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I wanted to make / fabricate an instrument for my railroad project which will have an input of 220 VAC and out put will be varying between 0 to 16 VDC (Potentiometer) and will have an arrangement to change the direction of train (forward and backward).

I will be very much thankful to you for guiding me in this regard by giving me a list of components and circuit diagram thereof.

I am having a ready made (purchased from the market) equipment of the same but it has become very old and hence I want to develop myself.

With warm regards.

Sher Dil

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I have cut some parts of the diagram because it was a little bit confusing so now it's simpler.
Components:
22Kohm Potentiometer
10Kohm resistor
BC107 or BC547 NPN transistor
1N4001 diode
TIP147 power Transistor Darlington IC and his radiator
16VDC or 16VAC and rectifier bridge
switch
OPTIONAL: Polyswitch FUSE
This circuit uses a "sinewave" wave form and the performance its better than PWM!! Also is better because it is very quiet!

Does your, what you call, "ready made (purchased from market) equipment", have a nameplate that tells us how much max current,in amperes, or max power, in watts, it can supply?

If it does not say, I am going to just sort of guess that a toy train does not want more than 5.0 A of current, or equivalently (5.0 A)*(16 V) = 80 W of power.

I think the appropriate strategy for throttling power to your train, is the same strategy as used by a cordless electric drill.

That is to say, you start with a DC supply, capable of supplying the maximum voltage you could want, and then use a fast switch, like a MOSFET, driven with PWM (pulse wave modulation), so that you get a DC voltage whose time-average is the voltage you want.

Mathematically this is,

Vaverage = D*Vmax, where D is duty cycle, where 0 < D < 1. Essentially D is the fraction of the time the PWM wave is "on". Conversely, the fraction of the time it is off is 1-D.

To change the polarity, just use a DPDT (double pole double throw) switch wired to do this, like explained here:

https://forum.digikey.com/t/polarity-reversal-usin...

This is also the same wiring in the direction-switch, found in a typical cordless drill.

And I think that would be cheaper, easier, than using an H-bridge. Although an H-bridge might be handy, if you have to change the train's direction using logic level signals, like from a microprocessor or whatever, rather than an actual human hand toggling a switch. A DPDT relay would work too, and that could also work with microprocessor control, if you do not mind the audible, "click" noise it makes, upon being switched.

Actually, if you happen to have an old cordless drill, it has all these parts, specifically the PWM module plus a polarity changing switch. Also the voltage range, and power throughput capacity of a cordless drill power throttle is a close match too, so it would probably work for throttling power to your toy electric train.

Although, I think the usual electric toy train throttle has a big knob potentiometer, rather than a slider-style pot, connected to a trigger, as found in a typical cordless drill.

Someone skilled in the art, could replace a slider style pot, with big knob pot of same resistance, although there is labor, and possible frustration or trickiness, involved in doing so.

PWM in the form of a module, made to work with DC power in this range, e.g. 16 VDC, at around 5A max, can probably be found via the direct-from-China eBay sellers.

Although some care is required when shopping for modules on eBay, because the specs and wiring instructions are sparse, Spartan, if that's the right word for it. I mean, basically you'll be alright if you can figure out what you're looking at, what it is capable of, and how it should be wired up, before you buy it.

I should also mention, a DC-to-DC converter with variable voltage output, or buck converter with variable voltage output, is pretty much the same thing as PWM, and that is worth mentioning since these are words for descriptions of these modules sold via eBay. Some will come with a potentiometer on the module itself. Some with a place to connect an, appropriate sized, potentiometer.

Thanks a lot Mr Jack A Lopez for your detailed reply. The instrument I am having is of Bachman grey coloured around 100 mm x 100 mm x 40 mm plastic box. I wanted to run two trains (2 Engines) which they could not do. A friend of mine suggested to go in for higher Amperage. I am basically a Geologist, retired now and wanted to pursue my age old dream hobby - the Trains (railroad). I am trying to learn right from basics where in your suggestion is of great help for me. I will try to do as advised by you. Thanks a lot once again

A train (model) is nothing more than a simple DC motor that gets its power from the tracks the train is riding on.
Lights are powered the same way.
I had a different brand back when I was young but the transformers for trains worked all the same back then

Inside the box is a center tapped transformer, rectifier and a wire wound potentiometer.
On the "advanced" models you also find a capacitor after the rectifier.
The only special part is that potentiometer.
Center tap of the transformer is zero voltage, and the two halfs provide with the diodes or rectifier a positive and a negative voltage.
On the potentiometer the center tap is connected to the output or what goes to power the train.
And well, the positive and negative connects to the windings of the potentiometer.
In return you get nothing with the knob in the center but increasing speed or voltage in both turning directions.

It is lossy, is is messy and it wastes a lot of energy.
A simple and cheap lab power supply might be far too high on the full voltage of around 30v but you don't have to crank it that high ;)
And as Jack said, if you add a three way switch you can select the direction of the train by reversing the polarity.
Added bonus is that you can see how much current the train draws.
Can be quite nice to see problems like worn brushes or similar before they become a real problem.

But as said, since you are dealing with just a normal DC motor checking a H-bridge controller might be n option too.
Ebay also has cheap replacements for the switches of cordless drills.
Not too hard to create something with a dial to press the know in to crank the speed up.
And with a bit of tricker you can also include the direction switch into the design - or just operate it manually ;)

Transformer or SMPS followed by a rectifier and H-Bridge.
The h-bridge you can even buy as ready to go components if you look around as they are a standard motor control element.
But at least for me nothing beasts the old transformer with knob on top.
Corroded contact or worn ones are fixable and can be cleaned as well.

Thanks Mr Downunder35m, I will try to locate the circuit diagram and do as suggested by you.