Power Supplies (transformers)

I figured I would write a few words about the power supplies people would / can use for running trains.

First, the "old" word is often used: "transformer". I assume this comes from the fact that the old toy trains mostly ran on AC, so all you needed was a tranformer, which just changed (transformed) the 120v AC to something lower.

In reality, for just DC power, you need more components, and also often people confuse plain power sources with ones that have a variable output (often called a throttle).

Indeed the old lionel transformers usually had a speed control, but it was often part of the transformer "winding".

So let me establish a few terms that will work for you AND will not give an electrical engineer a brain aneurysm.

Transformer: An actual physical transformer with a metal core, copper windings, which serves to change an AC voltage to another AC voltage.

Power supply: any device that takes your 110v from your house and convert it to some other voltage AND/OR power type, typically DC around 24 volts.

Throttle: A device that takes a fixed input voltage and produces some kind of variable output, it may be a smoothly varying DC or PWM (pulse width modulated), etc. PLEASE NOTE: on this page, I'm ignoring wireless throttles for remote control systems like AirWire, Crest Revolution, DCC wireless throttles, etc. They are indeed referred to as throttles too.

What does it take to "make" a power supply?

It depends, for an "ordinary" power supply, with a DC output, the minimum is a transformer and a rectifier(s).

The transformer converts your AC 110v to a lower AC voltage, like maybe 20 volts AC. Then the rectifier (diode) then converts it to DC.

At this point, because AC is a series of alternating polarity pulses (sine wave), converting it to DC with a rectifier gives you a DC that sort of pulses. Typically a filter capacitor is added to smooth out the pulses.

 

What is a "regulated" power supply, and why do I care?

The way a transformer works is that the output will vary under load. Our model trains have electric motors whose speed is very dependent on voltage, so when you present more load (a second train, going up a grade, more locomotives) the voltage of a transformer alone will drop, and you will see stuff slow down.

So, enter the regulated power supply, where the power supply gives constant voltage regardless of load (current drawn). This makes operation much more consistent, especially when electronics is involved, whether DCC or even just sound cards in the loco.

It used to be an expensive proposition, since regulating DC basically took what amounted to a large amplifier and a "volume" control constantly watching the output voltage and compensating.

But those days are long gone, and inexpensive "switching" power supplies well under $100 can be found easily. By the way, anyone telling you switching power supplies are bad and "pure DC" is the only way to go has information from 20 years ago and they know nothing themselves. When you meet someone pontificating about this, since they clearly do not understand electronics, you likely cannot educate them, just nod your head and walk away.

What the heck is VA and what does it tell me?

Basically it is a term to make systems appear "bigger" then they are. It sounds innocent enough when you look at it, but it is misleading.

The definition of Volt Amps is that there is SOME some combination of Volts and Amps that (multiplied) gives the VA rating.

Notice the word "SOME"? This means that volts times amps does NOT equal the VA rating at ALL voltages and amps.

This is a VERY common misconception, fostered by manufacturers that want to make thier stuff look better.

Let's assume that a particular power unit is rated at 48 VA, and the box also says "max 24 volts" and "max 2 amps".. so you would assume that 24 x 2 = 48 so you can have BOTH 24 volts at 2 amps at the same time, because 24 V * 2 A = 48 VA.

WRONG!

It turns out that somewhere volts times amps = 48 and it's almost aways NOT at max volts or max amps. I first learned this with a loco that needed a couple of amps and the VA rating of the "power supply" was 60 and max voltage 24... figured I would have over 2 amps at 24 volts, as this was an LGB track cleaning loco, and it needs at least 24 volts to spin the cleaning wheel. WRONG! I could not even get the unit to put out 24 volts because the load in amps was over 2 amps.

The best advice I can give is don't buy anything rated only in VA. If you cannot get this (which usually means a regulated supply), then measure the output voltage under load and see what happens.

WIth switching power supplies with 24 volts and 10 amps available for about $70, you are wasting your time and money buying junk that really has deceptive advertising.

Short list of notes on specific manufacturers and models

LGB starter sets have a VERY weak power output, often 1 amp or less. Since starter set locos draw very little power and may only have a couple of cars, this can work, but it's an often-posed question, someone goes out and then buys a USA Trains loco and oh gosh, it does not run! Of course! It's drawing too much current. Some LGB supplies are 5 amps, they are OK, but you get very little for your money.

MRC has been around a long time, but many of their smaller units are crap. Yep. I had a MRC6200 and learned how misleading their VA rating was. The smaller units are not protected well from overloads and die easily. I did buy the MRC "Power G" a big stomping "traditional" unit with 10 amps, a big throttle lever, that I recommend, the rest of their products are NOT appropriate for G scale. Here's an article by Dave Bodnar on repairing the MRC6200. Note that the output transistors are paralleled, which means if you blow one up from an overload, the second can follow quickly. Look at the construction and form your own opinion. http://www.trainelectronics.com/MRC_6200/index.html

Bridgeworks is a large "old style" unit with a big transformer, and an analog output circuit that runs the throttles. This is old school and many people tout "Pure DC"... a load of hogwash. It really makes me laugh because all modern locomotive receivers/decoders output PWM (pulse width modulated), so it's silly to believe you need "Pure DC" as an input. These are big honking, heavy, expensive units that are unregulated DC, and will often put out 35 volts to the rails, destroying electronics or lights. The guys that run long trains on DC swear by them, and of course there's that "status" factor in the hobby, of people showing off that they can pay $500 for something that could be bought for under $100. Do NOT use on any trains with electronics in them unless they are good to 40 volts. Do NOT use as a power source for DCC systems.

USA Trains makes a large power supply/throttle combination, but many people have them fail. They seem to be good about repairing them. Not really excited about this product.

 

 

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