RailPro > RailPro Specific Help & Discussion
Stay Alive Circuitry
G8B4Life:
--- Quote ---Electrolytic capacitors experience RUD
--- End quote ---
I can attest to that. Way back in high school in electronics class, a fellow student decided to put a cap into a power point (240v) and turn it on. The resulting instant very loud explosion scared the daylights out of everyone and the student was lucky to still have his fingers intact; there was a large dint in the roof where part of the cap hit it.
Anyway this thread is proving most useful with quite a bit of knowledge to be picked up, especially on the dampening of the ringing. Alan, is there a reason why your component values (power dissipation I'm talking about) are so high for this? Back when I was almost a DCC user I did some reading on this and the component values given to do this for DCC wouldn't have handled that amount of power.
- Tim
Alan:
--- Quote ---Anyway this thread is proving most useful with quite a bit of knowledge to be picked up, especially on the dampening of the ringing. Alan, is there a reason why your component values (power dissipation I'm talking about) are so high for this? Back when I was almost a DCC user I did some reading on this and the component values given to do this for DCC wouldn't have handled that amount of power.
--- End quote ---
Because I am using a rude and crude method! :-[ And also because of the difference between DCC and RP DC.
Adding a parasitic load (100W resistors) is a crude way of fixing the problem because it is power wasteful - electricity turned into heat. But, it is the simplest to design, easiest to implement, and has the lowest initial cost. The penalty is a few additional cents on the electric bill!
Relative to DCC...
DCC is an AC square wave on the rails. RP is steady state DC on the rails. This is an extremely important difference. AC buses react somewhat differently than DC buses when a load is applied or removed. While DCC also suffers from load change induced bus ringing, with RP the effect is more pronounced due to it being a DC system. On the flip side, DCC has to deal with all the waveform headaches while RP has no such issues.
Allan Gartner's Wiring for DCC site http://www.wiringfordcc.com/intro2dcc.htm explains the DCC waveform issues well. To borrow a couple of his images...
DCC waveform at the booster terminals:
DCC signal 25' from booster:
Closeup:
See the ringing in the DCC signal?
I found this trace online that, while not an actual DCC or RP measurement, is a close representation of what you might see if you put DCC and RP on the same scope. The green line = DCC, magenta line = RP. The DCC line rings on the leading and trailing edges of the square wave while the RP line rings when load is changed. Like I said, this is not an actual DCC RP scope trace but I'm hoping a picture is worth a thousand words.
It should be mentioned that none of this conversation applies to smaller layouts. Only large layouts, with long runs of wiring and power supplies (or DCC boosters) located large distances away, have to be concerned with ringing. Even then it doesn't seem to be an operational problem for RP, rather it is the fact Tim put that darn warning icon on the controller causing me to investigate further. It bothered me that I went to such trouble building a low voltage drop bus system and still got an occasional warning icon. Had to fix that. >:( Obviously the fix is optional, not mandatory. All other RP layouts are doing fine without it. For my own sanity I had to silence the warning icon.
Wattage-wise:
DCC layouts must accurately control the AC waveform shape in order to work correctly. Long wiring runs, parallel wire paths, and other model railroad realities play havoc on a square waveform. The various components, usually EMI filters and bus terminations, are [may be] added to a DCC track bus to better sharpen the square wave control signal that the layout wiring inductance is otherwise destroying. Minimizing the adverse effects of wiring on the waveform is the reason for many DCC best practices - keep booster distance to a minimum, twist the bus wires, etc. Shaping the AC waveform does not involve large current flow so high wattage components are not needed.
The components I added to my track bus are there to smooth a steady state DC line voltage. My crude method does this by placing a significant load on the line hence high wattage components. Adding new loads on the line (starting a loco) now represent less than 100% of the total current so they have less than 100% effect on the voltage regulation circuitry inside the power supply. Like driving a car with your feet on the brake pedal and accelerator at the same time.
G8B4Life:
Thanks Alan,
I had read Allan Gartner's site. Certainly a recommended site for those in to DCC.
So if I read you correctly, and in basic terms in DCC (AC) you need to "shape the waveform" with filters (resistor and capacitor usually) to dampen the ringing and in DC you need to "control the load" on the line (ballast resistor) to dampen the ringing?
I guess this might be one of the reasons Ring also makes a point about having short buses, "excessive heat generated by large resistors used for dampening at the end of a very long bus might cause a fire" ;)
This is actually good stuff to talk about as some might be contemplating long buses where ringing might become an issue. I know one of the major reasons for dampening the ringing in DCC is to assist in the prevention of the destruction of decoders (the voltage overshoot can and does kill decoders). I wonder if the same ringing issue could be a trap in waiting for us if one was not informed? Your trace back on page 2 showed a peak of about 25v, a significant amount above the max limit of 18v that Ring specifies. I'm sure that Ring has built some margin into it but I wonder?
- Tim
Alan:
--- Quote ---So if I read you correctly, and in basic terms in DCC (AC) you need to "shape the waveform" with filters (resistor and capacitor usually) to dampen the ringing and in DC you need to "control the load" on the line (ballast resistor) to dampen the ringing?
--- End quote ---
There are other methods but basically yes.
--- Quote ---I guess this might be one of the reasons Ring also makes a point about having short buses, "excessive heat generated by large resistors used for dampening at the end of a very long bus might cause a fire" ;)
--- End quote ---
Voltage drop is a common primary concern with long buses as well. Ring may advise against long buses for multiple reasons. The resistors don't reach the ignition temperature of anything found on a model railroad so fire is unlikely but melting styrofoam certainly is possible.
--- Quote ---I'm sure that Ring has built some margin into it but I wonder?
--- End quote ---
During my circuit breaker design and test process I exposed a LM2 to brutal conditions by repeatedly creating dead shorts on the rails with the loco at near stall current draw. I was dialing in my breaker design through trial and err. That one LM2 experienced several hundred shorts on a 15V 5A supply with no dampening whatsoever. It survived without damage and is operating inside a loco on the layout today. A DCC decoder would have checked out after the first couple shorts. LM2 is one tough cookie.
Josephbw:
"I guess this might be one of the reasons Ring also makes a point about having short buses, "excessive heat generated by large resistors used for dampening at the end of a very long bus might cause a fire" ;)"
So what would be considered a long bus run? I'm going to have a fairly large layout for a home, and will eventually have at least 2 power supplies, possibly 4.
Thanks for this discussion,
Joe
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