Using 1.5 volt bulbs with DCC decoders

Normally, decoders put out basically track voltage (usually 12 to 16 volts in the HO world) on the function leads. If you connect 1.5 volt bulbs directly to these outputs, their life is measured in a fraction of a second, perhaps even destroying the control transistor inside the decoder. There are several ways to control these lovely bulbs, each with advantages and disadvantages!

BEWARE: most folks apply more than 1.5 volts to these bulbs. When they have the correct amount of voltage and current, they are not very bright and only last a few hundred hours. If you apply MORE voltage, they will draw more current than they are rated for and reduce thier life significantly!

Consider an alternaitve to these small bulbs: surface mount LEDs. They are MUCH brighter than the 1.5 volt bulbs and measure their life in thousands of hours and are easy to tame. If you put about 2000 ohms in series with a surface mount LED, you will get a result that is at least as bright as a 1.5 volt bulb and easily 20,000 hours of life - that means that the LED will burn for TEN YEARS of 8 hours a day, five days a week! Drop the resistor to about 750 ohms and the brightness will blow you away and the service life will only be a couple of years of 8 hours a day, five days a week.

How much work is it to change the bulb you are connecting?

Still want to use 1.5 volt bulbs? Here are some ways:

Decoder

Manufacturers are starting to make decoders with 1.5 volt support included. This is the easy way to tackle these little bulbs!

The TCS A6X HO decoder was the first. It is built on an Atlas Light Board sized circuit board. It can control up to 6 functions with an internal 1.5 volt regulator, providing 100 mA of current - enough for 6 standard 1.5 volt bulbs drawing 15 mA each .

The SoundTraxx TSU-AT1000 diesel Tsunami is built on an Atlas Light Board sized circuit board. It can control up to 4 functions with an internal 1.5 volt regulator, providing 150 mA of current - enough for 10 standard 1.5 volt bulbs.

The QSI Revolution A is built on an Atlas Light Board sized circuit board. It can control up to 6 functions. Each function is supplied from an internal 5 volt regulator with a series resistor designed to allow the direct connection of one LED or one 1.5 volt bulb drawing 15 mA.

Train Control Systems also offers an external 1.5 volt regulator to interface with some of their other decoders. CLICK HERE

Resistor

If you know your track voltage and will NEVER operate your loco on another layout or change the track voltage on your layout, then you can simply add a resistor in series with each 1.5 volt bulb. The formula for calculating the resistor is:

Resistor (ohms) = 1000 * (track voltage - 1.5) / (bulb current in mA)

For a 15 mA bulb with 15 volts on the track, the value is:

Resistor (ohms) = 1000 * (15-1.5)/15 = 900 ohms - since 910 ohms is a standard value, select that. If you want to have a longer bulb life, increase the resistor a tad to 1000 (1K) ohms.

The advantage of this method is that it is quick and cheap. You MUST know the current draw of the bulb. The disadvantage is that the bulb(s) may vary from not lit to blown out depending upon the design voltage and the track voltage as you move from one layout to another. Also, there is a tendency to reduce the resistor value to get a bit more brightness out of the bulbs, reducing the bulb's life

Resistor and Diodes

The quickest and cheapest and most reliable system is to add a couple of diodes to the basic conneciton above to regulate the voltage applied to the bulb(s). A forward biased (activated) diode drops about 0.7 volts, so two in series will provide about 1.4 volts over a wide range of input voltages.

In the above circuit, it is critical that the banded end of the diodes are connected as shown. The BLUE connection can be to the blue wire or to a rail (red or black). The WHITE connection can be any function lead (yellow, green, brown, etc.).

For 15 mA bulbs, the resistor will vary with the number of bulbs as shown below. These values are calculated for any track voltage between 12 and 16 volts:

Bulbs

Ohms

Watts

1

240

1/2

2

150

1/2

3

100

1/2


The advantage of this scheme is that it is easy and inexpensive and provides good performance over a wide range of track voltages. The disadvantage is that the resistor can get rather warm and should not be in contact with plastic components.

Current regulator - LM334

A solid state current regulator can be used to contol as many 15 mA bulbs as you wish to light at one time (up to about 6). A LM334 Integrated Circuit can be connected with a 5.1 ohm resistor to make a current limiter that will deliver about 14 mA regardless of the supply voltage. This elegant solution runs very cool and is small, allowing for easy installations. Connect the IC as shown in the diagram below and cover with shrink tubing.

The single lead can be connected to the decoder blue wire or a rail (red or black) wire. The other end of the bulb (or string of bulbs in series) is connected to the function lead (white, yellow, etc.).

IMG_1929

A photo is sometimes worth 1000 words. Here is one that I built just before I put heat-shrink tubing over it (I used ⅜ inch diameter tubing).

Folks have noted that this circuit is outside the range specified for the LM334. I’ve built and used these circuits for at least 10 years with no failures.

Voltage regulator - LM317

An alternative method is shown below. It will provide 1.25 volts to as many bulbs as you choose to attach in parallel.

LM317-1.25Volts

Here is a photo of a 15 mA bulb being driven by the circuit chown above. The red wire in the photo would be the blue wire coming from a decoder and the black wire in the photo would correspond to a function wire, say the white or yellow.


Copyright © Bruce F. Petrarca 2007 - 2017; All Rights Reserved