I am planning to modify how I am supplying power to three blocks on my layout. Right now, each is an isolated section connected to a separate transformer. But the train sparks and hesitates crossing from one block to another. I plan to use two Lionel 95 reheostats instead. I will set the transformer to the highest speed I need and run that wire directly to the uphill block. Then I will run wires through each rheostat to the other blocks for slower operation. Are there any problems with this plan? Should I leave all three blocks isolated? They are completely isolated with fibre pins on the middle and both outside rails.
The rheostats I have have a bit of rust, but not too bad. What’s the recommendation
Rheostats with a single transformer are the best way to go. Since all the outside rails go to the transformer common, you don’t need gaps in those rails of course, but keep the gaps in the center rail.
Make SURE to check for phasing with the transformers. It can be as simple as turning the plug around in the wall socket to make the sparking stop. Jake
Let me make sure I understand. If I use one transformer and two rheostats, I should insert the fibre pins into the middle rails only to separate the 3 blocks. The metal pins can stay on the outside rails. Each “hot” or red wire will be run to the center rail with the rheostats running through the red to each block’s center rail. Correct me if I’m wrong.
Any suggestions for the rust, other than some fine sanding?
Jake, he’s planning to use a single transformer output, which eliminates any possibility of fault current.
However, if he were to continue to use separate transformers, the only way to eliminate the fault current entirely would be to have all three transformers putting out exactly the same voltage, waveform, and frequency. But he wants to have higher voltage uphill than on the level and higher on the level than downhill, which requires him deliberately to set them at different voltages. It is true that having the transformers in phase reduces the voltage difference between blocks, but it does not eliminate it in his case.
I got a little confused reading this, but thats not hard for me. I have two of these reostats, And i am just starting my layout . what is the main purpose of these. Do i need to use them? I am just trying to get a grasp on all this electrical part of the layout. btw i am using pw zw. mainly just trying to understand their purpose i guess…
Rheostats were commonly used in the early days before variable transformers were available. They were the only practical way to adjust train speed continuously, whether you were running off a battery or a fixed transformer. Long after toy trains made the transition to variable transformers, scale modelers continued to use rheostats with 12-volt DC supplies.
The problem here is that it can be downright dangerous to power successive blocks from variable transformers set to different voltages in order to change the track voltage to accommodate grades. Rheostats offer a safe way to adjust speed along the track: Using a single transformer with rheostats eliminates any possibility of fault current when the train crosses a block gap.
The fault-current problem is mostly a nuisance (arcing and voltage spikes) when you use separate transformers for the blocks, because the transformers’ circuit breakers shut them off (one hopes) before they get too hot. But, with the blocks powered from a multiple-output transformer like the KW, Z, or ZW, the circuit breaker does not trip because of a flaw in the design; and the transformer or the layout wiring or both can burn up.
There is actually an article by Neil Besougloff in the current CCT showing how to wire up this exact arrangement that I am telling you is unsafe. I wrote a letter to them about it. The new editor, Carl Swanson, thanked me and replied that they are going to have the technique tested.
I’m not sure of the resistance range of the Lionel rheostats. I’m guessing they were 0-10 ohms. Check with a multi-meter (power off) to see if I’m correct.
The issue with modern replacements is power rating. It’s very difficult to find (and spendy when you do find) wire wound variable resistors of hefty power ratings any more. It’s the short circuit current that drives the power rating.
As an example, say you have the rheostat set to 3 ohms for a train drawing 2 amps (6 volt drop at the rheostat from 16 at the transformer). At that setting, the rheostat is wasting 12 watts of heat. When the train derails and causes a short circuit, the full 16 volts is applied to the rheostat. You are now putting 5.3 amps through the rheostat (assuming the transformer circuit breaker trips at higher than 5.3 amps - normal for anything bigger than a 1033). The rheostat now needs to be rated at 86 watts to sustain the short circuit until corrected. The fix to keep the rheostat power rating reasonable is to add a 3 amp circuit break or fuse in line with the rheostat. The 3 amp value may have to be adjusted upward if you have a particularly power hungry train. The 3 amp circuit breaker drops the required rheostat power rating to 50 watts - a more viable solution.
If there are errors in my math or assumptions, please correct.
The Lionel rheostats, like the 81, 88, or 95, have a maximum resistance of 5 ohms. Lionel didn’t specify a current or power rating. It is the current rating that is important for a rheostat, since it is independent of the wiper position. From their size and construction, wound a porcelain tube and surrounded by a perforated steel cage, I estimate that one could carry 3 to 5 amperes. It should of course be protected by a circuit breaker or fuse at that level.
I don’t know about the Z1000, but if you do use an external fuse or circuit breaker, one in series with the transformer output terminal feeding the whole rheostat-center-rail business is enough. I would use a lower current rating, however, perhaps 5 amperes.
You can indeed wire two rheostats in series. But there won’t be any advantage unless one (the one for fine adjustment) has a much lower resistance than the other, perhaps 1 ohm. There won’t be any toy-train rheostats that meet that requirement.
You might be able to make something like this work: Put a 10-ohm resistor in series with a standard 5-ohm train rheostat. Then wire this combination in parallel with another 5-ohm rheostat.