"Cheap" alive

[Kiha66]

Inspired by Gavino's work with lighting circuits, I've been playing around with the idea of retrofitting a home made keep alive into a decoder and locomotive not designed for one.  "Cheep" as you can probably do it for under $5, and probably under $2 per locomotive if you order the components in bulk.  

The circuit is very simple.  A capacitor to store energy, a resistor to limit current while charging, and a diode to ensure full power when releasing the energy.  The circuit will attach to the internal DC bus on the decoder, after the diode bridge.  This ensures that both the motor, lighting and logic components all are provided for.

For this trial run I am using spare parts I found in my electronics box.  I am using a 25v 1000uF capacitor, a 100 ohm resistor, and a 1N4007 diode.  Any of the 1N400# series diodes will be more than sufficient for this project, I just happened to have a #7 type handy.  

The locomotive being used is an HO Bachmann 0-6-0 porter tank loco, with factory unlabeled Bachmann DCC.  As the decoder is not designed for any external keep alive some circuit tracing will be required to figure out where to attach the wires we will be adding.
(A huge thanks to Marcus Ammann whos write up on this topic covers much more on the subject than I can explain here http://www.members.optusnet.com.au/mainnorth/alive.htm)

In this case the orange glass diode in the center of the decoder is attached to both ends of the DC bus, so I will be using that as my wiring points.  The line on the diode indicates positive power, but to verify I put the loco on DCC track and measured with a volt meter to ensure that I knew which side was positive.  Be very careful not to short anything with your test leads while doing this!  If you do the decoder is likely toast.
 

[Kiha66]

First we need to assemble the keep alive circuit.  To make things easier I first attached the diode and the resistor to each other.  As the components are quite small this minimizes the amount of fiddling we'll have to do later with these small components.

 

Next we attach them to the positive lead of the capacitor.  The negative end will be marked with a silver stripe, we want to use the other lead.  Also be sure that the silver band on the diode faces away from the capacitor, or it will be doing the opposite of what we want it to do!

 

Now we'll attach the wires to the deocder.  This probably the hardest part of the operation, as a misstep here will possibly melt/slag/short/ect the decoder.  Be very careful and use the smallest tip soldering iron you have!  

 

 

 

[Kiha66]

Now we can attach the keep alive we made to the wires we attached to the decoder.  Be sure you know which end is which!  Polarity matters here, so be sure the positive end of the capacitor goes to the positive bus of the decoder.  I chose to use green and purple wires here as they are not used elsewhere in this locomotive making it easier to know which wire is which.

 

 

Now make sure noting is going to short, and give the loco a test.  I've found the easiest way to do this is to have the locomotive headlight on and tilt it off the tracks.  It should hopefully stay on a bit longer, possibly dimming before going dark.  You can also test with the motor but the power will be drained much more quickly this way.



Now that we know everything is working, its time to start cleaning up.  Make sure everything is properly insulated and cant short out as you reassemble the locomotive.  Here I used kapton tape to wrap the decoder to ensure nothing can short on the loco weight.

 

Next the joy of routing!  Before you do an install you should ensure that there is room in the loco for the keep alive components to go.  In this case they will go in the cab so a slight notch in the water tank was necessary to pass the wires into this section.  In this case the cab is all plastic so there is little worry about any shorts, but best practice would be to insulate them anyway, and maybe paint it all black so its not noticeable from the outside.  In this case I will probably come back later to add another capacitor for better keep alive performance so I left the wire ends bare for now. 

Edited February 7, 2019 by Kiha66

[Kiha66]

All done!  While this was for HO, such an installation is possible wherever there is room to fit the circuit.  In an N scale locomotive you may have to use smaller capacitors, but otherwise works the same.