Running across gaps

[lionelsoni]

Chris "Birds" sent me this by e-mail. I am answering him here because this topic seems to be generating a lot of interest lately.

"I was reading your comments on the CTT forum about trains running across blocks set to different voltages. I am trying to understand this and have some questions if you don't mind.

"I have read through both volumes of Peter Riddle's "Wiring Your Lionel Layout" but don't recall this being covered.

"As an example:

"I am running a KW transformer.

"I have a loop of track (Block A), and one turnout off that loop.

"This turnout leads to a very long siding (Block B) isolated from the turnout with a fiber pin on the middle rail.

"Block A comes off post A of the KW.

"Block B comes off post B of the KW with the intent of allowing the engine speed to be slowed down and controlled separately as it crosses into this block.

"If I understand what you are saying, you do not recommend this setup because at some point an engine or lit car can straddle both blocks A & B, and if the voltage between the two blocks is set differently, then this could short at the winding coil, and is a possible fire hazard.

"My confusion comes because in the owner's manual for Lionel's new ZW there is an example of single throttle train control for a simple loop using different blocks set to different voltages to maintain a constant speed over grades.

"Would this setup have the same concerns as my setup with the KW, or is there a difference between vintage KW and ZW transformers and the new ZW in this regard?

"What would you recommend doing to protect the transformer in this case if I wanted to have separate throttle control?"

Enquiring minds will want to look at the wiring diagram for the KW on Olsen's web site: http://pictures.olsenstoy.com/cd/transfmr/pskw2.pdf There you can see that, except for the whistle and reversing switches in series, terminal A is shown connected to the right end of the 14-volt secondary winding and terminal B to the left end. So there are 14 volts between those terminals. If you trace the path between them, you can see that it never passes through the circuit breaker. Therefore connecting A and B together is literally shorting out a 14-volt transformer secondary. If you do this, you will find that a lot of current flows and, after a little while, the transformer gets very hot; but the circuit breaker never trips.

Now, if the two controls are instead set to the same voltage no current will flow. If they are set almost to the same voltage, a smaller current will flow. But if the voltage difference is substantial, a substantial current will flow, whether this is done by accident, as by forgetting to set the controls to match when moving from one block to another at constant speed, or deliberately, as Chris proposes to do, to slow a train down as it enters a siding.

I am not familiar with the design of the new ZW; so I can't say whether they have done something new to make block transitions possible and safe. But the problem exists for old Lionel multiple-output transformers, which, as far as I know, never had more than one circuit breaker, located in series with the common. This includes the much admired old ZW and the Z.

These transformers can be made safe by adding fuses or, my favorite, circuit breakers, one for each output. I recommend automotive ones, which are easy to find, cheap, and very similar in principle to the original Lionel breakers.

I would not rely on fuses nor breakers to protect new electronic locomotives, but would put Transzorbs across the outputs for that. The problem is that any damage is done by high voltage, not high current. The harmful voltage is produced not by the short circuit but by the clearing of the short circuit, which acts with the transformer's output inductance to make spikes.

A safe way to make a slow siding is to feed it from the same supply as the main loop but through some voltage-dropping component, like a resistor or a string of back-to-back diodes. For this, I like the prewar Lionel rheostats (wire-wound variable resistors), which can be bought for just a few bucks each.

[Anonymous]

This whole problem can be avoided by using an insulated center buffer rail as long as or longer than the maximum train lenth, and 2 DPDT relays activated by 2 outside insulated rails on each side of the buffer. Hook up one relay coil through the contacts of the others so the relay latches and unlatches just after the train the passes through the buffer block. Hook the center buffer rail to the common of the other set of relay contacts one transformer to the NC and the other to the NO contact. Using this method the transformers can never be hooked toghether. The system is bi directional. As soon as the relay is thrown or canceled by the front train wheels the cars behind it are automatically switched to the other transformer.

Dale Hz

[Anonymous]

This wasn't as big an issue with post war trains and transformers (especially the larger units). There were even cases where this as offered as a "solution" for running trains up and down grades under "automatic" control. There would be some back emf when the train was crossing the "gap" as their would be some current flow from the area of higher potential to the area of lower and this would cause some additional heat build up in the transformer windings. Problems would occur in the event of a short circuit. The usual vicitm of these shorts was whatever wiring was intervening, usually illuminated passenger cars. The light weight wire could become a "fuseable link" and melt down depending on the amount of power involved.

This also won't be an issue with a modern supply running in a fixed voltage command mode. If the blocks are all the same at say 16 volts, no flow. I wouldn't be too keen on trying this stunt with modern supplies and post war train set let alone modern supply and modern train sets. Switching power supplies really don't like to see back EMF and modern trains aren't built to mil spec standards the way the old PW stuff often was. A nasty short on a PW train might result in some melted insulation. On a newer electronic set it will probably fry the board(s).

[cnw1995]

This reminds me of the automated warning on the Hong Kong MTR that calls out 'mind the gap.'

[Birds]

Bob,

Thank you very much for your detailed reply and the link to the schematics. It is much appreciated.

Birds