RailPro > RailPro Specific Help & Discussion
Stay Alive Circuitry
Alan:
Is the desire to attach the keep alive after the module's rectifier, thus necessitating the LM3 ground pin, just to avoid the 0.7V drop of an additional diode? Or is there another reason?
SD90:
RailPro's keep alive will be very cool. Maybe they will just build it into the LM4 and LM4S!
melarson:
--- Quote from: Alan on July 26, 2016, 06:48:54 PM ---Is the desire to attach the keep alive after the module's rectifier, thus necessitating the LM3 ground pin, just to avoid the 0.7V drop of an additional diode? Or is there another reason?
--- End quote ---
I only mentioned the diode drop in relation to connecting a battery to the module. Tim's suggestion would bypass one of the diodes in the path to the module's internal power supply lines (henceforth referred to as the IPS). I like that because that puts more of the battery's voltage into the module. Since the battery output is only 12vdc, I would like to avoid as much unnecessary loss as possible.
But the keep-alive is to be wired to the 6-pin's new negative lead (the IPS negative line) and the 9-pin's existing blue lead (which is actually the IPS positive line). This actually bypasses the entire rectifier (and according to Tim, any filtering, regulating, and protection circuitry) and as such would avoid a two-diode drop of around 1.3v, but that is not the reason the KA is to be wired that way. The KA must be wired the way Tim instructs because it is polarity-sensitive, so connecting it to the IPS (a point of known, unchanging polarity) is essential. There are other benefits, but that is really the big one.
The following is pure conjecture. Because connecting a KA directly to the module's IPS bypasses the module's internal protection circuitry, I believe the Ring KA will likely have some sort of output protection built in, something I don't believe the DCC KA's have. Which would explain the technical reason for Tim's disclaimer about using DCC KA's at our own risk. Of course, the disclaimer could just be marketing. ;)
Alan:
So avoiding the voltage drop is the only reason. Then the usual KA cap/bridge/resistor setup can be used on the supply lines. Could you add another cell to make the battery 15.7V (+3.70V) and then the diode drop would be just right.
William Brillinger:
--- Quote ---Then the usual KA cap/bridge/resistor setup
--- End quote ---
Can you elaborate on "the usual" please?
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