Lights
for DCC – How to get the best effects.
Introduction: One
of the great benefits of DCC is the lighting effects that can be added to enhance
our models. Fitting a decoder into a loco requires, for motor operation, only
four connections that take less than 1/2 an hour to do, once the body is
removed.
Operating headlights are normal for today’s
locos. Connecting or adding the headlights takes a little longer and can create
a few hassles along the way. Adding marker, ditch, beacons, mars etc lights
(some only on
Operation of
headlights in DC: Headlights in earlier locos were 12 volt
incandescent types and during operation would be very dim at slow speed and
progressively get brighter the faster the loco went. A diode was added for
directional control. Endeavouring to improve the effect of the lights, loco
manufacturers fitted constant lighting boards that provided some form of
constant brilliance for example the NR, 442 and the 44. These were basic
circuits that used diodes in series with the motor that created a voltage drop
that provided current to a 1.5 volt lamp. These circuits also reduced the loco
top speed. Thus for existing locos, there are two different voltage lamps
fitted, 12 - 16 volt and 1.5 volt.
Fitting
headlights in DCC: The function output voltage of a decoder can be
between, from 12 volts to as high as 20 volts DC, depending on the DCC system
used. This broad voltage range and a desire to get a bright light can cause
some installation problems, depending on the type of lamp fitted to an existing
loco. I have listed the characteristics of both the incandescent and LED types
of lamps. Using a low voltage type, will require a voltage dropping resistor to
“match” the decoder. To achieve the best brilliance of an
incandescent lamp, a calculation is necessary to determine the resistor value
that will require some test measurements. This makes fitting a low voltage
incandescent that is popular on later diesels, a little technical and a time
consuming, since a general value resistor cannot be stated, not like the
recommendation of a 1,000 ohm resistor for a LED.
Characteristics of both lights:
Shown in the photo are the two types of lights,
LEDS in 4512 and 12 volt incandescents in 4505. The “4505”
installation could have been “adjusted” as the plastic diffuser had
been heat damaged, but to get similar results to 4512’s LEDs it is too
much trouble – see later.
Incandescent
lamps.
- Numerous
types, 1.5, 12 – 16 volt versions.
- Sizes
0.75, 1.2, 1.7, 2.4 and 5.5 mm. See Miniatronics below.
- Different
current ratings for both types eg 10, 15, 20, 30 mAs etc.
- 1.5
volt lamps will require voltage dropping resistor.
- Small
voltage variations cause large variations in brilliance.
- Produce
a lot of heat and a little light.
- Inrush
currents (up to 5 – 10 greater) may effect decoders.
- Not
all equal brilliance at their voltage rating.
Light Emitting
Diodes – LEDs
- Solid
state low voltage, 1.2 – 3.5 volts depending on colour.
- Sizes
0.8, 2.0, 3.0 and 5 mm.
- LEDs
are polarity conscious.
- Require
a voltage dropping resistor.
- Brilliance
does not change markedly with voltage variations.
- Prototype
White or similar for a prototypical incandescent light.
- Produce
a lot of light and a little heat.
- Longer
life.
Incandescent
lamps- The importance of Voltage and current.
Lamps fitted to a loco with a diode board for
constant brilliance eg the NR, will be 1.5 volt types, otherwise they will be
12 – 16 volt types.
Since the function (headlight) output on most DCC
decoders is about 12 volts DC, the
operating voltage and current of “unknown specification” lamps has
to be determined.
This is as easy as connecting the unknown lamp to a
1.5 volt battery with a multi meter selected to milliamps, connected in series
with one of the leads:
- If
the lamp illuminates, it is a 1.5 volt type and the meter shows the
current rating.
- If
it does not illuminate, it is a 12 – 16 volt. Test with 12 volts for
current rating.
The current rating will be used to determine the
voltage dropping resistor value – see below. The NR is a 40 mA unit.
When 12 – 16 volt lamps are connected
directly to the decoder, some decoder manufactures recommend using a 22 –
33 ohm resistor to reduce inrush current when using these lamps.
If still wanting to install incandescent lamps, Miniatronics
have the following micro miniature incandescent lamps suitable for our locos
etc in all scales in packages of 10 or 20. Not even Miniatronics lists a
“current” of the lamps, so doing the “calculations” for
the best effect, may be necessary.
·
1.5 volt, 0.75 mm, 20
mAs.
·
1.5 volt, 1.2 mm, 15 mAs.
·
1.5 volt, 1.7 mm.
·
1.5 volt, 2.4 mm.
·
12 volts, 2.4 mm, 50 mAs.
·
12 volts, 1.7 mm,
·
14 volts, 2.4 mm.
·
16 volts 2.4 mm.
Determining the
function output voltage and DCC track voltage: The function
output voltage is not regulated and is determined by the DCC track voltage. To use
1.5 volt incandescent lamps, this voltage must be determined to calculate the
dropping resistor value. Connect a decoder to a decoder tester or use a decoder
in a loco already converted to DCC.
Measure the voltage between the blue common
positive lead and the white or yellow function negative lead. The headlight
MUST BE selected ON with the throttle. If you cannot get a reading then the
decoder may have to be programmed. The voltage indicated will be 0.5 to 1.0
volt lower than the DCC track voltage for almost all decoders due to decoder
electronics.
In my example with my NCE system, this shows 13.36
volts DC and a DCC track voltage of 13.8 volts with my DCC Pocket Tester. DCC
track voltage can only be accurately measured with expensive specialist DCC
meters. Knowing the FUNCTION voltage is more important than the DCC track
voltage, as all accessories connected to any function lead, will have this
voltage applied to them.
A difference as small as 1 volt in the function
voltage, can make all the difference in effect of the headlight if using
incandescent lamps. This is why I cannot just recommend ONE value for the
voltage dropping resistor for a 1.5 volt incandescent lamp. This is why you can
see a variation of the brilliance of incandescent headlights when operating on
layouts with different DCC systems.
Resistor value for 1.5 volt
incandescent lamps:
To determine the value of the resistor measure the
function voltage as above and subtract 1.5 volts. With the example of 13.5 less
1.5 volts equals 12 volts. This 12 volts, will have to be “dropped”
across the resistor.
Ohms Law will determine that the value of the
resistor will be: Voltage divided by Current. For an NR 0.04 A (40 mAs) lamp,
this will be 12/0.04 = 300 ohms. Wattage is Voltage multiplied by Current; 12 x
0.04 = 0.48 watts. A 1 watt resistor will be required. A ½ watt will get
too hot.
The table shows the required resistor for the DC
function voltage and I have included subtracting 1.5 volts for the lamp in all
my calculations and the closest E24 resistor value stated.
Using these values may give a lower than
“desired brilliance” lamp. It will depend on your personal
preference. Test the resistor lamp combination and if too dim, reduce the
resistance value (increases current) but not more than 10 - 15%. The lamp will
have a shorter life. This is one of the problems associated with incandescent
lamps.
See Resistor Colour
Code Chart for identifying resistor values.
Function voltages higher than 16 volts, consider
reducing the track voltage by fitting the diode arrangement that Andrew Krassay
suggests in AMRM Aug 04 issue. The nominal NMRA track voltage for HO is 14.25
volts DCC. For the NR use a Digitrax DH163LO without having to add resistors.
See AMRM Oct 05.
The incandescent
lamp problem: The
trouble to determine the correct resistor value and the voltage variation
effect on the brilliance of these lamps, has
led some modellers to fit LEDs. While the same voltage variation is still
applicable to LEDs, it has negligible effect. It is for this reason I suggest
that if going to the trouble of fitting headlights, you discard any
incandescent lamps and fit LEDs. The extra effort is worth it. Some other
lighting effects are better with incandescent lamps, so you will have to
“work out” the resistor value as above. Modellers have told me that
Mars lights are better if incandescent lamps are used.
Fit a LED for
the best headlight. Use the 3 mm Prototype White LEDs from DCC
Concepts (or similar LEDs) that come supplied with a 1,000 ohm resistor or
similar. These do not have the blue tinge that early white LEDs have and
produce a light similar to an incandescent lamp for steam and early diesels.
LEDs are polarity conscious and need the correct
polarity to operate. The decoder’s common “blue” wire is the
positive and connected to the LED’s long lead and the function white,
yellow, green purple etc wire are the negative and these must be connected to
the LED’s cathode that is identified by a “flat” on the
shoulder of the LED’s body.
The 1,000 ohm 1/4 watt voltage dropping resistor is
not a critical value, as is the case with incandescent lamp installations,
since LEDs will operate with 5 – 20 mAs of current with only a minor
brilliance change. It is this reason that the voltage variation doesn’t
impact on the brilliance of LEDs.
LEDs have a very narrow beam of light, that will allow
it to be mounted on the mechanism and pointing at the headlight opening or
plastic diffuser, but not mounted pointing vertically, like some incandescent
lamp are in the Powerline 48 for example. Mounting “lights” on the
mechanism makes for an easier installation – no mechanism to loco body
wiring to tangle up.
Wiring in the voltage dropping
resistor: Place the resistor in either leg of the lamp or
LED, but for consistency, I always fit them to the POSITIVE Blue lead. Do not
share resistors, one per lamp/LED. Locate the resistor for incandescents away from plastic as they may get warm
and use 1.0 mm heat shrink on all exposed connections.
Lighting effects:
While the above discussion has been on headlights that are the major lights on
a model, whatever light the prototypes have, the model can have, if
accessibility allows for it. The use of fibre optics may be suitable to fit
more lights in a model as was the case with the Trainorama 44 diesel model on
the left with the red and white marker lights.
With DCC these can be operated independently of
each other if a separate decoder function output is used. Each can be set up to
be on/off, reverse automatically (Rule 17) or manually and flashing lights can
have different flash rates. To enable prototypical operation, a six function
NCE D14SR or similar decoder is required. In some instances, incandescent lamps
may be beneficial for the extra lighting effect as with “Ditch”
lights. See decoder instruction manual.
Programming the
decoder: The
default values of the decoder may not give the effect you want. Decoders from
different manufacturers, use different CV numbers and values for setting the
effects, so use the manual supplied or for NCE decoders see Light Effects at
DCC for Novices.
For LEDs there may be an extra values as is the
case with NCE decoders by adding “128” to the light effect CV value
in CVs 120 121 etc. This additional “128” provides a LED with a
lower voltage (than the “standard” for an incandescent), that will
enable the LED brilliance to reduce enough for a DIM headlight setting when
using F4 to dim. Different manufacturers use different CV numbers. After a
decoder reset, these may have to be reset to get the loco back to normal.
Caution – Using incandescent
headlights with DCC.
When using headlights in DC they are ONLY illuminated when the loco is being
operated. This would not cause too much heat unless used for long periods.
With DCC the headlights can be illuminated with the
loco stationary and this could be for long periods, sometimes for as long as
the track is powered. Even when re-powering the layout, some headlights
illuminate even though the loco is not selected for use.
Heat has to be dissipated from the 12 – 16
volt headlights and the voltage dropping resistor for the 1.5 volt lamps. These
items become quite hot and this heat can damage plastic loco bodies. The photo
shows what can happen on a DCC layout. This is one more reason why I recommend
fitting LEDs.
The LED voltage dropping resistor remains cool due
less current and does not create a problem. Remember, unless yards and sidings
are “switched off”, all locos are powered on a DCC layout whenever
the layout is powered. Good operator practice can reduce headlights
overheating, if the headlights are turned off on locos after use, but how many
of us are this diligent, maybe you are.
