Lights for DCC
Introduction: Operating headlights are normal for today’s locos and using DCC allows for operation more typical of the prototype. Adding/connecting Head, Marker, Ditch, Beacons, Mars etc lights (some only on U.S. diesels), can create a few hassles but mostly “solvable” along the way.
Headlights in DC: Earlier locos had 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 and use 1.5 Volt incandescents. Thus for existing locos, there are two different voltage lamps fitted, 12 - 16 volt and 1.5 volt versions.
Fitting headlights in DCC: Power for the loco lights is provided in most decoders by the “non regulated” Function (Common) Blue wire/Pad of the decoder and the Light is switched on by supplying an “earth/ground” at the Function wire. The Function 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 when using Incandescent lamps but not an issue when using LEDs.
To achieve the best brilliance of an incandescent lamp, a calculation (Ohms Law), is necessary to determine the resistor value that may require some “test” measurements. This makes fitting a low voltage 1.5 Volt incandescent that are popular on later diesels, a little technical and a time consuming. This means due to the incandescent lamp’s characteristics, a general “single” resistor value cannot be recommended, not like what can be recommended for LEDs – a 1,000 ohm resistor.
While the already fitted Incandescent lamps work, in many installations they are very dim. If you’re going to the trouble to fit lights, let’s see them.
Incandescents also produce heat that can damage plastic bodies, more of an issue with DCC due lights can be illuminated for longer periods with them being illuminated at idle.
Brilliance issues related to varying track voltages that can be corrected to a degree by determining an “exact” resistance, (see below), makes what could be a “good” light on one layout, a “bad” light on another.
With these less than desirable effects, I replace most incandescent lamps with LEDs and remember, all that’s necessary is to use a 1,000 Ohm ¼ to ½ Watt resistor connected in “series” in either of the leads to the White LED of your choice. Along with SMD LEDs, the job of fitting lights is made much easier.
The above photos from Ulrich Models, clearly shows the comparisons between the “real” loco, LED & original incandescent lamp.
Use LED’s for your Loco lights, they’re the way to go.
Advantages for using LEDs over Incandescents:
· No noticeable changes of brilliance of the LED with varying track voltages from changing layouts/systems, whereas incandescents can be very dim to bright and possibly blowing.
· No heat, whereas incandescents can distort plastic bodies.
· Voltage dropping resistor ONE value – 1,000 Ohms, whereas for incandescents require you to “calculate” a value.
· Later SMD LEDs are so easy to install, just glue it to the light lens.
· LEDs are brighter and have a longer life.
LED Characteristics & Specifications.
LEDs are semi-conductors.
Shown from Top – 3.0 mm White from Jaycar Electronics Australia, 3.0 mm Golden White from Richmond Controls, 2.0 mm Tower from DCC Concepts & 0.8 mm Surface Mount LEDs with & w/o leads, from Richmond Controls.
LEDs are polarity conscious devices that require the longer Anode lead connected to the positive & shorter Cathode lead to the negative.
The “Flat” on the LED housing of the 5.0 & 3.0 LEDs, identifies the Cathode (negative)
Typical LED operating current is 20 mAs.
Current limiting resistor necessary – 1,000 Ohms for DCC decoder’s 12 – 14 Volts DC.
Later types of LEDs are brighter and can be operated with less than 20 mAs.
12 to 13.5 Volts DC at the Decoder
A 1,000 Ohms ¼ to ½ Watt is suitable for ALL LEDs (approximately 13.0 mAs).
If the LED is too bright, INCREASE the resistor value.
Photo courtesy Ulrich Models
Golden Whites have a more yellow, making them suitable for Steam & early Diesels.
The White LED’s blue tinge can be eliminated with Tamiya Acrylic Clear Orange X-26.
Recently, the Surface Mount 0.8 mm (603) LEDs have become very popular.
Some modellers purchase these small SMD 0.8 mm LEDs without leads and solder “magnet” wire to the tiny solder pads. I prefer to let someone else solder the leads on.
These SMD LEDs are easy to install, just glue them to the lens.
Terminate the fine wires on a piece of “sleeper” Circuit Board like in the photo.
The lens can be easily “coloured” with Tamiya Acylic X-26 Clear Orange paint.
When installing LEDs or incandescent lamps, use one resistor per LED/lamp in either lead to the lamp, as shown below.
Ulrich Models for many LED applications
DCC Concepts here in Australia has lots of LEDs and many construction articles as well.
The LED selection when configuring the Lights on some decoders.
The "LED" selection when configuring the Lights for NCE and Soundtraxx decoders, to name a couple, is for when the Light is "dimmed". The DIM currents are different between LEDs and Incandescents, so the decoder needs to know how much to reduce the current. Select “LED” in Decoder Pro or configure the decoder using the Manual when using LEDs, otherwise using the “incandescent” lamp “dim” current, the LED will not “dim” enough.
If you still want to use Incandescent Lamps.
DCC Track voltages are NOT all the same, also what scale is being used impacts on what voltage is at the Track. Measuring track voltages from my NCE system at 13.85 and a Roco (Lenz/Atlas derivative), I found it at 23.0 Volts. Voltage drops in layouts due to wiring, 1156s etc, also have an effect on lamp brilliance.
Incandescent lamps come in various sizes and “current ratings” e.g. 10, 15, 20, 30, 40 mAs etc. See Miniatronics.
To have Incandescent lamps as bright as possible in the loco, providing a “good visible” Headlight, the lamps should be operated at their specified current rating. To do this, you need to know:
1. Operating Voltage of the Lamp - 1.5 or 12.0 Volts.
2. The current rating of the Lamp - 15mA, 30mA etc.
3. The actual voltage produced in the decoder between the Common + and Function (White Yellow etc) -.
Checking the Lamp Operating Voltage
If you don’t know the “voltage” of the lamp installed in a loco:
· Connect the lamp to a 1.5 Volt battery.
· If it illuminates, it’s a 1.5 Volt lamp.
· If it remains extinguished, it’s a 12.0 Volt lamp or it’s “blown”.
· Connect to 12 Volts, to see if it’s blown.
· If so, throw it in the rubbish bin.
Lamp Current Rating
For 1.5 Volt Lamps from the above test, connect an Ammeter in SERIES with one lead from lamp to the 1.5 battery. The Meter will display the lamp’s “rated” current. As can be seen from the below Table, there is a different resistor used per “mA” rating.
Decoder Function Voltage
Measuring the “actual” decoder Function voltage, measure the voltage between the Blue Common positive lead and the White or Yellow function negative lead, as shown in the photo. 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.
My Measurements and Example
13.8 Volts DCC My NCE layout track voltage using my Pricom DCC Pocket Tester.
13.36 Volts DC Function Voltage from above desription.
40 mA lamp NSW NR (Australian) loco 1.5 Volt Incandescent lamp
Voltage dropping resistor necessary is determined by Ohms Law: Resistance = Voltage/Current.
Note: The voltage “to be dropped” by the resistor, is Function Voltage LESS the 1.5 Volts for the Lamp. That is 13.36 – 1.5 = 11.86
Resistance = 11.86/.04 = 296.5 Ohms. Closest value = 300 Ohms.
Resistors have a “Wattage” rating - ¼, ½, 1, 5 etc Watts, to dissipate the heat produced, “dropping” this voltage. The small Surface Mount resistors used on the decoders are “603” and 1/16 of a Watt while the larger “1206” are 1/8 of a Watt, while the “axial” resistor shown above, that we use with LEDs, are ¼ or ½ Watt.
Wattage rating of the resistor is equal to P (Watts) = E (Volts) x I (Amps) = 11.86 x .04 = .4744 Watts = .5 Watts.
The heat caused by 40 mAs of current flowing through this 300 Ohm resistor will need to be dissipated. A ½ Watt resistor will be get very hot and burn out, requiring replacement, something you don’t need after putting the loco together. A ONE WATT resistor will be necessary but it will get warm. If next to plastic be cautious. An alternative would be to use TWO 1 Watt resistors providing a total resistance of 300 Ohms – 2 x 150 Ohms in SERIES or 2 x 600 Ohms in PARALLEL. Just another thing to ponder it you need to install Incandescent lamps.
The Table below has these values.
See Resistor Colour Code Chart for identifying resistor values.
Getting incandescent lamps to be bright enough, is a "juggling" act. Increase the current above the nominal (40 mAs in this case) value and the life expectancy, reduces. If you increase it too much, the lamp will blow as soon as you connect power.
Once you have got your Incandescent “right”, use the loco on another DCC system, the lamp brilliance may be different depending on what the Track Voltage compare to the one you used, to ”set up” the lamp.
12 – 14 Volt
< 50 mAs
> 50 mAs
22 Ohm ¼ Watt
820 Ohm ¼ Watt
390 Ohm ½ Watt
300 Ohm 1 Watt
200 Ohm 2 Watt
Don’t want to make all these measurements,
If you know the mA current rating and operating voltage (12 or 1.5 Volts) of the lamp from the above test, you can use the resistor according the Table right, but don’t be surprised if the incandescent lamp is too dim or blows altogether.
It is suggested that you can operate incandescent lamps with higher voltages but greater than 10%, may severely impact on the lamp’s life.