Limiting Track Voltage / Train Speed

You can wire some back to back diodes in series with the transformer output to limit the power applied for every loop.  This will work pretty well unless there are grades, that could be a trickier thing to solve. 

Can drop transformer voltage with diodes (Use 25 amp bridge rectifiers),explained here

 

www.jcstudiosinc.com/BlogShowT...3&categoryId=426

 

Also can install voltage dropping diodes in the engine. This not only eliminates jack rabbit starts but also top engine speed. Explained here

 

www.jcstudiosinc.com/BlogShowT...8&categoryId=426

 

Dale H

Starting at the end and working up...

 

Resistors are for reducing current (amperage) not voltage. This is a common misconception.

 

If you use conventional trains and older, PW transformers, you can put an autotransformer (brand name, for instance, "Variac") to set the maximum percentage of line voltage going into the train transformer. Set it at 50% and the top speed of the train will be half of the maximum allowed in normal operation.

Bill,

 

I like Arther's solution. Simple, robust, and one setting for all the transformers. Elegent. Now to locate a Variac of suitable size...

 

Chris

LVHR

I would not use a Variac on the input of a CW mentioned or modern type transformer. It would be OK for PW type as Arthur mentioned. Variacs are also a bit expensive but a good solution.

 

Dale H

My idea would be to use Z1000 controllers connected to a multi tap transformer with a lower than 18v secondary.

So, determine the output voltage required by trial and error, and just buy the appropriate transformer. Maybe something like a 9 or 12volt secondary would be about right. And don't forget to add a circuit breaker.

I think a variac would work with the MTH Set style transformers.  Not sure they would work with the Z4000 or CS 80's though.

 

Could use variacs between the output of the transformer and the the track.

 

The diode / rectifier solution is also somewhat simple and I have a bunch on bridge rectifiers in my parts box.  I like that I can get nice steps of of .6 volt reductions...

 

I thought there was some reason that resistors were the wrong thing!

 

I don't think I want to mod any engines just for this one display, but dropping the transformer output is good.

 

Dave,

 

I could use the 14v output from the z1000 instead of the 18 volt output....

One issue I see with a variac is that different models of trains require different throttle settings for the same speed.  In order to get some PW AC motored locomotives running at a reasonable speed, you may already be derailing a can motor equipped locomotive.  The tuning with diodes allows you to deal with that issue.

John,

 

Yes a good point I am certain I would need independent control of the max on each of the 6 loops because of the wide variety of engines to be used.  

 

So 1 variac in from on all six transformers would not give me enough fine tuning during setup.

 

I think with variacs my best be would be to use them between the output of the transformer and the track.  I am not sure what the effect of some of the not exactly sine wave output would be there either.

 

Keep the good ideas coming!  I appreciate all the ideas so far!

I think finding variacs to put on the LV side is going to be a bit hard to do, and certainly expensive!  OTOH, a boat-load of 6A diodes will be pretty cheap, and they'll do this job quite nicely and allow fine tuning of each setup.

 

The issue of fine tuning each loop is somewhat of a sticking point here.  Short of buying a bunch of variacs or special transformers with a maximum output limit, I think the diodes offer the most cost effective solution.

PS2 engines in conventional default to the same speed vs. track-voltage profile of ~20 sMPH at 12V, increasing by ~20 sMPH per Volt above that.  So you can set a reasonably accurate speed limit - independent of load - by limiting track-voltage using diodes or whatever. No programming is required from command mode. You can turn off speed control using the Whistle/Bell buttons to make the behavior more similar to basic reversing units presumably in some of your other engines.

 

Stan,  Good info to have!  I did not know that!

 

I think the back to back diodes is the way to go here.  Especially since I have a bin full of appropriate parts already!  Two to three bridge rectifiers off of each transformer gives me the ability to vary from 15 to 18 volts as a max.  Easy to wire up and easy to adjust when I am setting up.

 

Thanks to All!

Sounds like a plan.

Can't you just tie a knot in the cord?

Don't know if MTH has a max speed function, but Lionel command engines can be programmed individually for a maximum speed.  You can get the process from the Lionel service video website.  I'm pretty sure you:  Address the engine, press set in the cab1, accelerate the engine to the highest speed you want it to run, press the boost button once.  That should lock in the max speed for that engine.

Uhh... from the original post.

 

But instead I want to put a few conventional transformers out and just let the kids control the trains. 

 

No TMCC in sight here.

Awhile back, someone here put mechanical stop on the CW-80 throttle. It was simple, but I don't recall what he used to keep his young-uns from flying the trains. It was removable. --Joe

You know the old adage that if you give a carpenter a problem all of the solutions will be wood and nails?

 

I love the fact that Dale H always comes up with back to back diodes and I come from another camp and all of my solutions are rheostats.

 

I recommend the use of a 50 Watt 10 Ohm rheostat. (I think that I have as many airplane and Ohmite rheostats in my spare parts drawer as Dale has diodes.) The rheostat would allow you to fine tune the current flow compared to the back to back diodes. They can be found reasonably priced on eBay and easy to insert in your circuit.

 

BTW - both solutions will heat up the joint, but will keep the trains on the track.

 

Denny

Some of the post war transformers vary the variable voltage maximum output depending on which binding posts their output is taken from.  But their circuit breakers are not very good and an external curcuit breaker would be advisable.

Well, the difference between diodes and rheostats are that with diodes the voltage is pretty constant, however with rheostats the voltage varies with current.  Worse yet, when you are coming on a downgrade with little current draw, the rheostat will give you more voltage, probably not what you want if you're trying to control speed.

 

Both will slow things down, I personally think that diodes are a better solution in this instance.