DC "power packs"

Overview:

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

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

Power Supply: (definition)

First is a power supply of some type, either a transformer that brings the house "line" voltage down where where we need it, or a "switching" power supply that uses electronics to convert the 110v AC line voltage to a lower voltage. Each of these have their pro's and cons. I have a page that goes into more detail about power supplies: click here

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.

Throttle: (definition)

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

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.

 

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.

There is one big gotcha here, and unfortunately I need to "point" at a particular manufacturer. Bridgeworks makes really large "power packs" up to 25 amps. Unfortunately this "pure" DC system, which has a beautiful, smooth, well filtered DC output is unregulated. Since it is designed for heavy loads, and is unregulated, when a light load is applied, the track voltage can exceed 30 volts, reports of up to 38 volts have been reported. This can and has "blown up" electronics in locos with decoders or sound boards. Most large scale system SHOULD be engineered to handle up to 27 volts, the NMRA DCC standard, but some only cna handle 21 volts (like Soundtraxx), and even a brief application of 30+ volts can destroy them. In Bridgeworks defense, when you put a loco on the track, normally the load of the motor itself, will load the Bridgeworks down enough to bring the voltage down to zero.

So for off the shelf, not electronic locos, a great supply. For DCC, do not use it. For most other locos that have a motor or good load across the rails, decoders that run well from DC (like MTH) you are PROBABLY ok. Sorry Bridgeworks, but this is the history here.

 

OK, I'm bored, what's the bottom line:  (this section is incomplete)

All in one power packs:

  • simple all in one box fewer wired

MRC AG990 "Power G" - an inexpensive unit for what it does, 22 volts and 10 amps. Solid, powerful, no PWM, recommended.

Bridgeworks

LGB xxx

USA Trains 10 amps system

 

Throttles that need power:

  • Advantage is that several throttles can be connnected to a single power supply ( good suggestion by Knut )

Piko

LGB

Aristo

 

Power supplies

Meanwell & similar

Aristo

LGB

 

From Knut:

1. MRC AG990 (Power G) ….Spec: 22 volts, 10 amps $US 265.-
2. Bridgeworks Magnum 5-SR …. Spec: 24-30 VDC, 5 amps $US 410.-
3. Bridgeworks Magnum 10-SR …. Spec: 24-30 VDC, 10 amps $US 470.-
4. USA Trains RTP 10 …. Spec: 20 volts, 10 amps (180VA to track) $US 190.-
5a. LGB 51079 (Throttle only) …. Spec: 24 volts, 5 amps $US 180.-
5b. LGB 52120 (Throttle in hut) …. Spec: 24 volts, 5 amps $US 125.-
5c. LGB 52121 (Throttle panel mount) …. Spec: 24 volts, 5 amps $US 90.-
5d. LGB 51095 (Switching supply) …. Spec: 230VA / 19VDC / 100VA $US103.-
6a. Piko 35002 (Throttle only) …. Spec: 22 volts, 5 amps $US 152.-
6b. Piko 35020 (Switching supply) …. Spec: 120VAC / 24V / 5 amps $US 80.-

 

Stuff to avoid

 

small MRC packs, low current, low voltage, not reliable when stressed

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

Bridgeworks when you don't need to have a non-PWM power supply or you intend to run DCC later.

 

 

 

 

 

 

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