Using resistors to slow trains on downgrades

[Anonymous]

Rather than a short I think it it is more like a backward flow through the path of least potential creating a hot spot in the winding. But there is a circuit flow.No matter what term you call it it is not desirable and can be avoided with a simple relay system,very easy to set up with 3 rail track systems. Anyway a lot of food for thought. Thanks for the imformative posts Lionelsoni.

Dale Hz

[lionelsoni]

It sounds like you understand the problem pretty well, Trevor. All that you are missing is that the practice is very common and that people do it either because it is about the easiest way to wire up the layout and they don't know that there is any danger in it or are willing to take the risk. If the train is moving at a reasonable speed and the voltage difference is slight, there is unlikely to be any damage from the fault current. This approach leaves it up to the operator to shut things off if these conditions are not met.

The business of using a different transformer voltage on a grade is a little flakier, in that the voltages are deliberately set at different values. Arcing and momentary transformer overloading are to be expected. The only saving feature is that, if it is done with separate transformers, the circuit breakers at least are functioning. I'll bet it has been done the other way, however, with dual outputs from the same transformer, in which case a stalled train could be quite dangerous.

When the multiple pickups are on a single assembly, as on a typical steam locomotive, the fault current will flow through that and bypass the inside of the locomotive. However, when pickups are mounted on separate trucks, as on many Diesels and passenger cars, the current will flow through the fairly small wire that connects them together. This wire may be the first thing to burn out, saving the transformer but damaging the rolling stock.

The diode scheme that Dale describes is the same one I referred to in connection with the bridge rectifiers. They are just an easy way to combine two pairs of diodes into a single package. For modest reductions, the diodes lower the RMS voltage by about 90 percent of the diode's forward drop. a bit less reduction with each additional diode pair. This varies a little with the current. There is not actually much difference in the power dissipation between the diode and resistor schemes.

Whether you call the situation of transformers connected together a short circuit or not is a matter of what you want to call a short circuit. Connecting together two taps on the same transformer secondary winding seems very like a short circuit to me. In fact, the last two type-Z transformers that I opened up each had a burned section in the middle of the secondary. I can't imagine another way that that could have happened.

[Anonymous]

The 2 pickup rollers essentially connect both power sources (via the center rail) since they are wired together with a common wire. One roller is on one block and one is on the other.You wont get a short unless the transformers are out of phase. If running a passenger train this would happen numerous times. On a freight train 2 times,once on the engine and once on the caboose.

Dale Hz

[Anonymous]

I am confused about this issue of shorting the transformer outputs if you run an engine or car with two pick-up rollers from one power block into another one. For example, if I have two main lines, each powered by a separate output of a modern ZW (ground return is common to all the outputs) and I have a train cross over from one main line to the other, across a switch with a fiber pin in the middle power rail where the switch ends and the other mainline starts. What I think you are saying is that unless the two mainlines are at the exact same voltage, I will create a short and there will be a problem. But don't people build their layouts this way all the time? Trains cross over from one power block to another. The outside rails will be continuous from block to block (i.e., metal track pins), and the "power blocks" will be separated by fiber pins. In the Wiring Your Lionel Layout, Intermediate Techniques book, he never mentions any problem with the train going from one power block to another. He also describes using two separate transformer outputs, set to different voltages to allow the train to go from a "fast" area to a "slow" area. He does not mention any problem with the train causing a short. What am I missing?

[Anonymous]

Dont use resistors use diode arrays. Diodes consume little power and drop voltage by a fixed amount. each diode pair drops AC voltage by .7 volts. You can make these up yourself by going to Radio Shack. Buy a jumbo European style barrier strip and some 3 or 6 amp diodes. Mount them in opposite directions up each side of the strip. Put this in series with the block and transformer power use the most suitable tap for your grade. A 12 position strip can drop voltage up to 8.4 in .7 volt increments. If you dont live near a radio Shack I would be glad to send you some diodes. Using 3 amp diodes would be good for about 100 watts and 6 amp 200 watts.

Another problem you may have is double roller cars jumping the voltage when they cross the center insulating track pin. You can over come this by making an isolated center rail as long as the maximum train lenth on the approach to the down grade. Install an insulated outside rail on the down grade the same lenth as the isolated reduced power center rail on the downgrade.. Install an DPDT relay activated by the insulated rail. Run the approach isolated center rail to the common of the relay contact. Run the track power to the NC contact and connect the downgrade center rail to the NO contact. when the train hits the down grade it will connect the 2 isolated center rails together and disconnect the approach center rail from the transformer.

Dale Hz

[lionelsoni]

I am assuming that the outside rails are all connected together to form a common or "ground". As you say, there will be a problem when the pickup rollers connect the two transformers together. Each will try to impose its own voltage on the other and a large current will flow through the pickup rollers from one transformer to the other.

The situation is very similar to a simple short circuit. If you connect a 5-volt transformer output across a wire, a heavy current, called a "fault" current, will flow, limited only by the capability of the transformer to supply current. The same kind of fault current flows when two transformers set to differ by 5 volts are connected together.

If the transformers are separate, then each one has a circuit breaker somewhere in series with its output that can react to the fault current and shut it off before the transformer burns up. This doesn't help the arcing at the track; but it does prevent a fire.

If both transformers are really one ZW transformer with two outputs, things can get dangerous. Lionel took a shortcut in their transformer design by using only one circuit breaker per transformer. The fault current that we are talking about unfortunately does not flow through the circuit breaker. So, if a train should stop while bridging the gap, the transformer can just get hotter until it burns up, possibly along with your house.

The nice thing about the rheostat is that there can be no fault current across the gap, since the open-circuit voltage to the track (the voltage present when the train is not there) is exactly the same on both sides of the gap, since it comes from the same transformer output.

It is also possible to reduce the voltage safely using a trick with bridge rectifier modules, which are not expensive. They have the advantage that the voltage reduction does not increase as the load increases. Search the forum for "bridge rectifier" to find the many postings on this topic.

[Anonymous]

"but there will still be a fault current as the train crosses the gap"

Can you elaborate on this? I read what you posted on that other thread, and I am kind of lost on this. Can you try to explain it again. Also, I think what you are saying is that as the train passes from one power block to another, with one pick-up roller in one block and another roller on the same engine or car in the other block, there will be a problem of some osrt (sparks? A short that will trip the circuit breakers on the modern ZW?). Can this be prevented by having a common layout "ground"? Sorry to be sort of clueless, but can you try to explain it to me?

thanks, I appreciate these answers and the great info.

[lionelsoni]

I think the resistor is a better idea than the transformer. Lionel made several rheostats (variable resistors) for controlling prewar trains, before the advent of variable transformers. These are not hard to find for a couple of bucks and are ideal for just this sort of operation.

I just described the problem with transformers in this topic:
http://www.trains.com/community/forum/topic.asp?TOPIC_ID=49966
If you do use a transformer, a separate one is much safer than using a second output from the main transformer; but there will still be a fault current as the train crosses the gap.

[Big_Boy_4005]

I like Fred's idea of a second small transformer. Of course, what goes up must come down, though in this case it's the other way around. If you had another small transformer, you could use it for the uphill portion of the run. Kind of a "poor man's" cruise control.

[fwright]

Trevor

I like the idea of wiring a toggle switch in series so you can disable the resistance circuit when desired.

Roger's idea of a variable resistance is good, too. One of the inherent problems with added resistance in series with a locomotive is that the voltage drop achieved depends upon the current being drawn by the locomotive. A smaller, modern O27 train set engine running light might draw 1.5 amps. With 1 ohm resistance the voltage will be reduced by 1.5 volts, which may not slow the engine sufficiently. On the other hand, a twin motor postwar F3 might draw 4 amps. This means a 4 volt reduction, which may or may not be the right amount of reduction to reduce the speed to where you want it to be. So a 5 ohm variable resistor would probably work very well.

The other downside of a resistor is getting a big enough power rating to handle short circuits in the block of track it is regulating. A 1033 transformer will put out at least 4 amps before the circuit breaker even thinks about popping. Larger transformers would be even worse. In the event of a short circuit, your resistor - if set at 2 ohms - would be generating 32 watts of heat until the circuit breaker pops. The higher the variable resistor setting, the more the heat load, and far less chance of the circuit breaker ever popping. At 4 ohms and 16 volts, the resistor will a red-hot 64 watt heater, and the circuit breaker will never pop. It's unlikely you would be at 16V and 4 ohms - and less resistance is going to pop the circuit breaker, so a 25 watt variable resistor would be reasonably safe.

A better solution is to use separate, small train set transformers for your downgrade sections. Phase them with your main transformer, and put a SPDT toggle to select which transformer powers the downgrade section. Now you have a variable voltage source for the downgrade block that can be preset to the voltage you want, and is reasonably independent of locomotive current draw. Temporarily overloading a small transformer with a heavy train on a downgrade is not as likely, and not really a big deal. You would want to switch back to the main transformer if going up the grade anyway.

The main drawback to the phased transformer approach is that it may cause momentary small voltage spikes as wheels cross the block boundaries. These will be of no concern to non-command locos, but could cause problems with the more sensitive command electronics. I would hate to see you let out the factory installed smoke in these electronics - it can't be put back in!

As always, choices and decisions. I wanted you to know the downsides before you went ahead.

yours in transforming
Fred Wright

[Roger Bielen]

Hi Trevor,welcome to the forum. [#welcome][#welcome][#welcome]

I haven't used this system, I run TMCC, but there is no reason that it should not work. Essentially, you're creating a block, remember to isolate the stretch of track from the rest of the system using insulated pins, or a gap, on the center rail at both ends of the block. Also, resistors generate heat, mount them where this won't create a problem. Mounting them on a piece of alum. with heat transfer cement (from Radio Shack) and using a ceramic resistor, if the right size is available, would probably work best.

Rather than a fixed rated resistor they may have a variable resistor with the WATT rating you need, this will allow you to "tune" the power.

Let us know how it works out.

[Anonymous]

I've been reading the Greenberg's book Wiring Your Lionel Train Layout, Intermediate Techniques. http://www.amazon.ca/exec/obidos/ASIN/0897783727/701-0548369-8985157


I have a few questions. First one is about using resistors to slow down the engines so that they do not go crashing down a downgrade and then de-rail. This is non-command control stuff. I am building my layout without command control, and I know it is all better and easier with TMCC or DCS. OK. But I want to do this without command control. So, has anyone tried these ideas? Basically, you wire in 1 or 2 resistors (1 ohm, 10 watt) in line with the power feed going to the downgrade so that it decreases power as the engine enters the downgrade. You can use toggle switches to route the power through the resistors or directly to the track so you can run trains in any direction, but still have the "resisted power" for the downgrade. Has anyone tried this and does it work? Many of my engines do not have cruise control. Some of them do, but many of my engines do not, and I do not want to use command control.

Thanks !!