Model railroad lighting can be daunting if you don't know about resistors. This is true with or without DCC, but particularly important with DCC.

With DCC, there are three voltage ranges: N scale, HO scale, and O/G scale. Over the years, various manufacturers have changed the voltage slightly, and different manufacturers use different voltages. To confuse the issue further, there's no easy way for the typical model railroader to measure it. We'll discuss all these issues below. And at the end of it all, it's all academic in comparison to finding the correct resistor value for your particular application. While this all seems daunting, the long and short of it is: it's as easy as trial and error - with an educated guess. That said, let's get started.

First, you need to know a little bit about resistors. There are several kinds of resistors, but the ones we will be working with are carbon resistors: little ceramic- looking (actually epoxy) cylinders with a wire running out each end from a carbon filler. The best part about them is that you can't install them backwards - it doesn't matter which end goes to which connection.

Resistors have color bands painted on them that indicate their value. I've never been able to remember which color means what - I always have to look at a color chart. There's a little ditty that some EEs used to help them remember what color means what, but I can't even remember the ditty. Color charts are usually available at better electronic stores, usually as a free promotional item, but I think it's easier to just measure the resistance with an ohm meter. By the way, if you don't have a multi-meter (combination volt, ohm, amp, meter), get one. With model railroading, there's no excuse for not having a multi-meter.

Resistors come in various watt ratings. The most common is 1/2 watt. However, 1/8 watt, 1/4 watt 1, watt, 2 watt, and on up are available. 1/8 watt, being the smallest, can be handy for creating a current draw on a freight car for detection purposes. 1/4 watt is what you'll need most for model railroad lighting. However, in some cases, a bulb can draw so much current that a higher wattage resistor may be needed.

One hint here: if you find that you need more than 1/4 watt, change the bulb. The bulb will be drawing so much current that it will be hard on the decoder, and could create so much heat that it could melt the plastic shell of the loco. This is not always the case, but is a good indication.

You will need to add a resistor to your loco's lighting most often will be when you're installing a decoder. Many locos come with 1.5-volt bulbs, LEDs, or other voltages anywhere between 1.5 and 12 volts. But since the decoder will be putting out anywhere from 12 to 18 volts, hooking them up directly would burn them out immediately, or at least shorten their life span.

The amount of resistance that is needed depends on three things: the voltage requirements of the bulb, the amperage needs of the bulb, and the voltage on the track. We'll talk about track voltage first.

As mentioned before, there are three different track voltages available with DCC. While this varies from manufacturer to manufacturer, and from year to year, N scale will have about 12.5 VAC on the track, HO scale will have about 15.5 VAC, and O/G scale about 21 VAC.

Because the voltage on the track is square wave AC, your typical AC volt meter won't measure it accurately - but it will be close enough for your purposes. All you really need to know is that the voltage that comes through the function to the bulb (or other device) is DC voltage about 1.5 volts lower than the track. This means that N scale will put out about 11 volts, HO about 14 volts, and O/G scale about 18 volts - generally speaking.

Anyway, a lower voltage bulb can't handle this amount of voltage. To make it easy to understand, think of your mouth as the bulb. You are capable of drinking a certain amount of water per minute. Now, stick a hose in your mouth and turn it on full blast. You're gonna have problems. But all you have to do to make it manageable is kink the hose. You just added resistance. You being an adult can drink water faster than a small child. The small child will require more resistance in that kink than you do. So it is with bulbs.

Loy's Toys offers two different types of bulbs: 1.5-volt Grain-of-Rice (GOR, very tiny), and 14-volt Grain of Wheat (GOW, a little larger). We'll discuss the 14-volt bulb later.

The 1.5-volt Grain-of-Rice bulb we carry draws 15 mA. mA stands for milli-amp. To give you a better idea about how much current this is, it takes 1000 mA to make 1-amp. If you're operating on the HO-Scale setting, it takes 670 ohms of resistance to reduce the function voltage down to 1.5 volts for that bulb. However, if running on the N-scale setting, it only takes 560 ohms - which brings up another issue.

Some people recommend operating HO-Scale layouts on the N-scale setting. They have their reasons that I don't agree with. But be-that-as-it-may, if you chose to run your HO-Scale layout on N scale, you will need to set up your bulbs with that voltage in mind - 560 ohms for our 1.5-volt GOR, for example.

The problem arises that if you take your loco to another layout, one that is operating on the HO-Scale setting, your bulbs will be excessively bright. It won't blow them immediately, but it will reduce their life expectancy. On the other hand, if you set your HO-Scale layout up to run on the HO-Scale setting, you won't have this problem. And if you take your loco to another layout that is running on the N-scale setting, it will still work. The lights will be a little dim, but it won't damage them.

But what if you're dealing with a bulb that you don't know about? Let's discuss Life-Like bulbs, since they now recommend changing them if using DCC.

Ever since Life-Like started making provisions for DCC in their HO-Scale locos, they've been plagued with dim bulbs. The dimness comes from the fact that they use a series of diodes to provide constant directional lighting for those that aren't using DCC. The bulbs are the correct brightness without DCC, but removing the dummy plug and plugging a decoder in makes them dim.

I once sent them a simple circuit that would correct this problem, but they chose to try other remedies - none of which worked. I presume that's why they now recommend changing the bulbs. The only problem with that is that they recommend changing them to 12V bulbs. As we've already discussed, if you're running your HO-Scale layout on the HO-Scale setting, that bulb will be getting about 14 volts - again needing a resistor to keep them from burning out pre-maturely. This is why we carry the 14-volt GOW bulb. You can install these bulbs directly without a resistor, and they'll work fine for a very long time. However, some people still install a 47-ohm resistor just to provide a little surge protection for the decoder and make the bulb last even longer.

Just because Life-Like recommends changing the bulb, that doesn't mean you have to change it. In some cases, you can simply add a resistor and use the bulb that's already in the loco. But what resistance is needed? Unfortunately, I can't tell you. That's because Life-Like doesn't use the same bulb in all locos. Of the ones I have used resistors on, the value has ranged from 120 ohms to 270 ohms. Since these are 1.5-volt bulbs, this means that those bulbs are high-current.

To use a 120-ohm resistor to drop the voltage to 1.5 volts would mean that you'd need a two-watt resistor. This being the situation, I'd recommend changing the bulb - which is probably why Life-Like also recommends changing the bulb. So, how is that wattage calculated?

Ohm's Law. Volts times amps = watts. But we didn't know the amperage of the bulb. We only knew the resistance needed (only because I figured that out before) and the amount of voltage we'd be dropping. With these two values, we can find watts with volts squared divided by resistance. If you're interested in more of these types of calculations, an Ohm's Law chart is something else that many of the better electronic stores give out as promotional items.

You don't really need to know about Ohm's Law, how much voltage is going in, being used, bulb amperage, or any of this other stuff we've so painstakingly laid out to confuse you. If you have a few trinkets, you can easily find the correct resistor value through trial and error.

The first thing you'll need is an assortment of resistors. It just so happens that we provide a resistor assortment for this exact purpose. You get 5 or 6 resistors of about 17 different values (placed in separate plastic bags and labeled) from 47 to 1000 ohms - 100 1/4-watt resisters in all for $8.

You can do the testing by connecting the red and black wires of a decoder to the track, and using the blue and white wires to connect the bulb and resistor to. Don't solder anything together. Start with a resistance you think will be too much, turn the function on, then just quickly touch the resistor to the bulb wire to see how bright the bulb will get. If it didn't come on too bright, try lower-valued resistors until you get the brightness you want. Just remember that the brighter the bulb, the shorter the life. So the key is to get the bulb as bright as it needs to be to look right, but no brighter. But there is yet an easier way.

Loy's Toys offers a Decoder Tester. While not absolutely needed, it is a real time- and frustration-saver. With this device, you can quickly clip the decoder into the Tester, clip the resister in, clip the bulb in, and test. Better yet, use a decoder that has an NMRA plug so you can plug the decoder in. This leaves the spring clips empty for bulb and resistor - you don't have to clip the resistor and decoder wire into the same clip. Oh, by they way, you can test decoders with it too.

So where do these resistors go?

Ideally, you'll use one resistor on each function wire, as shown below. Some people put them on the blue common, which all functions use, but bringing multiple resistors off of that one wire isn't as clean as one resistor on each function wire.

Some people are tempted to use one resistor for both front and rear lights. As long as you have directional lighting, this will work. But if you set your decoder up for non-directional lighting, it won't work well. If you turn both lights on at the same time, they will both go out - a resistor that is correct for the current draw of one bulb will be too much resistance for the needs of two bulbs: which brings up the next issue.

Some locos, such as Athearn's SD70MAC, have two bulbs in the front, and two in the rear.

If wiring two bulbs yourself, it's best to wire them in series. This is the old Christmas tree string of lights where when one goes out they all go out. The reason this is best is that it increases the circuit voltage instead of the amperage. Athearn's MAC has them wired in parallel. This is OK if you don't want to take the trouble to change it. But if you ever have to change a bulb, it would be best to rewire them at that time.

Whether wired in series or parallel, the resistance that will be needed will be less than for just one bulb. Two 1.5-volt bulbs wired in series makes a 3-volt circuit. This means that the function voltage only has to be reduced to 3 volts instead of 1.5 volts. Two 1.5-volt bulbs wired in parallel makes a 1.5-volt circuit that will require twice the current of a single bulb. It, therefore, requires less resistance because the higher amperage will cause a higher voltage across the resistor.