Hello everyone, this isn't my usual kind of forum but my latest hobby project has turned into quite the rabbit hole. Long story short, I have a transformer that is functioning properly and steps the voltage down from 120 Volts to 18 Volts AC. I need to find an AC motor control circuit that can be used to regulate the voltage at the track to control the speed of the trains. I can only find stock AC motor controllers that have a working voltage of 110 Volts. Does anyone know of a way to have AC voltage regulation at lower voltages? I have found schematics on the internet for these types of the motor controllers but the DIACs used in these circuits only trigger at 30 Volts. Thanks for the help, I am curious to see if this can be done.

 

You might take a look at this collection of reviews of train controllers, with circuit diagrams. Nothing like what you're doing!

https://www.scottpages.net/ReviewOfControllers.html

 

You might take a look at this collection of reviews of train controllers, with circuit diagrams. Nothing like what you're doing!

https://www.scottpages.net/ReviewOfControllers.html

Thank you for the reply. While that is an interesting read it deals with train controllers that are strictly DC. The trains in question need to run on AC. I am obviously no expert but I m guessing the circuitry would be greatly different for AC and DC regulators?

 

Who is the manufacturer of the trains you are trying to control?

The old Lionel controllers were an auto transformer variac. They used a transformer whose secondary was tapped by a sliding contact which varied the output AC voltage. You can use a 110VAC version to vary the output voltage from 0 to 110 and feed that into an 110-18VAC transformer.

 

Who is the manufacturer of the trains you are trying to control?

The old Lionel controllers were an auto transformer. They used a transformer whose secondary was tapped by a sliding contact which varied the output AC voltage. You can use a 110VAC version to vary the output voltage from 0 to 110 and feed that into an 110-18VAC transformer.

Lionel is the manufacture, and I did not even think to put the regulator in front of the transformer, thank you for the suggestion. The only concern I would have is the safety of doing something like this with the higher voltage through the controls. The modern controllers regulate the voltage after the transformer unlike the the older ones as you mentioned. I'm sure there is a way to do it with the lower voltage I just can't seem to figure out how.

 

Varying AC voltage is not a trivial task if you want to maintain a sinewave..
Using an autotransformer as djsfantasi mentioned is likely the easiest way.
How exactly do you want to control the voltage, do you want to use a rotary knob?

 

Hello everyone, this isn't my usual kind of forum but my latest hobby project has turned into quite the rabbit hole. Long story short, I have a transformer that is functioning properly and steps the voltage down from 120 Volts to 18 Volts AC. I need to find an AC motor control circuit that can be used to regulate the voltage at the track to control the speed of the trains. I can only find stock AC motor controllers that have a working voltage of 110 Volts. Does anyone know of a way to have AC voltage regulation at lower voltages? I have found schematics on the internet for these types of the motor controllers but the DIACs used in these circuits only trigger at 30 Volts. Thanks for the help, I am curious to see if this can be done.View attachment 225774

Replace the motor in the diagram above by the primary of your transformer.
The motors in the trains will work fine off unsmoothed rectified DC so the transformer manufacturers won't have included a smoothing cap in order to save money.
If there IS a smoothing cap, then it might not work quite so well as a speed control.

 

Why not put the auto transformer on the secondary 18 VAC side? This way any 1:1 auto transformer will work, I think it would be easier to find one specifically designed for running Lionel. I like Lionel, there are some interesting accessories designed for them and wouldn’t change the system (motor) out.

 

The old Lionel controllers were an auto transformer. They used a transformer whose secondary was tapped by a sliding contact which varied the output AC voltage

That's not an autotransformer - that's a variac. An autotransformer is a transformer where input and output are taps on the same winding, which is something you really don't want on the mains because it has no isolation.

Why not put the auto transformer on the secondary 18 VAC side?

Presuming that you mean "variac" - where were you hoping to get a variac with a 18V input? ( A 1:1 autotransformer is just a rather pointless large value inductor connected between live and neutral)
If you use a 120V or 230V input variac on 18V then it needs to be rated for the output current, so it's VA rating will need to be 7 or 14 times the power required.

 

Hey @Ian0 i think we are having a language situation, it’s ok my wife is from the lower midlands... what we call an autotransformer has taps on the secontrary which allows you to change the output ratio...

 

That's not an autotransformer - that's a variac. An autotransformer is a transformer where input and output are taps on the same winding, which is something you really don't want on the mains because it doesn’t provide isolation

Thanks, Ian. I indeed mean variac. I just couldn’t remember the name and erroneously chose auto transformer. I’ve edited my original post.

 

I was guessing it must be one of those trade names that find their way into common usage like Hoover, Tannoy and JCB.

 

Yeah my wife calls all vacuums Hoovers, usually when I hear autotransformer it’s implied that it’s variable. They’re usually tapped otherwise you just have a huge inductor. ... and xerox

 

Thanks for all the help everyone. One note on the wave for coming from the train transformers, here are two popular and modern AC train transformers on the scope, pictures provided by some friends over on a model railroading forum. The first is a Lionel CW-80 and the second an MTH Z-1000. I am guessing these do not represent very clean AC signals?

 

Those waveforms come from a phase-fired triac dimmer circuit just like the one in Post #1! (The right one rather better filtered than the left)

 

Thanks for all the help everyone. One note on the wave for coming from the train transformers, here are two popular and modern AC train transformers on the scope, pictures provided by some friends over on a model railroading forum. The first is a Lionel CW-80 and the second an MTH Z-1000. I am guessing these do not represent very clean AC signals?
View attachment 225805View attachment 225806

These are not very motor friendly, that steep leading edge from phase control can cause excessive motor heating and commutator damage, in the case of model trains wheel pitting and track dirt as well. An old fashioned variac is much kinder producing a sine wave output. Of course it somewhat depends on the scale/gauge and hence the motor size as well as any filtering components attached to the motor, in my case in N-scale the above waveforms caused severe damage.