltage drop will depend on the current that the train draws and therefore may require readjustment for different trains, you can use anti-parallel connected diodes, which will have an almost-constant voltage drop.
The basic unit is two rectifier diodes with rated forward currents at least about 5 amperes. Connect them in parallel with the diodes pointing in opposite directions. Put this in series with the center rail of the level track to get about 1/2-volt drop. Put more of these pairs in series to get a greater drop.
You can make the equivalent of two of these pairs, with a total drop of about a volt, from a bridge-rectifier module: Connect the + and - terminals together and use the other two (~) terminals to wire the module in series with the track. As before, add more modules in series to get a greater drop. If you need a finer adjustment, use the ± junction of one of the bridges as a 1/2-volt tap. (Note that the bridge-rectifier module is not being used here as a rectifier, just as a convenient way to get 4 diodes in one package.)
It would make the setup safe, assuming that the track feeders and any internal wiring connecting pickups together in the locomotive or lighted cars is heavy enough to carry the rated current of the added circuit breakers. It’s the way Lionel should have designed their transformers, rather than skimping by putting a single circuit breaker in the common return wire.
But the overcurrent would still be there; and so would any arcing or welding caused by it. An intermittent short circuit is also a good way to generate high-voltage spikes, of the sort that often do in modern electronics-heavy locomotives.
I can understand two different power sources for two different isolated blocks.
It’s my understanding that lionelsoni is referring specifically to the issue of straddling blocks where pickups will, at least momentarily, reside in two different isolated blocks.
Perhaps I do not understand this, but it sounds to me the only way to avoid the issue, referred to by lionelsoni, is to create a scenario where power would be regulated to on/off almost immediately. A locomotive or powered car entering block A from block B would need special handling as it crossed between blocks A and B.
When the locomotive enters block A (the lead pickup crosses the line), power to A would need to be off until the trailing pickup enters block A. At the moment, and not before, the trailing pickup enters block A, the power to block A would need to be on. I could see this happening manually be coasting a locomotive over the dividing line and only providing power to block A when all locomotive pickups are in block A. This would be impossible to manage manually if lighted passenger, or other powered, cars were part of a consist.
I “think” I understand lionelsoni’s objections, but am not positive. I am sure I don’t know how to implement a solution to the problem I “think” he is describing. If there were a
This is a problem for prototype electric railroads too. They have to go between sections powered from different sources. They avoid connecting those sources together by coasting through an unpowered stretch of track.
George posted a very practical solution above. That is to drop the voltage on the level track with a series resistor. I further suggested replacing the resistor with diodes, to keep the voltage drop constant. In either case, the important thing is that connecting the two center rails together between blocks does no more than place a short-circuit around the passive voltage-dropping element, whether resistor or diode network. This is harmless.
The more common situation, which is not the one here, is moving from block to block between two supplies set to the same voltage, or intended to be set to the same voltage. It is better to avoid this situation entirely, by doing what lion88roar suggested, switching the blocks to be powered always by the same source. With two sources, this is easily done with a single-pole-double-throw-center-off switch per block.
John,
There is no ‘safe’ way to move between two tracks powered by separate transformers (as far as I know) on the fly.
My layout has two mainlines on the lower level connected by 4 switches in a crossover. Two switches on the outer loop and two switches on the inner loop, these switches are at the end of one block on each track. There is also a reversing loop inside the inner loop that is contained within the same block as the crossover switches with a shutdown toggle, and a switch leading to a grade to the second level. Each lower level main line also has a separate block.
When I want to transition from the upper level to the lower level I have to:
Park the train on the inner mainline inside the non-reversing loop block - disengage power to this block
Change the source for the reversing loop to the source for the second level
Change the source for the transition grade block to the source for the second level
Engage power for the grade and reversing loop
Throw the switches to the transition grade and reversing loop
When the train is on the lower level
Park the train in the reversing loop
Disengage power to the grade block
Disengage power to the reversing loop
Throw the grade block switches to normal
Change the source for the reversing loop to that of the inner mainline
Engage power to the reversing loop
Engage power to the secondary inner mainline block
Move both trains on the inner mainline
You have to pay attention to what you are doing, but having the parked trains’ blocks powered down ensures you will not ‘cross’ transformers.
My setup is somewhat simpler. I have a yard and two loops with a double crossover. The yard is a block; and each loop is divided into two blocks. So I have 5 toggle switches, one for each block. Whichever of my two transformers a train starts with, it stays with that transformer as it moves about the layout, as I throw the toggle switches to assign the blocks ahead to that transformer.
The upper picture is one of the sides where the rollers go, as you can see by the two circular tracks. The opposite side, for the other two rollers, is similar. The lower picture shows the (once) insulated edge of the winding that is almost vertical at the bottom of the upper picture. As you can see, the insulation is completely incinerated in the middle section where the fault current flowed. I am confident that the current was well over the 15-ampere rating of the circuit breaker, but was not interrupted by it since it did not flow through the circuit breaker.
Thank you very much for your patience and efforts to help me clearly understand some of the issues surrounding multiple power sources, multiple blocks and the effects upon my trains. Your recent posts have confirmed to me that I do understand, at least to a degree, the difficulties imposed by electric conventional control.
My most current set-up, which I am intending to expand, indeed allows me to take a locomotive through multiple blocks followed by the same transformer. Essentially, it seems to me, I must wire my layout to allow complete layout access, modified by block on/off switches, for each power source. I’m speculating that this might manifest itself with a series of hierarchical block switches for each power source.
I’m not sure what you mean by hierarchical block switches. How many transformer outputs are you planning to use? A single toggle switch (SPDT-CO) per block works well with two; but the concept can be expanded to more sources. You can connect to 4 with 1 SPDT-CO and 1 DPDT, 6 with 1 SPDT-CO and 2 DPDT, and 8 with 1 SPDT-CO and 2 3PDT per block. Some folks prefer a rotary switch to a cluster of multiple toggle switches per block; but, unless the rotary switch is augmented with an overall SPST on-off switch, you can get in trouble switching an occupied block from one source to another past active sources. This is why a toggle-switch cluster should always include a center-off switch at the center-rail end of the switch tree.
You may be thinking of on-off control of sidings and yard tracks. This can be done with just an SPST connecting the track to the associated main line or the yard lead. Instead, I have gone to the trouble of modifying turnouts to route power in these situations, in the American Flyer fashion; but control-panel switches work too.
I have 2 separate blocks on my layout, operated by 2 separate transformers. Both transformers have permanent volt and amp meters in line. One of the blocks is an incline, so there is always a difference in voltage as the train enters. I have never seen any noticeable spike in voltage or current as the train passes across the blocks. I have operated this way for years without any problems so I will have to agree completely with the above comment from BIGAL.
