I have an O-scale 3-rail track model train. I would like to be able to control this with a computer.

The normal model train transformer/controller goes from 0VAC at "stop" to 18VAC at "full speed".

My questions are:

1 - Could I technically control this with an electronic 120VAC dimmer pack, if I set a max level on the dimmer to 15% (thereby technically being 18VAC)?

2 - For safety and not frying components by accident, is there any sort of power supply that could 'scale down' the voltage, so 120VAC -> 18VAC and 60VAC -> 9VAC?

Thanks for any advice as to whether my thinking/approach is valid here, or if there are better ways of controlling this via a computer.

  • \$\begingroup\$ Incorrect assumption. Lighting dimmers do phase/duty control not voltage regulation, so your tracks will be at 120VAC. Sounds dangerous. \$\endgroup\$ – shuckc Sep 11 '13 at 11:47
  • \$\begingroup\$ Besides the problems of lighting dimmers others have mentioned, how would you try to comfortably reverse the direction of your train? 3-rail AC locomotives typically reverse the direction with a short impulse of approx. 20 to 24 V. A digital decoder might be a better investment, you can control it with a model rail booster and a PC software, you can find free and open-source programs on-line, like rocrail. \$\endgroup\$ – vsz Dec 5 '14 at 15:51
  • Use of a dimmer pack could kill you - even if voltage never went above 18 VAC.

  • Use of a dimmer pack could kill your train as if something is able to output 120 VAC but is "limited" to 18 VAC then Murphy has ways of arranging a burst of "more than enough" VAC for "more than long enough" mS to produce magic (train) smoke

There are a number of ways to get 18 VAC or 9 VAC - and it may also be worh finding out why the train com=ntroler needs AC, as it MAY be possible to use DC. It is like that the AC is converted to + DC and - DC supplies by the controller so, while there are ways of feeding + and - DC to the correct points in the controller, if you do not wish to venture into its insides with cutters and soldering iron then using AC will be easier.

You are shown as being in the US.
There are many sources of low cost transformers that would produce 18 VAC from 120 VAC and it would be a really good idea to acquire one. I don't know what Wattage or amperage (W = V x I) you need for your layout, but it is likely that you can acquire a transformer at low to no cost if you are keen enough.

Once you have 18 VAC control of voltage by computer is "relatively easy." The two methods most likely to be attractive are:

  • You can use phase control with a TRIAC.

  • You can use a MOSFET inside a bridge rectifier.

As we have covered the question material well enough I won't launch into model control. Let's make sure your needs are met in this area and we can then move to computer control. Note that the hobby community has been active for a very very long time with such things and much will be available on the web.


For 120 VAc: 18 VAC you can use -

18V secondary 2 x 9V secondary

12 + 6V secondary

3 x 6V secondary

Any of these MAY be available on an off the shelf transformer.

Digikey in stock 18 VAC

0.13 A $4.55 111 :-(
2A for $17.39 ~~~


Jameco 18 VAC


$9.95 each AC-to-AC ADAPTER

18V @ 2200mA, BLACK, STRAIGHT 2.1mm FEMALE PLUG Part no. 2076631
Manufacturer AULT INC. Manufacturer no. T57-182200

Basic data sheet

Aha - Ohio. Cheaper than China :-)


US$2.48 18 VAC 1.1 A. !


This video from Lionel explains how their modern and postwar transformers work. A typical train transformer will provide 18V at about 5A. The modern transformers work by turning the voltage on partway through the sine wave, while the older transformers work by controlling the peak voltage of a smooth sine wave. http://www.lionel.com/CustomerService/ProductInstructionalVideos/video.cfm?documentID=6534

There are a ton of commercially available ways to control trains by computer, including putting the speed controller in the engines themselves. http://wiki.rocrail.net/doku.php is a good gateway.


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