| 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. |
