I have a 120-V AC (input) variable output transformer, commonly known as a 'Variac', which I use for various electronic applications. I've been thinking of using it to power model trains. This would require adding a diode to the transformer output and perhaps a filter capacitor to smooth out the DC voltage. Have any of you tried this? What is the typical DC voltage range required to operate most model train engines?
That would be like playing with dynamite while roasting marshmellows over a campfire. The problem is that the Variac uses the same windings for both the input (high voltage) side and output (controlled voltage) side, commonly called an "auto-transformer". Here are the major drawbacks:
Any insulation failure could send full line voltage (120 volts) to the tracks, with all sorts of dangerous and possibly lethal results.
The Variac could accidentally be cranked all the way up, sending high voltage to the tracks.
Any short on your layout, such as a derailment or loose wire, will draw current up to the maximum capacity of the Variac, usually at least 5 Amps, enough to melt wheels and start a fire.
The solution, if you really want the Variac to control voltage, is to use an 18 volt isolation transformer connected between the Variac output and your model railroad applications. Buy a conventional 120V primary/18V secondary transformer, and connect the 120V side to the output of the Variac before your bridge diodes. And by all means include appropriate size fuses.
Incidentally the old toy train transformers were auto-transformers (Lionel and AF), but the maximium output voltage was limited to 18 volts. Toy train transformers haven't been made that way since the 1950s because of the hazzards.
Are they all auto-transformers? I agree those shouldn't be used, but there have been several variable transformer throttles marketed which weren't. The UK company Hammant and Morgan was one - they had primary and secondary windings, with the outer face of the secondary uninsulated and a sliding contact. I think the German motor manufacturer Buehler made one more recently.
Our O-gauge club has used similar non-auto variable transformers for years. It's a pretty old set up, but electrically safe.
Before the days of voltage regulator or transistor throttles they were the best way of getting hight current at low voltages, which was the problem with traditional rheostats.
A Variac is perfectly safe for model railroad use IF you use an isolation transformer between the Variac and any other circuitry, as I described.
A rheostat (variable resistor) is a very inefficient way of controlling a model railroad locomotive, since the voltage drop is caused by resistance. It was the best option we had 50 years ago, now there are many better choices.
=>I have a 120-V AC (input) variable output transformer, commonly known as =>a 'Variac', which I use for various electronic applications. I've been =>thinking of using it to power model trains. This would require adding a =>diode to the transformer output and perhaps a filter capacitor to smooth =>out the DC voltage. Have any of you tried this? What is the typical DC =>voltage range required to operate most model train engines? =>
Don't. Too dangerous, as other poster has explained.
I just ran the vairac to a cheap train power pack and set it to max.
The tortise requires 12 vdc, and i think 16v dc will be about all you need. some say leave it unfiltered and make it half wave because the 'spikes' of power will jolt the motor at slow speed. I have a transistorized power pack that does this. motors tend to be noiser when run on half wave at slow speeds.
Your idea will work, but I DON'T advise it. Most model trains run on 12 volt DC, so a 10:1 step DOWN is required to run off 120 VAC ... PLUS the diode as you correctly surmise to make the DC.
While this CAN be done with the Variac alone, you'll only be using the bottom 10% of the control range. Resolution will be VERY poor, and there's always the possibility of accidentally setting the voltage too high and 'cooking' your loco ... or yourself. *NOT* good
To use the Variac, you SHOULD install a 120 to12 volt 10:1 step down ("filament") transformer behind it. The transformer needs an adequate 'secondary winding' (output) current capacity ... typically 2 or 3 amps. These are not hard to find.
So. the arrangement would be: Wall outlet (120 VAC) to Variac (0-120 VAC), Variac to step down transformer (0-12 VAC), Transformer to diode (making "half wave" DC), diode to output (0-12 VDC, "half wave") .
Instead of a simple diode, consider using a 'bridge rectifier' (4 diodes) to get 'full wave' DC. A single diode only gives half wave DC). This costs only a few cents more, and will give improved performance. A filter capacitor across the output is a possibility, but probably not needed.
Most any book on model railroad electronics will cover these possibilities.
That will likely work (except with newer and sophisticated electronic power supplies), but all you're using the train pack for is the diodes. Not a very efficient use of either your money or the train power pack. What do you see as the advantage to what you're describing? Has the speed control part of the train pack gone bad?
To be safe, your solution also assumes that the train pack includes an isolation transformer. Most do ... but *NOT* all! Some old ones just used an autotransformer like a small Variac. Two Variacs in series still provide *NO* power line isolation. NOT good!
The H&M Powermaster" wasn't a Variac. It was a conventional transformer with the secondary windings exposed and a carbon brush wiping across the turns to vary the output voltage.
Variac is a trademark and can only be used to describe a particular type and make of variable autotransformer.
It would be very dangerous to use such a transformer as a model train controller - one of the rails would be effectively connected to the live conductor of the mains supply with the other rail connected to the neutral conductor. Very, very dangerous.
The standard is 12 VDC. Your Variac is not recommended for direct control because the useful range of the dial will be too narrow. What CAN be done is to feed the output of the Variac into the primary of a second 120 to 12 volt transformer and then rectify the output on the secondary for use on your layout. As you vary the voltage on the imput of the step-down transformer using the Variac you'll get a proportional variable AC output on the secondary. Make sure that both the Variac and the step-down transformer are rated sufficiently high for the purpose. 500 VA should be more than enough; 250 VA may suffice for 1 Athearn loco or 2 or 3 locos with can motors.
I like the idea of using the isolation/reduction transformer after the Variac, not only for safety but to provide a full range of control with the Variac. The control knob on the Variac has a nice feel to it, so I think it would make a good model train controller.
The combination of Variac and step down/isolation transformer still makes a pretty good, if heavy and bulky, train controller. Prior to the modern electronic power packs this was about the best you could do, and vastly superior to resistance (rheostat) type packs.
I've not done this, and I've never varied the supply voltage to a stepdown transformer. Are stepdowns linear up and down their operating envelope. Will a transformer designed to output 12V with a 120V input, supply 6 volts when fed with a
when fed with a 60V supply, i.e., is it always a 10:1 ratio?<
I'm not sure about the voltage ratio but do know that this combination works well (for it's time in history). Unlike normal power packs (pot controlled, i.e. rheostat) the output was effected very little by the current draw of the engine so before electronics it was the best approximation to variable constant voltage supply. There were a couple of power packs on the market that used a type of variac control. I believe one of the MRC dual power packs was this way, from the '50s.
To first (and good) approximation, and within ratings, the ratio of the number of turns of wire in the primary and secondary windings is the ratio of the voltages in those windings. This is true over a wide range of voltages.
In extreme cases the relationship fails ... those cases where load (current) exceeds ratings, or the field magnetic field in the core saturates, operation at frequencies below the design limits of the transformer, 'meltdown', etc.. This is not a problem in normal operation within ratings.
So, to answer your question, yes, a 10:1 step down transformer will produce 12 volts out with 120 volts in, or 6 out with 60 volts in. The relationship is linear for all intents and purposes applicable here.