RailPro > RailPro Specific Help & Discussion
Stay Alive Circuitry
Alan:
Bridge resolves track polarity issue and prevents capacitor from discharging to the wheels.
Resistor limits capacitor charge/discharge rate.
Capacitor supplies current only when none coming from wheels.
Could eliminate the resistor if not too many locos on the layout. Large amounts of capacitance may make it difficult or impossible to startup the power supply/breakers.
Effectively a filter too so not usable with DCC. Would wipe out the digital packets. This is RailPro. We play with pure steady state DC on the rails. Filter doesn't have anything to filter out.
William Brillinger:
Wouldn't the diode bridge prevent feedback of power from the capacitor to the rails anyway?
Other than using an existing Keep Alive, can you suggest a simple capacitor to use for this?
Or do you really need a bunch of caps to store enough energy for 2 or 3 seconds of travel?
I proposed this some time ago, but never acted on it.
http://model-railroad-hobbyist.com/node/22101
Alan:
--- Quote ---Wouldn't the diode bridge prevent feedback of power from the capacitor to the rails anyway?
--- End quote ---
Answer
--- Quote ---Could eliminate the resistor if not too many locos on the layout. Large amounts of capacitance may make it difficult or impossible to startup the power supply/breakers.
--- End quote ---
Alan:
--- Quote ---Or do you really need a bunch of caps to store enough energy for 2 or 3 seconds of travel?
--- End quote ---
Yes, you need a lot of capacitance. The advent of low cost super caps make it practical in size. https://en.wikipedia.org/wiki/Supercapacitor
If you promise not to reverse the track polarity during the test, put a capacitor (correct polarity) across the rails of an isolated section of track. Run the loco. Kill the power. See how far the loco moves. Electrolytic capacitors experience RUD, a SpaceX term ;D, when polarity is reversed. Rapid Unscheduled Disassembly can be very dangerous to you and your train.
melarson:
--- Quote from: Alan on July 27, 2016, 07:27:05 AM ---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.
--- End quote ---
First, I stated, in the case of connecting a KA, that avoiding the drop was NOT the reason to bypass the module's internal rectifier. It is a polarity issue.
Second, yes, you could add an external rectifier and put the KA after it, and then connect that to the red/black rail input wires of the module (as Bill has demonstrated in his post). But then you are introducing yet another 1.3v drop before power ever gets to the module (where if you will recall it has its own internal 1.3v drop). The more voltage you lose, the more you are going to start flirting with under-voltage alarms from the module. For the LM-3 it is best to use the 9-pin blue (+) and 6-pin yellow (-) to connect the KA. I agree that for LM-1's and 2's there is no choice; an external rectifier would be required.
And lastly, I think there is some confusion as to how I am implementing battery power. It is my intention to run the module on battery power exclusively, with no power on the track, AKA dead rail. The battery is not intended to act as a keep-alive. Implementing KA's or Batteries are two very separate issues.
That said, the battery system I'm using is from Dr. Neil Stanton of S-Cab.com. The battery itself is a single cell LiPo 3.7v pack with protection circuitry. It is connected to an S-Cab BPS circuit board, which steps the 3.7v up to 12v. Adding another battery in series to either the existing battery or the BPS output is not an option. 12v is what I have to work with, so if I can save a diode drop here or there I will do so, because I don't want to get under-voltage alarms all the time. The BPS also acts as the charger when a supply is applied through separate charging inputs, and it has all the requisite output current and voltage protection built in.
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