DC "power packs", power supplies, "transformers", what the heck?

Overview:

This page is devoted to the various power systems for people running "DC" (Direct Current) track power, i.e. unmodified electronics in their locos, the way all Large Scale locomotives will work as delivered (not live steam of course).

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

Definitions first:

Transformer: (almost always misused) - constant output

An actual physical transformer with a metal core, copper windings, which serves to change an AC voltage to another AC voltage, and in our case sinusoidal AC at 50 or 60 cycles per second.

As with many things, this usually started out defined correctly, as the first toy trains usually ran on AC (Alternating Current), so all you needed was a transformer, which just changed (transformed) the 120v AC from your wall outlet to a lower voltage, like 18-20 volts, but still AC. (other countries can use higher voltages, but the principal is the same)

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

Nowadays, virtually no one purchases just a "transformer", this is a good term to AVOID, since it is technically not ever used in normal large scale.

Power supply: - constant output

Any device that takes your 110v from your house and convert it to some other voltage AND/OR power type, typically DC.

Power supplies are usually in 2 groups:

• a unit that is basically a transformer with a device to change the AC to DC (rectifier). Old school and the output power (volts and amps) will vary widely with the load. Fine for smaller systems, but not real consistent, and not good in high power situations. Note well: some of the "power packs" still use this UNREGULATED style of power, and it is not great for high current or multi train operation. The reason it still exists is that it is CHEAP.

• For higher current, more trains, better operation, you normally want a REGULATED power supply. This means there are extra electronics to Besides being smaller/cheaper/lighter, virtually all switching supplies have a regulated output voltage, meaning the voltage does not vary as the load changes. This results in better operation for a number of reasons.

Bottom line: There are some electronic components involved, we will get into what they are and what the result is. For now "power supply" alone means nothing other than the fact the output is fixed in voltage, it could be AC or DC, but normally DC.

Throttle: - ability to vary output voltage

This is another term that is often confusing. Unfortunately it us used in 2 very different ways:

• A power supply with an speed control
• A box that can change the speed and direction of a loco. This can vary from a simple potentiometer  to sophisticated electronics - NO POWER SUPPLY. It can be hand held or in a box.
• A box that can likewise change the speed and direction of a loco WITH an integrated power supply.

Power pack: - ability to supply power and vary output voltage

Normally this means a power supply and throttle all in one unit, plug it into the wall, and connect the output to the track, and . Another term to avoid.

Bottom line: I will use THROTTLE to mean WITHOUT an integrated power supply, unless explicitly mentioned.

More details on the individual components:

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

Basic 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 (diodes) 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.

Note well: without some filtering, this is really not DC (because DC implies a constant voltage), but pulsating DC.

What is a "regulated" power supply, how is it different from a transformer, 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, etc.) 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. There are still a few products out there that use transformers and then some regulation circuit. (Bridgewerks)

But those days are pretty much 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.

More on "basic" throttles:

The term throttle can mean many things to many people, but I'll stick to the definition above, the circuitry used to receive a constant voltage, and vary the output power going to the locomotive so you can control it's speed. Notice I used the term power, not volts or amps.

Basic:

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 / potentiometer / 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 Bridgewerks unit does, and they are indeed big and expensive.

PWM throttles

Another method to vary the output power is by not controlling the voltage, sending part to the loco and the rest as heat in the transistors or potentiometer, 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 gets 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).

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.

OK, let's get into some details that impact YOU in terms of how your trains run.

Warning: 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 their 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 always 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:

So you need both "power supply" and a throttle, whether they are in one box or separate.

I've gone over the power supply issue, and hopefully you understand the transformer based supplies and the switching supplies. So it does not matter which you use with modern technology, but unregulated transformers are inferior to a regulated switching power supply, and regulated supplies are inexpensive.

Now to the throttle: With few exceptions, using a throttle with PWM is normally what you want on an unmodified loco running on DC. (The exceptions is if you are running a locomotive that has a DCC or DCS decoder in it... the PWM is often misinterpreted as control signals).

PWM is an inexpensive way to control the speed of DC locos, and also helps in slow speed operation. Also typically lights will be 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.

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

Throttles that need power:

Advantages:

• • several throttles can be connected 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

Recommended: (note throttles & all in one)

G Scale Graphics - top value - LINK

"No Frills" 5 amp Track Throttle - great deal, PWM only. You can buy as just the electronics or in a box.  $69 - $89, and they ship international.

You can buy their 5 amp power supply or a separate 10 amp one.

MRC AG990 "Power G" - throttle and power supply combination

• Sort of expensive, used to be cheaper. Solid, powerful, but unfiltered DC, I have two.
• It outputs rectified AC, with minimal filtering, works well, but not really good for a DC power supply for other electronics.

To match your nes, 20 amps and 24v….

SE-450-24, has a fan, and 18.8 volts

https://www.meanwell.com/webapp/product/search.aspx?prod=SE-450

I use the HEP series, no fan, completely sealed

HEP-480-24, 24 volts and 20 amps

https://www.meanwell.com/webapp/product/search.aspx?prod=HEP-480

OK, not a great value

Aristo - was a staple, but no longer made

No longer made, but some on the used market, PWM output and good price

Solid, last for years

I need to find the part number and a picture

Bridgewerks - 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 most of this is about the larger, high current models, not necessarily the 5 amp units.

It is important to understand that this is an unregulated power supply, i.e. the output voltage can vary under load. Of course, with the larger capacity (amps) of these supplies, the voltage does not vary a lot under load. The analogy is like a car with a high horsepower engine. Go up a hill, and you don't need to really push the throttle more, the load (hill) is not much of a load to the motor (power supply).

So, this kind of power supply works very well with the common DC loads of motors and lights. Motors can draw wildly varying amounts of current.

But it is not regulated, i.e. the voltage is NOT locked depending on the load.

There is another important point that must be made, until recently, these supplies had the strange characteristic of outputting high voltages under light loads, voltages of 34 to 38 volts have been reported. Normally, the load of a motor in a loco, would absorb this "leakage voltage", so there was no harm to a DC trains locomotive, where the motor is always connected to the rails. In this case the 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, destroying the decoder itself.

In recent history, an "over voltage" 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 Bridgewerks 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.

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. Usually bought when people want stuff to "match" or have been frightened into purchasing it.

LGB - nice but expensive

LGB 51079 (Throttle only) …. Spec: 24 volts, 5 amps $US 200

LGB 52120 (Throttle in hut) …. Spec: 24 volts, 5 amps $US 180

LGB 52121 (Throttle panel mount) …. Spec: 24 volts, 5 amps $US $160

Piko - expensive, similar to LGB, not as much track record

Piko 35002 (Throttle only) …. Spec: 22 volts, 5 amps $240, super expensive for just a throttle.

USA Trains 10 amps system

RTP 10 …. Spec: 20 volts, 10 amps (180VA to track) $US 304.- not a great track record on this product, you read a lot about them being repaired, but usually available.

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.-

Do not buy!

MRC 6200

Do not buy one! They fail often, poor circuit design, and under 2 amps and max 18 volts. I bought one and it was worthless except for tiny LGB locos.

Dave Bodnar has a page on repairing them (hint hint)  http://www.trainelectronics.com/MRC_6200/

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