All about power supplies

All about Power Supplies and Packs

By Roger Heid

Technical Advisor: Ulrich Albrecht



Believe me, this is a more complex issue than what you might expect, at first glance. There are a number of different types of power adaptors, power supplies, transformers and such on the market. They all serve their own purpose. The following dissertation is geared for model railroad use. Hence, be at hand, and let us start from scratch.

Let us assume you just got your new starter kit, conventional or analogue mode, not digital. You may wish to read the REI Blog covering digital (DCC) issues. You will find some useful info there.

Anyway, your kit came with a power pack/speed control. Usually, there are two separate power outlets found on these power packs. One is labeled DC (Direct Current). This one you connect to the tracks. It supplies power to your locomotive motor and all lights that may be installed in your loco and rolling stock. You do NOT connect this to anything but the tracks. You leave track power alone, all by itself, period!

Then you decided to add a couple of turnouts and a couple of internally lit buildings to your layout. Now the AC (Alternating Current) power outlet comes into play. AC is more efficient for electrical power transfer through wiring. Just take this for granted.

The turnouts and the building lights are connected to this AC outlet on your power pack. Everything will work just fine, at this point. You are a Happy Camper! Indeed! Things could not be better. As time goes by, you keep adding turnouts and lights, more and more. This goes on until you run into some nasty problems. Those are caused by your power pack’s inability to deliver sufficient power to accomplish the imposed task.

Here are some of the symptoms you will most likely encounter. Your locomotive does not run as fast as it used to. All the lights are not as bright as they used to be. When you engage a turnout switch, all the lights will temporarily get very dim or go out completely. A running train will suddenly become very slow or stop completely, followed by a sudden acceleration to the previously set speed. This jolt may cause a longer train, negotiating a tight curve, to derail.

More grief is added when your power pack goes into thermo shutdown. It has become quite hot to the feel, enhanced by overloads when turnouts are engaged. It takes a relatively long time for this power pack to cool down sufficiently before it resumes operation. During a railroading session, the times between shutdowns will become shorter and shorter. Obviously, this is not a good thing.

The time has come to do something about this dilemma. But first, let me explain a few things, for better understanding. Most power packs, such as made by MRC, supply 25 to 30 Watts. A typical locomotive consumes 10 to 15 Watts. The typical model railroad light bulb uses about 0.6 Watts, meaning ten of them use up 6 Watts. The turnout switch solenoids are power hungry. The ones made by Atlas, for instance, use 20 Watts, while engaged. Doing the math, you will see that you can install something like 30 lights, no turnouts, leave alone running two locomotives simultaneously. The alternative would be: All the turnouts, but only a few lights, not a good solution.

For best overall results, I strongly recommend you break down your power supply system into three sections, therefore requiring 3 power packs. One, you already have it, is for track power, one for all off track lights and one for all turnout switches and other solenoid driven devices. Take note that, both, incandescent bulbs and solenoids, can be used for AC and DC, something that can be of advantage.

The needed type of supplement power supply, or power pack or adaptor, can be found in hobby shops, hardware stores and computer stores, plus Radio Shack., et al.

What you are looking for is a device that puts out 12 Volts AC at no more than 3 Amps. AC is preferable. It is more efficient, as mentioned above. Employing a device that puts out more than 3 Amp is not recommended for our purposes, as it may cause some problems. Just take this for granted. With such a device you can theoretically employ 60 light bulbs, but I would keep it at 50 to prevent overheating. Add another pack, they are not expensive, you can have 100 lights. Wow! For the solenoids, one of these packs would do. You only engage a turnout briefly. There is enough juice to do this, without affecting the rest of the works, you see.

Other off track power consumers are signals, namely their solenoids and lights.

If you decide to use LEDs for your building and street lights, caution must be taken. This is altogether a different issue, going outside the boundaries of this article.

You may have come across a type of power supply that is referred to as ‘Switched Mode Power Supply’. Those you need to stay away from, as they are dedicated to certain purposes. They are different in so far that they do not put out neither smooth DC nor 60 Hz (cycles) AC. Instead, they put out a pulsating square wave at a much higher frequency, AC or DC, depending on the type. They are primarily used in DCC systems. They are too expensive to use them for just off track lighting. The typical turnout solenoids will not work because of the supply’s inherent high frequency. Never use a switched mode power supply for conventional track power. It can cause serious damage to your locomotives.

If you are using a DCC system, relax. Any electricity consumers, such as building and street lights, do not need to be integrated into the DCC circuit, at all. This section of supply circuitry does not need to carry digital information. It should be kept electrically separate from the DCC network, for best results.

If you are a beginner in the DCC world, be careful selecting additional power supplies. It is best to stick with the system dedicated devices. If in doubt, seek help on the Forum.

Happy Railroading!

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