Buffering a Marklin PIC decoder. Note there are many different layouts of these; the picture below shows the version as originally installed in the “fire engine” (Feuerwehr) set. This decoder can easily be buffered as shown:
The picture shows the underside of the decoder. The main connections are all on the left except for the orange wire (U+), bottom right, which is used for the return from the lights. The solder pad immediately above that (back line) carries the negative current that we require for our circuit.
The circuit is not my own invention – it’s well documented on the web elsewhere. The basic operating principle is that you use a capacitor to store energy whilst the decoder is powered from the track. The moment the locomotive hits a “dead spot”, the charge from the capacitor is released back into the decoder, allowing the decoder to “stay alive” for a brief moment until track current is restored.
The resistor limits the load current for the capacitor (a completely discharged capacitor is effectively a short circuit). The diode ensures that when the circuit is in discharge mode, no such limit needs to be overcome, and the full charge of the capacitor is available to feed the decoder.
Marklin Loft 