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Stay Alive Circuitry

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William Brillinger:
My layout has 3 runs of almost 60ft on one PWR56 and I don't have any where that fails the coin test.

G8B4Life:

--- 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.
--- End quote ---

I know, I was having a bit of a poke of fun at Rings repeated deceleration of "it might cause a fire if it's not our product" in his manuals. I rekon big resistors that can generate a lot of heat would get this warning from Ring.

Voltage drop is the most probable reason for the "short" bus length that Ring advises as the PWR-56 can only take 16AWG wire. I do wish the PWR-56 had bigger terminals; 14 would be better and 12 even more so.

Joe, The PWR-56 manual give a bus length of 30 feet. That can translate into 60 feet (30 feet each way) if your supply is in the middle of the 60 feet but you'd need to make a splitter to do that. Alan might be able to give a better idea of the most practical length you could go to before ringing might become a problem that needs treatment.

- Tim

melarson:

--- Quote from: G8B4Life on July 29, 2016, 09:35:43 AM ---Voltage drop is the most probable reason for the "short" bus length that Ring advises as the PWR-56 can only take 16AWG wire. I do wish the PWR-56 had bigger terminals; 14 would be better and 12 even more so.

--- End quote ---

I'm sure I'm not the first to think of this, but you can come out of the PWR-56 with a short length (1, 2, 3 inches, whatever you deem necessary) of 16AWG and then either solder it to your 12AWG bus or connect it to a conventional terminal block with your 12AWG bus on the other side.

BTW, if 30 feet is the recommended length for 16AWG, then simple math will show that your 12AWG bus can be approximately 2-1/2 times longer (right close to 76 feet) for the same voltage drop.  For large layouts this would seem very beneficial.

(Length of 12AWG bus derived from the ohms per thousand feet rating of both wire sizes.)

Alan:

--- Quote ---So what would be considered a long bus run?
--- End quote ---

That is the $64,000 question for which you will never receive a 100% reliable answer. There are too many variables for one answer to be the right answer everywhere everytime. However, rest assured everyone will ask it!  ;D

Because people will ask, manufacturers like Ring attempt to give a single answer that will work 99.99% of the time. That makes the answers very conservative. Look at Bill's situation for instance. Ring says 30' Bill has 60' without problems. Shucks, I have 150' and it works fine. Abiding by artificially low recommendations may place an unnecessary constraint on your construction or dramatically increase your cost. Throwing caution to the wind by totally disregarding electrical design is equally dangerous. At the end of the day, the physical arrangement of your layout space, your appetite for wiring, and your wallet will force certain electrical realities upon you. Fortunately, RP seems to be extremely forgiving. That is a good thing. (to the DCC guys we say neener-neener)

Rather than a single answer it may be better to simply use best practices and let the chips fall where they may in any one situation.

Before I go on I want to say this again - we are talking about large layouts. None of this matters a lick on a smaller railroad. For sake of argument only, let's say a large layout is one where the wire between the power supply and the track is longer than Ring's recommended 30'. And we are talking RP, not DCC.

This much we know for sure - our power systems are effectively this:



with L = wiring inductance
with R = wiring resistance
with C = wiring capacitance
with V = power supply voltage
with A = load on the circuit (train)
Our goal is to minimize L, R, and C while stabilizing V. Our trains dictate A.
We also know the load on the circuit will vary so it is given L & C will always be in flux.
We also know the voltage regulator in our power supply is >30' from the load so assume regulation will be poor at best. This means not only will L & C change constantly, so will short-time V relative to A.

See how complicated the "right answer" to your question quickly becomes?

Off the cuff shot at best practices:


* (LRC) Use the shortest practical wiring length - route wires in most direct path.
* (LRC) Use the shortest practical wiring length - power supply in middle of wire run instead of at the end.
* (LRC) Use isolated multiple power supplies dispersed around the layout if practical.
* (R) Use the shortest practical effective wiring length - if using one power supply position it electrically central to the layout.
* (R) Use largest practical wire gauge - effective upper limit of 14-12ga as they are the largest readily available economical copper wire sizes.
* (R) Use a lot of short feeders - short <12" small gauge 24-20 to every rail is bulletproof while not being obtrusive trackside.
* (R) Use a feeder for every 3-6' of contiguous rail - refer to ohm's law and the rail resistance of your specific brand track.
* (R) Make high reliability low resistance electrical connections throughout.
* (LC) Avoid parallel wires - Twist bus wires a turn per 1-2'.
* (C) Don't deliberately add capacitance - avoid twisting wires into candy cane tightness.
* (L) Don't deliberately add inductance - avoid small circular wire bundles.
* (C) Limit the charge time of keep alive circuits.
* (V) Possibly add a ballast load at mid-point furthest from power supply - big honkin' resistors.  ;D
* (V) Possibly use a power supply(s) with remote sensing.
* (R) The rails are equally part of the power system equation - solder joiners.
The are no doubt many more. My head hurts.

Josephbw:
At this point, I probably only have about 100' of track laid with only 6 pairs of track feeders. I am using 12 ga bus wire w/18 ga feeder wire, and I make sure all my track joints are electrically sound. I can take voltage measurements anywhere on the layout and get the same reading.

I only have about 1/4 of the potential track laid, but it may be 2-5 years before I am ready for operating sessions.

So much fun, so little time.  ;D

Joe

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