Hi everyone:
I am working on a fleet of eight cabooses that will have working marker lights and an LED inside to simulate the conductor's desk lighting. I am using Mark R.'s caboose constant lighting circuit which basically involves a 0.10 F 5.5V capacitor, a voltage rectifier and a 1 K 1/4 watt resistor to lower the voltage to less that 5 V. I will also have a magnetic latching reed switch so I can turn the lights off.
I have two choices when it comes to wiring the reed switch. One is to control the LEDs only, so that the capacitor is always charged and the lights will come on immediately. The other is to control power to the capacitor so the lights won't come on until the capacitor is charged, which takes about 15 - 20 seconds. If I have power to the capacitors constantly, will this cause a problem with the DCC system falsely sensing a short if all eight cabooses are on the tracks when I turn the system on. NCE Powercab by the way.
Mark R., if you are following the thread, please chime in.
Thanks
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
I wouldn;t have the reed switch control power to the capacitor, they aren't meant to handle much current and the inrush may be too much. However, with the resistor limiting current - if it takes the capacitor 15-20 seconds to charge up - then there shouldn't be much inrush current and there should be no issues with the DCC system seeing it as a short. That 1K resistor is limiting current to around 14ma at 14V, which is well below the 'short' current for the Powercab. If you had just the rectifier and a capacitor, it would charge up in a fraction of a second, and the current at that point may well be several amps instananeously - this is where there are inrush problems.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Randy has the answer Dave. The resistor before the cap plays a dual role - to limit the input voltage for the 5 volt cap and also to limit the inrush current.
Never considered needing an on/off control on my cabooses. Most all my cabooses are on the ends of trains which end up in staging at either end of the layout. If any cabooses are left in the terminal, it never really bothered me that they sat there lit up. Just never saw the need for the extra work to manually turn them off.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
I do want to get a nice brass one and do up the lights with a decoder and all - so I can turn it all off when parked, or maybe just have one lamp on by the conductors's desk. On my prototype, they used oil lamps inside right until the end, the markers they did change over to battery ones. So what I want to do is use a couple of SMD LEDs to be the oil lamps in the proper locations inside the caboose, and use a decoder to control them so I can use the flicker effect to make them look like oil lamps. Plus also turn the markers on and off, since I really don;t want to attempt to make actual light up but removable markers in HO.
hon30critter...I have two choices when it comes to wiring the reed switch. One is to control the LEDs only, so that the capacitor is always charged and the lights will come on immediately...
Mark R....The resistor before the cap plays a dual role - to limit the input voltage for the 5 volt cap and also to limit the inrush current...
I have not seen this circuit, is it simply the rectifier, resistor in series with the capacitor, and LED in parallel with the cap? If so, then I would not have a switch between the capacitor and the LED. The forward voltage drop of the LED works with the resistor to limit the voltage on the cap. If you break the connection to the LED, then you will be applying the full rectified voltage to the cap. The current limiting function of the resistor may prevent damage to the cap, but I would not rely on that.
Howdy, Randy,
Since you'll have a decoder onboard, have you consider changing the marker colors when the train takes a siding? Incidentally, did the Reading use green or yellow for the, "Clear of switch, in siding," indication?
I don't have that problem. All of my markers are just plain red (as is the banquet-plate size reflective disk that surrounds them.)
As for turning off the caboose lights, I simply de-power the track the brake van is standing on.
Chuck (Modeling Central Japan in September, 1964)
Hmm, neat idea. Doable, too, with SMD LEDs, they are small enoughto put in marker castings. As to which color, no idea, but I do have a rulebook I can look in - though it is newer than my modeled era.
Randy, Mark R. and CSX Robert:
Thanks for all the responses.
I think I will follow CSX Robert's suggestion and use the switches to control power to the whole circuit even though there will be a delay in the marker lights coming on. Hopefully the magnetic switches will respond reliably each time because I won't know if they worked for several seconds. Hopefully the reed switches will survive the current going to the capacitors because it is limited by the resistor.
The reason I want to be able to turn the lights off is that the cabooses will reside on the visible part of the layout most, if not all, of the time. I have three separate caboose tracks. I don't think it would look right to have them lit 24/7.
Randy:
Unfortunately I couldn't find the specs for my specific switches, but I checked out the specs for a number of similar switches of the same physical size and the minimum they would handle was 250ma @ 100volts. Most were 500ma @ 200volts. Unless mine are way under those specs I think should be fine. Would you agree, or am I missing crucial information?
For the way your circuit is, these should be fine - the current your switch would see in the circuit you described would be less than 1/10th of the rating of the switch contacts. Doesn't matter where you put the switch in the circuit, it will be the same.
In the other type of circuit, where the capacitor was directly after the rectifier with no resistor - in that case, the instantaneous current could easily exceed the switch rating - by 4x or more. High enough and those thin slivers of metal in there burn like a fuse. A lower levels of overload, they end up welded together and stuck permanently closed. But your circuit is different and you will only be switching LED current levels which are well below what the switch can handle. If you got anywhere near the switch's rating, the LED would pop like a flashbulb.
Dave,
FWIW I occasionally run the 1938 Century and the Empire State Express on my layout using the MTH cars with the keep alive lighting. I have 13 cars in each train and when I power-up the layout all 26 cars with their capacitors are being charged and I've never had a false-trip using the earlier PSX breakers and Digitrax DCS200s. Usually both trains are occupying the same power district at startup. My PSXs are set to trip at 6.2 amps at 10 ms.
With all the pluses and minuses of MTH they DID make these cars pretty well. The 6 leds stay lit for a good 60 to 90 seconds after power is removed. I haven't cracked one open yet but I'd love to duplicate their circuitry for the lighting.
Ed
Thanks Ed
Sounds like I am pretty safe.