Prime Power Supply Tips

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This page describes how to create and wire a prime power source for a track powered system. Simple examples are shown for basic safety precautions, wiring multiple tracks and a method for avoiding having to bring AC power to your layout.

simple wiring diagramThis is a basic diagram of a track power system. A "power pack" has all of this stuff in one enclosure. If you are using a control receiver of some kind, each box probably represents an individual piece of hardware.

The Power Supply is the source of your power. It might be a transformer, or a DC power supply, or a power pack set to full output, or even a storage battery.

The Controller is the device that controls the power applied to the track. It might be an Aristo Train Engineer receiver, an RCS trackside receiver, a DCC command station and booster or an LGB 5012 remote throttle.

The Throttle is a handheld wired or radio device. Examples are the Aristo Train Engineer Transmitter, an RCS transmitter, a DCC throttle or the knob on a power pack.

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Safety Information

If you use the AC power line as a source of prime power, then there is some important safety information that you need to know. There are simple precautions that you can take to save your trains from damage, and perhaps SAVE YOUR LIFE!

safety_diagramIf you run outdoors, the most important thing that you should do is make sure that the power outlet that you use is protected by a Ground Fault Interrupter (GFI) device. A GFI is cheap insurance against electrocution. This device is installed either as a replacement for your regular power outlet or in your circuit breaker box. All power circuits outside, in bathrooms, kitchens and garages should be protected by a GFI.

A GFI senses the currents in the hot and neutral lines. As long as the currents are equal, it does nothing. As soon as it senses an imbalance between the currents, it trips the hot line breaking the circuit. This current imbalance is often caused by current running to earth ground through another path, such as through your body if you manage somehow to get connected to the power circuits. It will trip so fast, that you might feel only a little tick instead of being dead.

If you don't feel competent to install a GFI yourself, hire an electrician to do it for you. The money that you save by not doing it will do you little good when you are six feet underground.

The next thing to do is find a place for the power supply that is safe from the weather. Water and AC power do not get along well. Install the power supply inside a structure where it can't get wet and run the low voltage AC or DC output out to the yard. If the low voltage run is too long, see the section on battery power below.

The next thing that you need to do is properly fuse both the primary and secondary circuits of your power supply. The secondary fuse should be a fast blow type of the lowest current rating that you can get away with, but not larger than 10 amps. This fuse will help protect your power supply, your controller and your trains in the case of a derailment that results in a short circuit. The primary fuse should be a slow blow type of not more than 3 amps. This fuse will help prevent a fire in the case that there is an internal fault in the power supply itself.

An Aristo ART-5401 10 amp controller can be used as an electronic fuse at about 11 amps for a constant track power system. Hook it up like it was to control regular track power, but set it all the way up. It'll shut down and retry automatically at a low enough duty cycle to prevent serious damage to derailed equipment.

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Wiring Multiple Power Supplies or Tracks

one power supply, two loopsIf you want to run more than one train at a time, you'll need one controller for each train. That 10 amp power supply is probably loafing along and you'd like to use just one power supply for more than one track. This will work AS LONG AS THE TRACKS ARE COMPLETELY SEPARATE as shown in the diagram. If the loops are connected, even with a double gapped crossover track, they are not separate. There will be a time when a metal wheel crosses the gap and problems will occur.

one power supply, one loopYou CANNOT get away with this setup for very long. All of the controllers currently available REQUIRE that the input circuit be isolated from the output circuit. You can't cross connect between both the input and output at the same time. Doing this will result in immediate problems such as smoke, burned up controllers or power supplies. As soon as a metal wheel crosses the rail gap with the polarity switches inadvertently set against each other, you'll get smoke. Don't even try it this way.

two power supplies, one loopWire it this way instead. Get a second power supply so that one power supply is dedicated to each controller. Then the output terminals can be cross connected (through the common rail in this case) and it will work fine.

simplified controllerMany people are dissapointed to find that they have to purchase one source for each controller but without special wiring (see below) its a fact of life. This is a very simplified diagram of a controller such as a TE receiver, a CRE-55401 trackside controller, an LGB trackside controller or any other available controller for that matter. All that is important is that there is some kind of voltage controlling device (the rheostat in this case) and a polarity switch. If the controller has some more fancy type of circuitry, such as an H-bridge for example, it doesn't make any difference, the problem is the same.

shorted sourcesThe red and green paths show the two short circuit paths that will occur if a metal wheel crosses a track gap with the polarity switches reversed to one another. If one side of the rails isn't gapped, one of the paths will occur even without a wheel involved as soon as the polarity switches are set opposite to each other. At this point, the power source is shorted out. Unless the source is protected somehow against short circuits, something will fail.

switched sourcesContradicting the restriction described above, there IS a way to use one power supply and two controllers to power two physically connected but electrically separated loops. It involves adding some switching so that when the loops are arranged to allow train passage between them, BOTH loops run from ONE controller and the other controller is completely disconnected. This method doesn't violate the restriction described above because when the loops are connected to allow train passage, the secondary controller is completely disconnected from the track so it can't participate in a potential short circuit.

This can be done by routing power to the secondary loop (the one that looses its own controller) through a set of DPDT contacts such as provided by an LGB1203 accessory switch installed on one of the turnouts that connects the loops. When the turnout(s) are set to allow train passage where a wheel short could electrically connect the loops, the power for the secondary loop is derived from the primary controller so that they really aren't two loops at all. When the turnout(s) are set to prevent train passage, and therefore the possibility of a track gap short can't exist, each loop runs from its own controller. Note that the connecting track MUST have a double gap.

The same technique can be extended to more than two loops. In each case of a connection between loops, one track of the pair is designated as the primary and the other as the secondary. When the turnouts are set to connect the tracks, the secondary controller becomes disconnected and both tracks run from the primary controller.

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Batteries as a Power Supply

Some people are blessed with a lot of land where they can build an outdoor railroad. Often the prime location is far away from any AC power and running low voltage wires will result in too much voltage drop.

battery prime powerTwo old and tired 12 volt automobile batteries make an excellent prime power source for large scale trains. They might not have enough soup to start your car, but they'll run trains for a very long time. If the controller is an ART-5401, it will provide some short circuit protection electronically. You won't necessarily blow a fuse for each track short.

There are many ways to charge the batteries. The diagram shows two cheap trickle chargers that are installed near the AC power inside a structure. Only three small gauge wires need to run out to the layout. If you can't even run these wires then you can either carry a regular battery charger and an extension cord out to the batteries and charge them one at a time, or you can carry the batteries inside once in a while to charge them. Note that storage batteries can evolve a little hydrogen gas when being charged so make sure that they are well ventilated during charging.

Note the fuse. This is very important. Even a tired 12 volt battery can generate an amazing arc if shorted. The fuse is important to prevent damage to your trains or wiring.

voltage reducerSome control receivers have a maximum input voltage of 24 volts or less. Two fully charged batteries will have a total voltage of 26 volts or a little more. If you need to reduce the output voltage, then use a circuit like this in series with the controller's input. Each bridge rectifier will drop the voltage about 1.5 volts, use as many in series as necessary. Using a bridge rectifier is preferable to using a power resistor as the voltage drop will be essentially independent of load current. This circuit will drop voltage in either direction so it can be used on the track leads as well. In the battery leg, a stack of simple high current diodes will work, but they are usually harder to find, are more expensive and harder to mount. Note that at 10 amps, each bridge rectifier will dissipate 15 watts so it will need some form of heat sink. The bridge rectifier should be rated at 20 amps or more so that in the event of a fault, the fuse will blow before the diodes do.

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Build Your Own Power Supply

You can either build or buy a prime power supply. If you choose to build, you can use one of the diagrams below as a guide. Most DCC boosters and some R/C receivers will operate on either AC or DC power. There is no need to go to the trouble to provide DC power if AC is acceptable. Loy's Toys provides a 10 amp AC power supply in kit form.

The Aristo ART-5470 Train Engineer receiver MUST have DC power of the correct polarity or it will blow its input fuse. In this case, you can either buy an ART-5460 or you can build your own.

simple ac power supplyAn AC supply is just a transformer and the necessary fuses. This is adequate for virtually all DCC boosters and the RCS trackside receiver.

simple dc power supplyFor use with a Train Engineer or an Aristo 5401 wired throttle, you need a DC power supply. The transformer is the same, but a rectifier and filter are added to convert the AC of the transformer to DC. The current rating of the rectifier bridge should be higher than the required average current to allow the bridge to survive momentary current spikes before a fuse blows. The rectifier must be bolted to a heat sink, the metal case of the power supply will probably be adequate. The filter capacitor can be any value of 20,000 microfarads or higher, use whatever is handy. The 1K resistor is simply a bleeder to discharge the capacitor over time after the AC power is turned off.

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Metering Track Voltage and Current

power supply meteringMany power packs provide metering of output current and voltage, but if you roll your own, you might want to add those features. The appropriate meters can be hard to find at a reasonable price, but you should be able to find them for $15 or less. The high current bridge should be less than $2.50 and the small bridge much less than $1. Try web sources such as MPJA, Hosfelt, American Science and Surplus or All Electronics.

The rub is that the meters are usually unipolar and the track voltage and current is bipolar (has both polarities depending on direction). You'll need to adapt the meters a little to make them work right. By using a couple of bridge rectifiers, you can make both work right. The schematic shows how to do it. The cost is a loss of 1.5 volts of track voltage. The bridge rectifier around the current meter routes current through the meter in the correct direction no matter what the actual track current direction is. The bridge around the voltmeter does the same. Since both bridges have about 1.5 volts of total voltage drop and the voltmeter is wired on the power supply side of the current meter (where the voltage is higher than the track voltage), its voltage drop is compensated by the voltage drop of the current meter's bridge and both meters will read actual track voltage and current.

The 20 amp bridge (or as big as you can get) will get warm so it might need to be bolted to a small heat sink. A larger current rating is better so that track shorts will not overstress the bridge and possibly cause it to fail. Both bridges need a voltage rating of 50 volts. Higher voltage ratings are ok, but a higher rating don't buy you anything and the parts will usually cost more. The low current bridge should have a small current rating, an amp or less. The actual current drawn by the voltmeter will be very small, certainly much less than 20 mA, most likely about 1 mA. Smaller bridges have less of a chance of being leaky and causing accuracy problems.

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© 1999-2008 George Schreyer
Created Nov 18, 1999
Last Updated September 2, 2008