Does anyone have a circuit diagram for a Keep Alive for lights only?
I don’t need to keep power to a decoder or loco motor, I’m simply trying to stop passenger cars lights from flickering.
I use this one that has been kicking around for some time now.
DCC_light2 by Edmund, on Flickr
In most cases I use strings of SMD LEDs that have very little current draw and they stay illuminated for 20 to 30 seconds after porer is removed.
With the “tape” style of LEDs and resistors built into the strips the LEDs stay lit for maybe ten seconds or so. Still enough to eliminate flicker.
Use the largest capacitor that will fit your space. IF I’m sure all the LEDs are the same I’ll wire them in series and adjust R2 accordingly.
Good luck, Ed
Thanks Ed,
How well do you think this would work with incandescant lamps? I guess I can just give it a go and see 
I gave up on powering my passenger cars from the rails. It is more than just dirty rails, wheels and turnout frogs. I had more problems with axle and wheel wipers because of plastic truck frames not working good enough to pick up power. I tried all kinds of keep alive capacitors and still had some flickering. I finally went with an onboard battery from the lead car (in my case Baggage car) and using super flex wire with micro connectors to connect all my cars flicker free.
I went with a single Lithium 1000ma 3.7 volt cell with an onboard charging board. The lousy wheel pickup doesn’t have any problems keeping the battery charged, the battery takes care of any power loss from the rails. I haven’t seen one single flicker since I did the conversion to battery on board. A real plus is I can park my passenger trains for long periods without power with the train fully lit up.
The car diaphragms hide the wire and connectors. My entire consist is 10 cars in each train and the current draw of the LEDs is well under 40ma at 4 volts.
I know my way isn’t what any other model railroader does but for me it has been the best solution ever. I rarely uncouple my passenger trains so running the power the full length of the train isn’t a problem. Another plus is I can remotely control the on off with t
Quite simple really. Assuming you are using DCC (will not work on DC) install a bridge rectifier and a capacitor. The trucks attach to the two AC terminals of the bridge rectifier and the + and - outputs connect to your lights / LEDs. The capacitor attaches across the + and - leads of the bridge rectifier. Be sure to observe the polarity of the capacitor. For capacitor rating, use a minimum of 25 volt rating. For capacitance, use the largest that will physically fit.
If you want to use a super-cap, I’m sure Dave will chime in with that circuit. 
(EDIT) - Sorry, little slow on the keyboard - looks to be covered already …
Mark.
Thanks everyone, good info [:D]
Well, the higher current draw of the incandescents would “dump” the capacitor much quicker unless you have room for a larger capacitor. You would probably get some reduction in flickering but certainly not much in the way of “keep alive”.
There was some discussion here about it. Keep alive is a popular subject:
http://cs.trains.com/mrr/f/744/t/268873.aspx
Cheers, Ed