I have plenty of room as the circuit will be in a passenger car with tinted windows.
background:
I have 4 Walthers Horizon cars that I custom painted to SEPTA livery, I installed a light at each stairwell (16 total) and they look great when driven with a battery, but I want DCC Control and track power going to them (not a battery). I wired them in parrallel so if one burns out the rest stay lit. I ran the control wires connecting the 4 cars to the rear cab control car hosting the dcc module (it also controls the head/taillights in that car too) hence the need to drive 16 lights from one function output.
Thanks for any help, I’m good with a soldering iron, just not with the math of figuring current/bulb resistor needs etc.
So, you’ll roughly be pulling a 1/4A if all the lights are on at the same time. That would be a nifty lightling effect, if it’s possible to run them all individually. Which DCC system are you using? How many amps do you have? Do you have any boosters?
Yes, but not by just hookign them up to a function output. Your decoder can only drive 40ma per function output, that’s only 2 of your lights. 16 x .015 amps = .24 amps. If you need tothem to all be off the same function, you need to connect a relay to the function output and then use the relay to turn the lights on and off.
If they are currently powered with a battery circuit - use the relay to turn it on and off. Dropping voltage with resistors is not good when wiring bulbs in parallel - if one burns out, the rest stay on, but sinc ehte current changes, the voltage to the bulb changes. Ends up being a chain reaction - one bulb blows, the voltage goes up, blows another bulb,t he voltage goes up more…etc. If your power supply is a battery, the voltage is fixed.
The TCS FL2 decoder has 2 funciton outputs only, and each one can drive 250ma max. (although the manual says 200ma max - the web page says 250ma per function - need to clal them to see which is is). That’s pushing it, running 16 bulbs with a total of 240ma, but you can remap the functions so both turn on with the same function. That way you can split the load. TCS also makes a nice voltage regulator module, the VR1.5. These can drive up to 120ma, but according to the documentation need to be touching the metal loco chassis for heat sinking if driving the full 120ma. Better yet would be to connect several of them - in your case, 4, 2 per function, 3 lights to each one. The VR1.5 delivers 1.5 volts regardless of the load (within its capability). If this is starting to sound expensive - it kind of is, a relay and a protective diode is going to be a lot cheaper, and you cna retain the existing battery circuit.
I seriously would just buy one of the Walthers or Kato’s interior lighting kits (prewired PC board) and just install Digitrax Function only decoder to turn on and off the lights if that’s all you want. Because for that many LED’s, things get a bit complicated not just to calculate the right resistance but you have to to consider installing capacitors etc with that kind of ampage draw with such small output.
With lots of incandescent bulbs, you need to consider the inrush current. Each bulb will conduct many times it’s rated current (like 10-20 times!) during the first few milliseconds when it’s turned on. LED’s do not have this problem. I suspect this would blow a decoder function output in short order. I think some of the decoders I’ve used have a degraded current ratings specifically for bulbs.
You might want to stick with the battery power as suggested. While requiring a small amt. of maintenance, it is the easiest way to go.
The rest of this information is theoretical and untested.
As for using track power … unfortunately, I don’t believe that there are any 1.5volt regulators on the market.
Might I suggest using a 6v regulator (LM7806 at less than a buck) and wiring four bulbs in series? (4 parallel sets of 4 in series)
Pick up power from the track, run it through the relay then a bridge rectifier then the 7806.
You might want to include capacitors on the input and output of the regulator (100uF and 1uF should work). I would also add a heat sink to the 7806 but since the total current is small so can the heat sink be.
We are using the Lenz DCC System (5 Amps) and we’ll be adding a booster soon (an additional 5 Amps)
I think I agree with the third poster that I’ll need a relay, I Suppose I can use diodes to rectify the track power off the wheel pickups, in fact I may beable to half the track voltage that way, wow my little mind is thinking up crazy ideas all of a sudden, This will require some testing. I just need to know what kind of resistance I’m looking at to offset the inrush when the relay pulls in with 16 40 ma bulbs.
I wish I knew which one it was on, but on one of the Yahoo DCC groups there is a nice circuit for connecting a voltage regulator to DCC track power. Complete with a large capacitor PROPERLY hooked up to give decent holdover time if contact is lost, yet not cause a current inrush problem with the track power is switched on.
The LM350 adjustable voltage regulators WILL go down to 1.5 volts but it’s the absolute limit and operation may be flakey. The 4 bulbs in series idea is better - put the 4 bulbs in a given car in series with each other, and wire each car in parallel to the next one. Use a 7805 instead of the 7806, that will give 1.25 volts per bulb, they will still be bright enough, and last longer. Heatsink, maybe. The combined load with the series/parallel arrangment will be 60ma and the 7805 is good for 1 amp with a heat sink, 60ma should barely warm it even without one. Plus they are self-protecting, they shut down when overheated.
OK, forget that, maybe. The LM150/350 can go down to 1.2 volts, see the data sheet for a schematic, replace potentiometer R2 in the data sheet schematic with a wire and it will develop 1.2 volts. Perfect for driving 1.5 volt bulbs. And up to 3 amps properly heatsinked. I don’t think 240ma worth of bulbs will overload the device.
I do not recommend using a relay with a decoder without using a diode across the coil to supress the BEMF that occurs when the relay coil is de-energized. The diode is wired cathode to the BLUE common and anode to the FUNCTION output. Remember in this case it is polarity backwards because the back emf will flow in the opposite direction of the incoming current
There is another suggestion I would like to add - TCS X series decoders allow the motor outputs to act as a high current function output when configured with the proper CVs. This output is not as sensitive to the lamps inrush of current at startup. However you will also need a voltage regulator and a resistor capable of handling the larger current. (56 ohms at 3 watts).
If I were you, I’d forget bulbs altogether and go with LEDs. You can get the smaller T1 size that are warm white and water clear and are extremely bright. I lightly sand the entire LED case which diffuses the light. I’ve illuminated (brightly) the entire interior of a diner with this method using only three of these LEDs !!!
The following source has them for VERY cheap - 10 for $6.56 !!!
Seems to me that you could use the decoder output function to control a single-transistor as a digital switch. It’s been a long time since I worked with that stuff, but as I recall, the decoder would be used to drive the base-emitter junction, and then the lighting would be connected across the collector-emitter path. A quick google found this additional info:
By selecting the right set of transistors and current-limiting resistors, you should be able to control the lighting without resorting to clunky relays. I’d recommend putting a separate transistor in every car, so you would divide the load among the transistors and only have to carry the signal current between the cars. (Although you might want to have a parallel power bus, too, to let you use all the wheels on all the cars all the time.)
Another option that hasn’t been brought up is magnetic reed switches. This would give you individual control of each car, without having to physically touch the cars. You won’t be able to throw them from across the room, of course, but my guess is that you’re really going to leave them all on most of the time, anyway.
Randy, hook the track leads to a full bridge rectifier. Hook the + side of the rectifier to a resistor (prevents current inrush) then capacitor. Hook the other lead of the cap to the - side of the rectifer. Hook in parallel the light circuit to the + and - leg of the cap. It works a lot better with LEDs as the current draw is minimal and therefore increases your time you can be disconnected from the circuit before the lights go out.
To turn them on and off using a regular decoder, I suggest you use the motor leads of a regular decoder which can handle up to 1 amp in many cases. Problem is motor leads are pulse based. If you aren’t running full tilt the lights will flicker. You could hook a capacitor across the leads, but I heard this is a no no with certain decoders.
You could use a 12V transistor as stated above and hook it to a function output.
Hook the one of track leads to a diode. Hook the + leg (the diode one alowing in current) to leg 1 of the transistor. Hook leg 2 to the function output of the decoder. Hook leg 3 to the rectifier circuit listed in my first paragraph. Hook the right pass through of the full rectifier bridge to the other track lead.
Solid State Relays are a potential option hooked to a function decoder. But even they take significant space. Don’t even think of using mechanical relays. These would NEVER fit.
Go with the adjustable VR as mentioned above, set it up to run at minimal (1.2V).
Hook a Full Wave Bridge Rectifier up to track power, then route the DC output through a NPN switching transistor. Output of the NPN should then go to the VR. VR to bulbs.
The base lead of the NPN can be attached to your decoder output (via a 1Mohm resistor) and all should be well. I used a very similar lighting circuit in a caboose a while back… just not 16 bulbs!