DC "power packs", supplies, "transformers"

Overview:

This page is devoted to power systems for people running "DC" track power, i.e. unmodified electronics in their locos.

Also there is some information on basic power that can be used for DCC.

To do this, there are 2 basic components needed, they may be in a single box or two separate boxes.

Power Supply: (definition)

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

In reality, for just DC power, you need more than a transformer, 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.

There are also regulated power supplies that use a transformer, and virtually all switching power supplies are regulated. More about regulation later.

So, we have power available to us, but unless we want our locomotive to run at full speed all the time, we need another component, which I call a throttle.

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.

Throttle: (definition)

 

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.

The term throttle can mean many things to many people, but on this page, it means a way to vary the power going to the locomotive so you can control it's speed. Notice I used the term power, not volts or amps.

This is because there are many ways to vary the power to a locomotive. The very earliest units actually used a big variable resistor to control the voltage and current to the motor. This did not work too badly, but since all the power goes through this resistor, it had to be big, and made heat, and there are other issues.

Often a transistor or two is used and a smaller variable resistor controls the transistor and controls the voltage and current coming out. This is how inexpensive power packs are made today, for example the common (and not good for G scale) MRC 6200... a transformer brings the 110v down to about 20 volts AC, then a rectifier assembly converts the AC to DC. Then 2 transistors controlled by a small variable resistor (also known as a pot, potentiemeter, rheostat) then vary the output to the locomotive.

Unfortunately, when you start running large amounts of current, you need bigger transformers, bigger transistors, and fans to remove heat. To get a nice variable DC output gets big and expensive. This is what a Bridgeworks unit does, and they are indeed big and expensive. More about this later.

PWM throttles

Another method to vary the output power is by not setting the power, sending part to the loco and the rest as heat in the transistors or potentiemeter, is to use pulses of full voltage.

This is called PWM (Pulse Width Modulation). Basically think of it this way: if you went to a light switch at home and turned it on and off rapidly, you would get maybe about half brightness on you light bulbs. Now if you only turn it on for a short time, and leave it off a longer time, it get's dimmer. Conversely, if you leave the switch mostly on, and only briefly turn it off you get brighter.

This is exactly the way that PWM works. There are transistors that send the power to the loco, and a simple circuit varies the on and off "Times", i.e. Modulating the Width of the Pulses (width in time) you are sending.

Why is this done? It's cheap and does not make a lot of heat. The transistors are either off (no heat), or fully on (very low resistance, i.e. heat). Just like your home, you light switches are never warm, but have you felt a "dimmer switch" make heat? Same thing.

So we have another way to control power to the locomotive that is cheaper, smaller, less heat. This is the reason it's used in many systems, not just trains. All DCC decoders and all R/C controllers output PWM to the locomotive motors.

 

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.

My opinion is that if you have a pack that is only rated in VA, run screaming. It is a specification that is misleading, and even people who "understand" electricity don't really get it.

So what does this mean? It means that typically under heavy load, you cannot achieve 24 volts or max current. So under heavy load you train may not perform, even though you THINK you have enough amperage.

Find equipment that rates the current output at full output voltage. MRC is a prime example of this, where you see it rated at 2 amps, and 70 VA, but your 2 amp train can never go fast enough... you find out the hard way that you don't get 2 amps at higher speeds.

Putting it together:

OK, so we have several ways to make a power supply, and several ways to control that power to go to a loco.

Before I give examples of systems, and make recommendations, we need to address "pure DC" and PWM, and "regulation".

"pure" DC is direct current (which we normally measure as voltage), meaning it is a constant level, except when we raise and lower it. It's the most compatible power. Old school is a transformer and a rectifier and lots of filtering, because the AC line voltage is contantly varying, not always at full plus and full minus voltage. When you get to high amperage supplies, this stuff gets big, heavy and expensive. "pure" DC is popular with certain groups because PWM can confuse some electronics. Notably the MTH DCS system does not like pulses, nor do some older sound cards, and DCC systems normally don't like it either.

But you can easily make "pure" DC from a switching power supply, and due to the frequency used of the "internal" AC in this design, it is easier to filter to "pure" DC. So for the last 20 years, a switching power supply as opposed to huge transformers and filter capacitors is a better cost and heat wise.

PWM is an inexpensive way to make high power systems and works well with current production locomotives not running electronics inside. The pulses give better low speed starting, also will make lights brighter at low speeds, and will also make smoke units perform better. But, it can drive on board sound cards or remote control systems crazy.

Regulation - this is where the output voltage of the power supply part is controlled to be the same all the time, no matter what the load. In the last 10 years people have finally figured out that this is a good thing. In G scale, the locomotives, sound cards, smoke units, lighting can all draw a lot of power. In an unregulated supply this can result in a wide variation in the voltage on the track when the load (current / amps) varies. This does not make too much difference when you are using a power pack, since you just turn up the throttle more, and you are usually just running one train.

Regulated power supplies make the most difference when you are running a system that wants a fixed track voltage, like DCC or MTH DCS or R/C where the locomotive power is pulled from the track.

OK, I'm bored, what's the bottom line: 

All in one power packs:

Advantages:

  • simple
  • all in one box
  • fewer wires
  • smaller
  • usually less total cost

Disadvantages:

  • less flexibility
  • have to buy one per throttle
  • more money to replace or repair if failed
  • can't pick the throttle you like and then add the power you want

 

What's available:

MRC AG990 "Power G"

an inexpensive unit for what it does, 22 volts and 10 amps. Solid, powerful, no PWM, recommended, I have one.

LGB - various models

Quality stuff, but usually has output much lower than you think. There's even a 7 VA power supply that can barely run one tiny loco on level track at a snail's pace. Bottom line, pretty darn expensive for what you get.

Bridgeworks - various models

Considered the "Cadillac" of DC power. Huge linear power supplies, with tons of current, built heavy duty, and because they are linear (not switching) heat, bulky and fans. Best high current supply for MTS DCS systems, beautiful smooth filtered DC.

Note: until recently, these supplies have been unregulated, and under light loads could put out 35 or more volts. On DC trains, where the motor is always connected to the rails, this high voltage is quickly reduced to almost zero, and everything is fine. BUT, for electronics, like DCC decoders, where there is very little load from the decoder itself, this overly high voltage can and has blown up dcc decoder equipped locomotives.

But: In recent history, a regulation circuit has been added to protect from this. Mark Sauerwald replied to me in an email:

"Greg - we added the regulation specifically because of the issues with DCS and DCC.

The regulation circuit is a simple, purely linear circuit - we use a voltage reference (LM4041) along with an op-amp (OP07) to generate a reference voltage which is used to drive the bases of four power transistors in parallel.   Each power transistor is limited to 4A with a second small transistor, and the outputs of the four power transistors are summed together to provide an output.   The default output voltage that we set is 20V, although it is a simple enough change to go to 18V at the output (one resistor)."

Great news, and now I can recommend the Bridgeworks without reservation, if you are buying a used one, you need to look to see if it has the updated regulation circuitry (which can be retrofitted).

Again, Mark helped with this information:

Greg

There are a few ways that you can identify a newer unit:

1)   On most of them, on the top of the case where is says 'Mag-15' we have added a '-R' using a sharpie.

2)   On the current production, on the back of the unit where it says

'25-32 Volts DC' we have put a sticker over that which says '20 V'

3) if you look on the bottom of the unit, normally there will be the 4 feet, and nothing more.   On the regulated ones, there will be an additional 4 screws which are holding the power transistors to the base (we use the base as a heatsink).

If you have an older unit, we can retrofit the regulator.  We charge the same for that as we do for a repair, which is $125 plus the cost of return shipping.

USA Trains 10 amps system

RTP 10 …. Spec: 20 volts, 10 amps (180VA to track) $US 190.- not a great track record on this product, you read a lot about them being repaired

 

Throttles that need power:

Advantages:

  • several throttles can be connnected to a single power supply
  • you can pick your throttle and match it to your power requirements
  • better use of space (power supply can be put remotely, throttles on control panel

Disadvantages:

  • usually more overall cost (unless you find Meanwell power supplies!)
  • more wires and space

 

What's available:

Aristo

Piko

Piko 35002 (Throttle only) …. Spec: 22 volts, 5 amps $US 152.-

LGB

LGB 51079 (Throttle only) …. Spec: 24 volts, 5 amps $US 180.-
LGB 52120 (Throttle in hut) …. Spec: 24 volts, 5 amps $US 125.-
LGB 52121 (Throttle panel mount) …. Spec: 24 volts, 5 amps $US 90.-

 

Power supplies

I'm only going to mention regulated power supplies here,

 

What's available:

Meanwell & similar

Aristo

LGB

LGB 51095 (Switching supply) …. Spec: 230VA / 19VDC / 100VA $US103.-

Piko

Piko 35020 (Switching supply) …. Spec: 120VAC / 24V / 5 amps $US 80.-

 

Stuff to avoid

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. LGB 5003/50030 starter supply, rated about 0.7 amp, just no good except for small LGB locos with Buhler motors.

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

 

 

 

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