FL4 KA connections, and "KA --> Paralleled Decoder" applications

Dear Ryan,

In inverse order:

- Per the FL4 manual
https://tcsdcc.com/sites/default/files/FL4.pdf
/>
the FL4 comes ex-factory with a Black/white striped wire which is the decoder GND.
That plus the "Function Common +ve" is enough to connect a KA to the FL4.
(IE no direct-to-decoder SMD soldering required).

- Connecting a single KA to 2x decoders means you would be coupling the GND and +VE lines of both decoders together. Without knowledge of the specific voltage-levels of the (2 or more) decoders in question, this poses a pretty significant risk of parasitic and/or "leak" voltages between the dis-similar decoders. Multiply by the parallel wheel-pickup connections, and the risk of "ground loop" and similar "sneak path" connections between the decoders is quite high, and very-much not-reccomended. (Warranty almost-certainly voided).

If space is at a premium, but there's enough space to fit a "sidecar" light-only decoder like an FL4,
then I'd strongly suggest reviewing the "N scale Keep Alive" articles previously posted here on MRH. The power-requirements of a FL4 are waaaaaaaaaaay less than a "full Motor + Sound + Lights" decoder, so can be kept "up" by a much (physically and electrically) smaller KA. Tantalum capacitors can provide a few 100uF of KA storage in "fits between the hood and cab-side of a Bachmann 45 tonner, below the window line" space, inc protection zener diode, and charge-limit diode/resistor combo, likely for less $ than a commercial KA.

Reccomend reviewing:
https://model-railroad-hobbyist.com/magazine/running-extra/2018-12/stay-alive-n
/> https://model-railroad-hobbyist.com/node/31852
/> https://model-railroad-hobbyist.com/magazine/mrh2019-06/electrical-impulses

Happy Modelling,
Aim to Improve,
Prof Klyzlr

. . .

TCS has a video where the gent does the FL4 + KA  soldering.  part 2

So you simply wire the decoders together to the KA.. piece of cake.

Marc

Thanks!

Thanks for the quick responses!

I didn’t know if small auto cutout circuit had been devised using diodes, a small transistor, etc. where the stay alive is charged through one decoder only, but could provide power to both decoders in the event of lost track power. I assumed connecting GND of two decoders would be okay, but still couldn’t figure out a simple solution.  Since the FL4 is lightly loaded with only LEDs, an additional small capacitor might take up less space than whatever would be needed to link the decoders!

Thanks again for the help!  I’ll try to be less of a lurker and post more often.

- Ryan

Since you aren't powering the motor...

Take a look at a the https://www.iascaled.com/store/ModelRailroad/Run-N-Smooth - it's about the minimum stay-alive circuit out there. I just ordered one to test. 

A KA1 is 27mm x 8.9mm x 6.2mm according to the TCS website. 

The R-n-S is 10.4mm x 9.1mm x 4.65mm. I'm very interested in seeing where I can hide it.... and how long it will actually keep an HO scale locomotive going, which it admittedly isn't designed to do.