Stored Energy Is the Key – How You Use It Makes the Difference
If you have any kind of light in a rail car, LED or incandescent, and the car runs over an unpowered frog or dirty track or a big gap in a rail, the light inside the car may well flicker off then back on. It is incredibly unrealistic. LEDs will turn on or off faster than an incandescent lamp so they will be more noticeable in their reaction to gaps. Please note: this is not about keep-alive for DCC decoders. This is only about car lighting. While the power is temporarily removed, the LEDs/lamps must get power from somewhere to prevent flickering out. Batteries and capacitors are the typical electrical storage devices. Besides battery power, there are at least four prior approaches to flicker-free car lighting: using a resistor and large capacitor with or without a voltage regulator, and using a super capacitor circuit with or without voltage regulator for lighting. All typically use a small diode bridge connected to the rails to provide a DC voltage. My friend wanted a dead simple, low cost solution. This eliminated the battery and supercap alternatives for both complexity and cost. Non-rechargeable battery solutions were ruled out for long term maintenance and replenishment cost.
Basic Flicker Free Diagram
The problem with the basic circuit focused on the amount of energy you could reasonably store in a capacitor that could fit in a rail car. Bigger was always better, and would even work reasonably well for a single lamp that did not draw much power. The problem was powering passenger cars that might have 3 to 8 LEDs in them, or a caboose that might have 2 LED marker lamps and 2-3 lamps inside. Even limiting the current draw per device, the total current might be above 30 ma. That kind of power draw would have a noticeable flicker for a mere 2200uf 25V capacitor, and larger capacitors or groups of capacitors consumed much space inside the models, or just didn’t fit at all. The use of a regulator allowed a drop in voltage in the keep-alive capacitor, but was itself a load on the keep-alive, ultimately limiting the capacity of this approach. But it was a good idea nonetheless,
A Different Technology Changes the Game
There is another, newer kind of “voltage regulator” that takes a very different approach – a “switching regulator” that actually acts as a DC to DC converter. “Big deal” you say? Well… functionally no, but it can operate with near 96% efficiency, and in this application it does make a difference. Years ago switching regulators were complex and expensive. The advent of small integrated circuits that accomplish nearly all functions changed the game. You can find quite a few on the market. The “Battery Powered Models in HO Scale” article in the November 2014 issue of MRH http://mrhpub.com/2014-11-nov/land/#83 used two examples.
A new, much smaller (physically) version came to my attention:
Small Cheap Completely Contained DC-DC converter/Voltage Regulator
It can handle up to 28 Volts input (from the track) and has an adjustable output from 0.8 to 20 Volts. In this application we will not be stressing the converter. You can see from the picture it is quite small. The largest component we will use will be a capacitor rated at 25 Volts. If you are running G gauge DCC then you should use a 35 Volt capacitor (but you will have ample room for the slightly larger cap). A small diode bridge less than 1 amp will do well too. I had a 1000uf 25 Volt capacitor and a 2200 35 Volt capacitor on hand for trials. That’s it—3 components! Here they are:
1/2A 400V MINI DIP BRIDGE RECTIFIER allelectromics.com CAT# FWB-16 .40
3A DC-DC Converter Adjustable Step down Power Supply Module .96
http://www.ebay.com/itm/291353891841
CAP ALUM 1000UF 25V 20% RADIAL Digikey 493-1305-ND .51
or
CAP ALUM 2200UF 25V 20% RADIAL Digikey P5157-ND .90
Putting these together is simplicity itself. The capacitor is soldered across the input to the converter – remember to connect plus to plus and minus to minus. Observe correct polarity for the tiny bridge rectifier too. It’s plus and minus connections go to the module input too. Before you connect the completed module to your car lighting, you must measure the output voltage and adjust it to as low as possible for the type of lighting you have. Different color LEDs have different voltage requirements. For my white LEDs I adjusted the output to about 2.8 volts, and used a 47 ohm resistor in series with a group of 4-6 white LEDs. You can find the tiny trim potentiometer (circular device) just below the largest black component on the module in the picture. Also an advisory to those who do use either module-- the little trim potentiometer is easy to turn, but has NO stops at either end. It would be a good idea to make all adjustments before you connect it to your model lighting. The module itself is tiny, The total volume will be dominated by whatever capacitor you use.
Parts & 2 Complete Units – 1000uf and 2200uf Caps
The “Small” 1000uf Keep Alive
There is an even smaller module available, which should also do exactly the same job: You can find the tiny one I used here: http://www.ebay.com/itm/291420624610
The “Tiny” DC-DC Converter
The above picture shows the relative component sizes.
Why Does This Work?
Track voltage is the largest energy source available for car lighting. The capacitor we added is charged to whatever is on the track after DC conversion, whether it is DCC, DC, or AC. The converter lowers the voltage to the minimum workable with incredibly high efficiency (87-96%) so most of the power stored in the capacitor will be used. We set the voltage out of the converter to be the lowest usable (or nearly so). As the capacitor is discharged to light the car, the converter will track the declining voltage and still put out (again with high efficiency) the correct lighting voltage. So the lights remain powered with no flicker. With the 2200uf cap at DCC voltage on my track I have measured about a 3 second keep alive for 35 ma lighting. With a 1000uf cap I get about a 1 second keep alive – all these are approximate.
In the following video you can get an idea of what this can do.
If you are running DC power, you will likely not have 12 volts on your track, 6-9 volts might be more likely. For whatever you set the output voltage of the module, it needs about 3 volts more to be effective. So if we set the output for 2.8 Volts it will need about 5.8 Volts on the track to be effective. The larger the voltage, the longer the keep alive will function. A larger cap for DC operation will also help in spite of the lower voltage margin. The same goes for variable voltage AC operation.
I hope this helps you light up your own cars along your right of way!
Have fun!
Best Regards,
Geoff Bunza