The amount of current the decoder provides for the loco's motor and functions is one of the two most important factors of your decoder selection (the other one being the physical size). If the decoder cannot provide enough current for the motor and functions, the decoder's life expectancy will be diminished - to what extent depends upon how deficient the decoder is.

The scale designation of the decoder does NOT provide you with ample information. Generally speaking, the physical size is a gauge of what scale of locomotive it will fit in. While the amount of current sort of follows the size, it is not an accurate gauge. So, along with physical size, manufacturers also publish various current ratings for the decoder.

DCC not only provides motor speed and direction control, but can also control the front and rear lights and anything else electrical on the train. Some people control their number board lights, add Blinking Ditch Lights, running board lights, truck lights, control the smoke units and even couplers. As such, everything has to be taken into consideration when choosing a decoder. A decoder that cannot handle the current loads will certainly have a shorter life span than intended, if not be fried on the spot. To control all these loads, there are many things a manufacturer has to take into consideration. First and foremost is the locomotive motor drivers - how much current will be needed? Then there are the light drivers (function outputs) - how much current for each? And lastly is the decoder's full-load (peak) current - how much for motor drivers and all the function outputs put together? We'll discuss all of this below.

Some decoder manufacturers provide only the continuous running amperage. Other manufacturers provide what's called a "Stall Current" rating - that is, how much current does the motor draw when the motor is provided full voltage while the motor armature is held stopped. Don't confuse "Stall" with holding the loco back while the wheels slip - that's how you test for maximum continuous running amperage. Stall means wheels STOPPED.

Many people argue that you'll never have this situation where the wheels are held stopped. At worse the wheels would just slip. But that's not the point. First off, the wheels ARE stopped (stalled) when the loco is stopped. The first few pulse(s) of power the decoder provides to the motor to get the loco started are at or close to stall current. The stall test is also a "gauge" as to how much current the motor will probably draw under normal conditions - with maximum continuous running being about 60% to 75% of the stall current.

The power pack needs to provide enough amperage for the test. That is, if you think the motor might top out at 2 amps, the power pack should be able to provide more than two amps. Most power packs provide only a "VA" rating. That stands for Volt Amps, not Amps. To get amps, divide the VA rating by the amount of volts you'll be using in the test. For example, if you have a 28 VA rating and you're testing HO scale, the power pack would have 2.33 amps available (28 ÷ 12 = 2.33).

Voltages:

Put the loco on the test track. Hold the loco down tight so the wheels won't slip. If you think the motor has enough power to damage the drive line by doing this, you'll need to remove the shell and hold the flywheel stopped while holding the loco on the test track. Quickly turn the voltage up to the rated voltage and read the Amp meter. As soon as you get the reading, remove the loco from the test track.

If your power pack doesn't have enough amperage to do the test, raise the voltage only to half of the scale's rating. When you get the amperage reading, double it. If testing HO scale, for example, raise the voltage to 6 volts. If the amperage reading was 0.8 amps, then the motor rating would be 1.6 amps.

Note: The stall current rating isn't something that the decoder is designed to handle for more than a few pulses. So even though a decoder's stall rating is higher than the stall current of the loco, you should NEVER do a stall test on a loco that already has a decoder installed in it. That's a sure way to damage the decoder.

The reason a continuous running rating is needed is for heat dissipation. We know that the components in the decoder can handle more current than the continuous running rating, because the stall test rating is higher. The problem is, with decoders being so small, there simply isn't enough surface area to dissipate heat fast enough to provide more current than that rating.

Take Digitrax's DH83 and DH83FX for example (both now discontinued). While this decoder is rated at 2 amps continuous, I was told that these decoders actually have a 5-amp motor driver. But because the decoder can't dissipate that much heat fast enough, the decoder is rated at 2 amps.

Many decoders have thermal protection built in. If you have a decoder that runs well, but just shuts down after a while, chances are that it's a thermal overload. In this situation you have two choices: install a decoder with a higher rating, or figure out a way to circulate more air through the loco and over the decoder. When you consider that the decoder is installed in a closed shell with the motor, the heat that is dissipated doesn't have anywhere to go. Many people open the fan vents on the top of the locos to let the hot air out. This is only necessary in rare cases, but people do it anyway because with all things electronic, heat is the enemy. The more heat you can get rid of the better.

Function output ratings range from 100 mA to 1-amp. The lowest function rating on a Digitrax decoder is 125 mA on the Z-scale decoder and many N-scale decoders, while the largest is 1-amp on some functions of Digitrax's G-scale decoders. All function outputs on Digitrax's HO-scale decoders are 200 mA. The function output rating on Train Control systems decoders is 120 mA.

It takes 1000 mA to make an amp. So, 100 mA is 1/10 of an amp, 200 mA is 1/5 of an amp, etc. You don't really need to know this, but many people are curious and ask. The key is that you know the mA rating of the device you're going to connect to the function, and that the function's rating is greater than that.

Incandescent bulbs draw a high surge current when first turned on. The surge current can be up to 10 times their rating. It isn't normally that high, but it can be. I'm told by some manufacturers that functions can cope with this surge, and that the rating is based upon the continuous needs of the device. But, just to be safe, I wouldn't use an incandescent bulb of more than 1/2 the function's rating - if only to avoid the heat a bulb of greater amperage would make. To take this one step further, heat is a good reason to replace higher amperage bulbs, such as those used in Life-Like locos.

The bulbs used in Life-Like locos are not consistent. They may use an 80 mA bulb in one run and a 110 mA bulb in another run. Note that I have never measured the amperage of any Life-Like bulb. I only know they are different because one loco will require a different resistor than another to have the same brightness - some requiring a 120 ohm, 1 watt resistor. For this reason, it's best to replace those bulbs. For this, we carry a 45 mA, 14-volt Grain of Wheat bulb. Since an HO scale decoder running on the HO scale setting will put out about 14 volts to each function, no resistor is needed. However, many people wire a 47-ohm resistor in series with them to increase their life and help damp the surge current when turned on

Coils such as those used in relays generate an electomagnetic field - that's how the relay pulls the contact points inside from one set of contacts to another. When power is turned off to let the relay relax, the collapsing electromagnetic field generates an electrical kick-back (Back-EMF).This kick-back can damage the decoder's function driver transistor. To avoid this, the installation of a diode in the correct orientation, as illustrated above, can direct that kick-back back into the relay coils for dissipation.

Note that the striped end of the diode is connected to the blue wire. With decoder functions, the blue wire is positive and the function wire is negative. When the function wire is turned on, it sinks the blue wire's current to ground through the device. When the function is turned off, the diode directs the back-EMF back into the relay coil rather than forcing it into the decoder function transistor. This diode can be any standard diode such as the 1N4001 or 1N4002 found at any electronic store - including Radio Shack. We also have them.

This is a total amount of current that will be used at any given time by all devices combined - motor driver and function outputs. The peak power rating will always be equal to or greater than the stall current rating, but could be less than the motor and all function ratings combined.

All power must go through a bridge rectifier before being used. The bridge rectifier converts the AC voltage from the track to DC voltage that the motor and light functions need. The bridge rectifier amperage rating is usually the peak power rating.

Let's take a typical Digitrax HO decoder, for example. Assume it is a 1.5-amp decoder - that's the continuous running rating. Stall current rating is 2 amps and peak current rating is 2.5 amps. Each of the four functions have a 200 mA rating, which is 0.8 amps for all four. If your motor really does draw 1.5 amps, and you have all four function wires drawing 200 mA, you have a total of 2.3 amps being drawn - well within the peak rating. But if your motor really does draw 1.5 amps continuously, chances are the first few pulses are close to 2 amps. Adding 0.8 amps if all light functions are turned on, takes it to 2.8 amps - 0.3 amps over the peak rating. Fortunately, most motors draw considerably less than 1.5 amps, and most function loads take much less than 200 mA, so you'll rarely even come close to the limits of this decoder.

But not all decoders are powered so generously. Some have more functions than the total amount of power the bridge rectifier can provide. The point here is that if you plan to load all of your functions, you have to be cognizant of the fact that there is a total load limit, as well as each individual function limit.

When power goes through a bridge rectifier, voltage is lost - about 1.4 volts. Provision for this is made in the fact that DCC rails have more voltage than that rated for maximum operation on an analog layout. By the time the excessive voltage goes through the bridge rectifier, and other components in the decoder, it comes out to the motor at about the rated voltage for the scale.

This voltage drop in the bridge rectifier is where a great lot of the heat is generated in a decoder. With Digitrax's 3.5- and 5-amp G scale decoders, for example, the biggest difference is the bridge rectifier. The 5-amp decoder has a much larger, exposed, bridge rectifier than the 3.5-amp-decoder. In fact, the bridge rectifier in the 5-amp decoder even has a hole in it so you can screw a heat sink to it if desired.

With Digitrax decoders, there's usually no need to be concerned about current needs for Z- or N-scale locos. Digitrax decoders that will fit in these locos can generally provide more than enough current to operate them. And many HO-Scale locos such as switchers and other small locos with good quality can motors can use Digitrax's Z- and N-scale decoders as well.

With Digitrax and Train Control Systems HO-scale decoders, there's usually no need to be concerned about current needs with most HO-Scale locos. Usually only older locos (more than 15 years old), especially older steam locos with Pittman open-frame motors, draw enough current to be concerned about. In fact, most S- and O-Scale locos with good-quality can motors will also do just fine with these decoders. There is one recent HO scale loco to be concerned about - Life-Like's HO scale Proto 2000 PAs. Their first run or two of these locos had motors that were not properly designed. The bad ones will draw about 3 amps on a stall test. On subsequent runs, good ones stall at about 1.1-amp. The only sure way to tell if you have a good or bad one is to do a stall test.

There is nothing wrong with using a G-scale decoder in an HO locomotive if it will fit. Some people use them simply because they have so many function leads, and want to add a lot of lights. But if you have one of the high-current Life-Like PAs, Digitrax's DG383AR decoder will fit and will provide the current that's needed for it. The drawback is that it does not come with the NMRA plug, so will need to be hard-wired by removing the existing Life-Like PC board. Fortunately, this if very easy to do. The motor is isolated and all the wires come back to that area.

American Flyer, Lionel, and most G-Scale locos require G-scale decoders. Which G-scale decoder you need (3.5- or 5-amp) depends on the currents we previously talked about. Most Lionel and many G scale locomotives, such as USA Trains, require the 5-amp decoder. Some American Flyer and most Aristo Craft can easily get by with the 3.5-amp decoder.

If there is a question about it, and you can't do a stall test, use the largest decoder you can fit into the loco. While the scale size rating of a decoder is a physical size guide, not a voltage or amperage rating, decoders that handle more current are generally physically larger. Any decoder can be used in any scale as long as it fits and puts out enough current for the loco.

Unless a decoder is made specifically for your loco, and except for the generalities above, decoder/loco amperage is something you need to consider when buying decoders.