2 Electrical Questions ... Bob Nelson

[CVette]

Hello all, new member here with two questions.

I'm setting up a temporary Christmas layout to run some MTH PS1 and postwar engines using an MTH Z1000 or maybe my ZW.  In order to use the ZW I understand I should use fast acting fuses and an in line fuse holder to protect the PS1 locos.  Should I be using 10A or 15A fast acting fuses?  I assume the fuse goes on the lead to the center rail?

The second question is in relation to this post for rheostats: http://cs.trains.com/trccs/forums/t/36556.aspx     I'd like to find a rheostat to reduce the max transformer voltage to have my 4 year old run the trains without my hand on the controls all of the time.  In the link, I've read I could use the Lionel #81, 88 or 95 rheostat units or buy soemthing like the Ohmite resistor.  Bob Nelson listed a 6ohm, 12.5W unit.  I'm trying to learn how to calculate the required rheostat performance needed.  Can anyone help me go through the math?  I don't know the proper assumptions to make - like current draw - for typical toy train operation.

Thanks!

[lionelsoni]

It's not unsafe to use fast fuses; but a thermal circuit breaker is more practical, since it won't trip on short harmless overloads.  You'll need at least a 5-ampere breaker for a train, maybe 10.  Use no smaller than 20 AWG wire with a 5-ampere breaker, 16 AWG for 10 amperes, or 14 AWG for 15 amperes (which is also the rating of the ZW's internal circuit breaker).

Don't think that fuses or circuit breakers will protect locomotive electronics.  If the locomotive draws enough current to blow the fuse, it is already done for.  What the electronics are vulnerable to is overvoltage, which a fuse cannot protect from.  Instead, wire a transient voltage suppressor (TVS) across each transformer circuit.  But, if you're using postwar locomotives, they have no electronics to be harmed by overvoltage.

A rheostat is a very practical way to control a train; but it does not directly regulate the track voltage and certainly not the train speed.  Those will vary with the current that the train draws, which is different for different locomotives and for different trains that they pull.  The Lionel rheostats are about right for controlling a variety of trains, but cannot provide you with any sort of consistent speed limit for your 4-year-old.

One option is to add some sort of mechanical limit to the transformer handle, which you probably don't want to do.  Another is to put a series string of diodes connected in anti-parallel in series with the transformer's output.  These will give you a well-regulated maximum voltage.  You can make them from bridge rectifier modules.  However, each module will lower the voltage by only about 1 volt.  Since the ZW can put out 21 volts, this might require quite a few modules.

[CVette]

Thank you Bob!  I really appreciate your help.

So should I not bother with a circuit breaker if I get a TVS?  Are the breaker and TVS something I can find locally at a box store or electrical supply store (I have one local)?  Can you provide some leads to online places if local isn't an option?  I've seen the parts on Scott's odds n ends but didn't know if the prices were competitive or if he was still in business.

Thanks for providing the options on the voltage reduction.  I was trying to avoid a mechanical stop but could if I had to.  What if I decided to forgo the variability and just bought simple resistors?  I could buy a bunch in different resistances and then test until I found a suitable rating.  That seems the most simple and inexpensive.  I'm guessing I could find those at an electrical supply store.

Chris

[TrainLarry]

  If you are using a ZW transformer, a quick and dirty way to limit the voltage would be to insulate part of the winding under the rollers. Dedicate one throttle for your child, and set the throttle to a comfortable running position. Note the position of the handle. Now unplug the transformer and remove the top case. Put a strip of electrical tape on the coil so the tape covers the remainder of the coil (just under the roller), and the roller will roll onto the tape. Make sure the end of the tape is sitting flat against the coil, or the roller will lift up the tape. Check the tape often to make sure it is still in place, and is not wearing through.

Larry