Reducing Light Flicker

"Lighting is everything" holds true. LED or Incandescent? I like incandescent for the scenery, like, street/highway lamps and buildings. For the confines of a passenger car, can't beat a surface mount (SMT)LED. It's small, easy to conceal, low low power consumption, little heat generation, long life and highly reliable. My dome obs has 17 LEDs (I said 19 in another post...another senior moment), and draws less than 40ma. Since the white LED as considerable bluish tint, I filter it with simple blonde masking tape. I like the Scotch masking tape. It gives pretty true colors, and diffuses nicely.

(See Interior Detailing for wiring materials, layout and construction in the car)

I use a full wave bridge rectifier and 3v-4v constant voltage regulator to power the car. The bridge is needed so the LEDs will be on in any direction off travel. There are a lot of low voltage regulators to choose from. One can go lower, but I'm not sure 1v would provide enough light through the tape. Whatever, but 2 things to think about: (1) Regulator with Low Drop Out. This is the minimum supply voltage required to maintain the regulated output voltage. There's not much point to having 1v regulated to the LEDs if it takes 5v in to get it. (2) Maximum Supply Voltage. Need a regulator that can handle the track voltage.

I installed each LED with 100 ohms, 1/4 watt in series to start with. At low currents, light output may very considerably from LED to LED, so I just adjusted by lowering or raising the resistance of some of the LEDs. By putting a resistor in series with the output of the regulator, the over all brightness of the car can be adjusted. The wiring doesn't look pretty, but LED placement is flexible, it's easy, fast, reliable, and it's covered up anyway.

Here's a partial parts list of key components. I get all my components at Mouser.com; reasonable and pretty fast.

(1)Bridge: CBRSDSH5-40 (SMT)

(2) Regulator: Sharp PQ15RW08J00H

(3)White SMT LED: Optek OVS9WBCR4 (big enough pads to hand wire)

Data sheets and application notes can be found at Mouser and on the net. I get ends and scrap copper tape at work, so I the only suggestion I have is to get on the net.

Schematic of lighting circuit with filtering capacitor

Bridge and adjustable regulator

The biggest improvement came when I converted all my lit cars to the Walthers trucks with 8 wheel electrical pick up.

To take it a step further, I went on a quest for a capacitor and/or a battery that would take care of any remaining dropouts. There's no rechargeable battery small enough or light enough that will work for me. The rechargeable cell/button batteries that I could find cannot delivery 20 ma continuously (average draw per car with 6-8 LEDS). The best I found could do 0.5 ma. Any rechargeable battery that can deliver that current is way too big and heavy for my interiors. I'm not interested in any battery that needs replacement after 10 hours of use either. I want to minimize handling and "popping open" my cars. Inevitably, something happens that I have to fix. So, that left capacitors.

I found a small, lightweight SMD capacitor that is 1500uF at 4volts dc. With the 8 wheel pick up, 3volt light rails and 20ma draw, it looks like a set of 3 of these caps in parallel, with 10 ohms in series with the set appears to be doing the trick. The caps easily fit in the roof area of my cars, in between the light rails. I could easily put a dozen of these caps in the roof area. The caps are small enough to put in lockers or dark rooms with the shades pulled, etc. The voltage on these caps has a limit of 4v dc. My regulators are 3v (gives the caps a little room to work). I AM STRONGLY AGAINST using even one of these high capacity caps across the output of a regulator without a series resistor. The cap will initially look like a dead short to the regulator when it is charging, and damage to the regulator may occur. Even regulators with over current protection may become compromised with the repeated charging and discharging of the caps.

3 volts and 10 ohms means my regulator will see a maximum peak of 300 ma, which is well below its 1 amp maximum. The regulator will only see that load when the cap charges, and for a relative short time. That maximum load time depends on how much total capacitance and series resistance there is.

300ma x 3v = 0.9 watts that the series 10 ohm resistor will see, but it is for a short time, so a 1 watt resistor is not necessary. I'm using 1/4 watt resistors on my cap circuits, and I feel no heat what so ever on the resistor after repeatedly charging and discharging. Now, if I was going to use 6-10 caps, I would go to a ½ watt resistor (longer maximum load time), or divide the caps up into more than one set. If more than one set of caps is going to be installed (so the caps can be distributed and hidden around the inside of the car) there has to be a resistor in series with each set of caps. For 2 sets, the series resistor for each set would be 2x10 = 20 ohms; 3 sets would be 3x10 = 30 ohms on each set, and so on. An option is to stay with 10 ohms per set, but the number of sets would then be limited by the equivalent parallel resistance of the 10 ohm resistors and the current capability of the regulator. In my case with a 1 amp max regulator, that would be 3 sets, i.e. (3) 10 ohm resistors in parallel = 3.33 ohms, and 3v / 3.33 ohms is under 1 amp, barely. One can have as many caps in a set as wanted, but the series resistor for each set must be determined so that the total current draw when all the caps are acting like shorts does not exceed the current capability of the regulator, and the resistors will not over heat. If this seems confusing, I'm sorry, but I'm trying to avoid technical impedance calculations involving AC theory, which would be even worse. If anyone needs help, please feel free to contact me.

The more LEDS and the brighter the LEDS, the more the current demand, and more caps will be needed to get the same filtering. My dome obs lounge will need at least 6 caps, because it has 17 LEDS, drawing about 40 ma. Also, for those cars with 4 wheel electrical pick up (wipers, all metal trucks) more caps will be needed to achieve the same effect has I did. This will require some experimentation by those users to determine how many caps are needed, cause I no longer have any cars with wipers or all metal trucks. A dc incandescent circuit will require a lot more caps cause of the large current demands, but remember, NOT OVER 4 volts dc on these caps and don't over heat the resistors. Any amount of caps will help reduce the duration of the dropouts. I say "do what you can". These caps cannot be used in AC circuits, cause the caps are polarized (plus and minus terminals).

Yep, there's a big down side. These caps are pricey; $6.50 at mouser.com, but the price goes down quickly with quantity. I got 25 for $4.25/ea; 100 was listed for $2.60/ea. A group of people could go in together and buy bulk, thus reducing the cost to each person. If anyone can find them cheaper, please let me know.

Part number for capacitor is 597D158X9004R2T. The data sheet is available at Mouser.

SUPER CAPACITOR

This capacitor requires a significant amount of space, and couldn't be hidden in my dome obs, diner or coach car. In sleepers it can be hidden in a darkened and shaded bedroom. Other places it could be hidden is unused/unseen vestibules areas, toilets, large lockers, aisles or mens/womens lounges. It is important to use a low ESR (equivalent series resistance) cap. The cap is pricey, but provides the best filtering so far. The part number is PA-5R0V224-R and is made by PowerStor/Cooper Bussman. I get mine at Mouser.

I got to say, that it has been a sweet experience to finally sit back and watch my train go around all lit up and see my interiors with no distracting dropouts. It's really cool to be creeping along at 5-10 mph, and no flicker. It's been worth every penny and minute of work. Still have to clean track and wheels once and awhile, but a lot less than before, and I have a small, reliable, low power, hands-off interior lighting system.

These are not intended to be final solutions. I will continue to look for smaller, lighter, cheaper devices and better solutions. I don't think it will be too long before we will see a small rechargeable battery that will deliver the current needed.