I am currently detailing an Athearn caboose and Im going to install interior lighitng and marker lights. My plan is to use a cheap TCS lighting decoder to control everything simply because I would rather not mess around with rectifiers and voltage regulators, the decoder will do it for me. But I want to install a capacitor or two to stop that annoying flickering that many people have with thier lighting. So there are a couple of questions actually.
First should I put the Capacitor(s) between the track pickup and the decoder or between the decoder and the LEDs. This would make a difference in my capacitor choice because if I put it between the track and decoder it would have to be rated 16V and AC. Im not even sure capacitors work with AC but I would assume they make some that do. Also will the capacitor interfere if for some reason I want to send commands to the decoder?
If I put it between the decoder and lights should it be before or after the resistor? If before, it would still need a 16V rating but for DC and if it were after the resistor it would only need to handle 1.5V, I dont even think they make them with that low a rating though. And where should I put the capacitor. do I need one for each polarity or only one? If only one which polarity should I put it on, and If I have more than one on one polarity should they be wired in series or parallel?
And maybe most importantly what type of capacitor should I be looking for. I know to find the lowest voltage that will work because low voltage = higher capacitance. But is the voltage rating how much it can handle coming in or how much I want going out?
Before going to all the trouble and expense of a decoder, or even a capacitor, just install the lights and connect them to the trucks. You will have to do all that anyway, so you won’t have lost much if you just “test” the system for a while without the bells and whistles.
I have a string of passenger cars, and I don’t notice any flicker most of the time. I’ve got a variety of lighting sources in the cars (incandescent, LED and flourescent,) and a pair of bright red LEDs on the tail end of the Observation car. Your results may not be the same as mine, but if you find the flicker is annoying then you can always just add some more stuff. I find that the DCC voltage itself works fine for my lighting, with no extra circuitry needed.
For each LED, you will need a 1K resistor, regardless of whether you go with a capacitor, decoder or straight connection to the trucks. The resistor goes in series with the LED.
Capacitors are designed for DC, unless they are being used as filters. As a charge storage device, you must use DC for them to be effective, so they should go on the light side of the decoder. A decoder puts out DC to the lights, by the way.
The resistor limits the current and resulting voltage by being placed in series with the light, meaning it is spliced into only one of the leads going to the light. If the the capacitor was connected in the same way, your light would never work. It has to be connected between BOTH leads going to the lamp. This permits the capacitor to build up a charge from storing electrons. When there is a momentary voltage interuption, the capacitor maintains the current flow until its’ charge is depleted. For this reason, you do not want to connect it between the track and the decoder. The capacitor will act as a filter and smoothe out the DCC (AC) signal if it is connected across the track voltage. It will again act as an insultor if it is connected to only one wire between the track and decoder.
When connecting to the lamp circuit, the voltage rating dpends on where it is placed in the circuit. If one connection is placed between the decoder and the resistor, the capacitor will see a 16 volt drop. If the capacitor is inserted after the resistor (connected in parallel to the led’s) it will see the same 1.5 voltage drop that the LEDs see. Substituting a a rechargeable 1.5 volt battery for the capacitor would also work.
Depends on what is causing the flicker: voltage fluctuations, or a ripple in the DC. A fair sized cap on the lamp might help. Put it across the output, with the lamp and the ballast resistor in parallel to it. The important thing is the time constant, which is R*C. If the lamp flickers because of a ripple in the DC, the cap should deal nicely with it. If power cuts out, it might not have enough of a charge to keep the lamp lit until the power comes back.
Im sure it wouldnt be much of a problem at home, I keep my track clean. I also have a pair of red surface mount LEDs in the tail of a passenger car, I have installed interior lights in all of my amtrak cars as well, and they are just fine. But a caboose would be a different story I’m afraid. Its shorter wheelbase wouldnt bridge as far if there are dirty spots, and the caboose will also be run over at the club where the track is DIRTY. I brought my pass. cars with lighting over to the club once and the flickering was very noticeable. The set of passenger cars the belongs to the club has the same problem with flickering. You probably shouldnt be running LEDs off of straight track power, DCC track voltage is AC and LEDs are designed for DC. The oscillation of the polarity is fast enough that they light properly, but all that switching of polarity might shorten the life of the LED. Theres really no reason to put more stress on components then you have to.
So, the cap. should go between the decoder and LEDs, would you suggest before or after the resistor? Also I think I will use more than one for extra power storage (lots of LEDs being put in) Multiple capacitors should be wired in parallel I assume, is that correct?
Normally the capacitor is ‘storage’ and the the resistor feeds the cap. Problem is whether the cap is sufficient to offset flicker, and can still fit inside the caboose.
Prpblem II is 4 wheels picking up power from the rails (cause of the flicker).
I spoke the the richmond controls guy at a train show this weekend, thats where this idea comes from. My problem is I couldnt remember what he had told me. Its pretty neat when he picks up one of his cabooses from the track and the lights keep shining for about 20 seconds.
I put marker lights on some of my cabooses. My solution for electric power was to mount the smallest regular-type battery in the caboose and put the smallest SPST slide switch so that the slide sticks through a hole in the caboose floor. I just turn it on when I place the caboose on the track, turn it off when I remove it or end the session. Even this little battery will run a pair of LED marker lights for qutie a few hours.
A similar solution that I’ve seen is to use the same idea of a battery, but instead of a slide switch, to use a magnetic locking reed switch that is glued to the roof of the caboose. Just run a cheap magnet along it to turn on and off.
Yes, that’s it. Be careful of polarity. An electrolytic or tantalum capacitor is polarity sensitive and will have a + or - sign on one of its leads. A reverse hookup can lead to a great explosion if available current is sufficient. [8D]
Also, voltage rating has no direct relationship to capacitance. However, increasing either increases physical size. I prefer a voltage of at least 150% of the supply voltage. (Across an LED, 5volts should be fine). Adjust the capacitance value to eliminate flicker and for whatever additional on time you need.
BTW, with large capacitance values it might take a few seconds for the LED to come on. There is a time constant equation but experimentation will work just as well.
You can also replace the decoder with a bridge rectifier or even a simple rectifier diode and save some money.
A charged capacitor is not like a battery in that the terminal voltage remains nearly constant during discharge. A capacitor discharges in exponential decay. It will be 90% discharged in the time constant (seconds) as set by the size and load. TC time constant = capacitor value in farad times the load in ohms. TC = f x ohms. Remember 1 uf farad is .000001 farad.
