RailPro and DCC discussion on Model Train Forum

[Homeless by Choice]

The topic of RailPro and DCC systems was revisited yesterday, Feb 17, 2017, on the Model Train Forum (see link below and goto page one).  Some of you may find it interesting and may care to comment.  This particular thread was started back on Jul 25, 2016 and that was when I became aware of RP.  Then I found this Forum and the rest is history.

If you are not allowed to post a reply/comment on the forum, you could email me what you would like posted and I can copy it as a reply stating your name or whatever direction you specify.

http://www.modeltrainforum.com/showthread.php?t=93649

One confusing point that is of no consequence to me is that one person called RailPro a 14v DC track system and another called it a 14v AC system.  Which is it?

LeRoy

[William Brillinger]

RailPro will run on 12 to 18V DC or AC, but the RailPro power supplies are 14V DC.

[Homeless by Choice]

RailPro will run on 12 to 18V DC or AC, but the RailPro power supplies are 14V DC.

Bill,
I have been thinking about this ever since I made the initial post and came to the conclusion that RP had to be DC because a reverse loop device is needed for loops.  I answered my own question, came back here to post that conclusion and found your reply.  It would be really nice if the track voltage was AC so reverse loop devices weren't necessary.  I have a very limited understanding of electronics and have no idea how an AC radio powered control system could be designed thereby eliminating the reverse loop requirement.

Thank you,
LeRoy

[William Brillinger]

I have a very limited understanding of electronics and have no idea how an AC radio powered control system could be designed thereby eliminating the reverse loop requirement.

Alan?, I'll let you explain...

[melarson]

RailPro will run on 12 to 18V DC or AC, but the RailPro power supplies are 14V DC.

I have been searching all morning on this because I have a recollection that Ring specifically states not to use AC, but I can not find it.  It might have been in an email or phone call with Tim.  Either way, I can think of two reasons using AC would be unadvised and detrimental.

AC sine wave voltage levels are stated in terms of Root-Mean-Square (RMS), which is the equivalent ability of the same level of DC voltage (12VAC and 12vdc can perform the same amount of work).  But the peak voltage of an AC sine wave (the instantaneous max and min of the sine wave) is the RMS value times the square root of 2.  For a transformer whose secondary output is 12.6VAC, the peak voltage would be 12.6 x 1.414, or 17.8 volts.  When rectified and well filtered, this is the raw, unregulated DC voltage that results.  As you can see, 17.8vdc is very close to the upper end of what is recommended to power an LM.  Saying that you can apply 12 to 18 volts DC or AC would lead the unsuspecting to think that 18VAC would be OK.  It really isn't.  Applying 18VAC to an LM would result in an internal DC level of 25.4vdc (sort of), well above the recommendation.  Now, I say "sort of" because this would be the level of a well filtered signal.  However, it takes fairly large capacitors to filter 60Hz AC to a nice smooth DC level; too big to fit into an LM.  So the second problem is the LM would have an inordinately large amount of ripple on it's internal DC supply.  Not desirable.

And then there is the question of whether the AR-1 would even work on AC.  My educated guess is it wouldn't, but Tim would be the better person to ask that.  I don't have a schematic to look at to determine the answer definitively.

It would be really nice if the track voltage was AC so reverse loop devices weren't necessary.

In a 2-rail situation, using AC or DC wouldn't matter.  There would still be a polarity (DC), or phase (AC), mismatch at the gaps, leading to dead short circuits when the gaps are bridged by your engine's wheels.  In both cases you would need to account for this.  The only way AC wouldn't also need a reverser is in a 3-rail system like Lionel or Marklin, where there is a center rail for one feed and the two outside rails for the other feed, and no matter how you set up your track, they never meet.  BTW, this would work for DC as well, but who wants to run center rail?   

Michael

[William Brillinger]

And then there is the question of whether the AR-1 would even work on AC.

The answer is no. The RailPro AR's and such are only designed to work with a DC power supply, such as the PWR-56.

It is my understanding that many DCC systems are AC on the track, which is why I said the LM's are ok on AC or DC. The LM Manual does not specify DC that I can find.

[Alan]

But the peak voltage of an AC sine wave (the instantaneous max and min of the sine wave) is the RMS value times the square root of 2.

Assuming when we say AC we are referring to the AC wave of a DCC system...

You are speaking to sinusoidal wave AC. DCC is square wave variable frequency AC centered around 10kHz. Root mean square still applies just that you cannot use the sine of the wave. The reciprocal of the frequency ¹/_f is used to calculate RMS voltage of square wave AC. Since a DCC signal is variable frequency (long pulses / short pulses) the effective DC RMS voltage varies constantly however it tends to be very close to the peak voltage because of DCC signal peak dwell. Within the context of RailPro it doesn't matter. The high frequency DCC power is easily rectified and filtered internally by the LM.

With respect to 50Hz or 60Hz house current your explanation is spot on. I surely hope no one is trying to run their train track on such. They won't for very long!

[Alan]

thereby eliminating the reverse loop requirement

As stated earlier, as long as we have only two rails reverse loop requirements will be with us. Reverse loops and frogs - one of the few advantages of battery power.

[Alan]

There may be some confusion about nomenclature. The model railroading community means old style variable throttle pack when they say DC. With RailPro we mean steady state voltage when we say DC.

Michael's AC explanation is applicable to throttle pack DC output since those units have no filtering or regulation. Even though the rheostat (throttle lever) varies the voltage/current, it does not smooth out the highly rippled waveform that is produced by the transformer inside the throttle pack. This works for DC motors because they don't need a filtered supply. The mass and momentum of the armature provides all the smoothing needed. Precise voltages (peak or RMS) are almost useless specs because the user simply moves the throttle until the train is moving at the desired speed i.e. getting the required voltage. If a second locomotive is placed on the same track it will reduce the speed of the first locomotive because the supply voltage is not regulated, it is throttled. Voltage varies with throttle position and load on a throttle pack DC layout.

For RailPro and DCC we are not directly powering motors. We are sending power to electronics that in turn power motors. The electronics require clean, steady state DC voltage. A RP layout has this available on the rails, a DCC layout has high frequency on the rails which is smoothed to steady state with small capacitors inside the LM. Additionally, with RP or DCC we expect one locomotive to not affect the operation of another locomotive. Therefore, the supply voltage must remain constant as load is varied. That is why both RP and DCC use regulated power supplies. Voltage does not vary with load.

The model railroad world generalizes into 3 categories: DC, DCC, and RailPro (or alt). All 3 use DC. The difference is what the DC waveform looks like. I whipped up an illustration to help:

[melarson]

I'm getting the sense that because DCC is an alternating current signal, we tend to use the terms DCC and AC interchangeably.  But electrically they are two totally different animals, as Alan has clearly shown.

Actually, all I wanted to do (in my typically long-winded way) was to keep people from using [traditional] AC on the rails.  We suggest to people that they can give RailPro a try with minimal investment by purchasing a CI-1 and one or more LMs.  They can use a power supply they probably already have on hand to power the rails.  So they look in their train closet and grab, for example, that DCC Specialties MF-615 power supply they used with their Digitrax command station.  They see its output is 15VAC, they compare it to the guideline they saw (12 to 18 volts DC or AC), see a match and off they go, never suspecting that it could be a problem.

Would it not be better to tell people that they can use 12 to 18 volts DC or DCC.  It's a small, but important, difference, and Ring's specification.  Hopefully that would alleviate any concerns about the compatibility of a particular power supply and keep people's LMs from over-heating, operating erratically, or possibly failing.

Michael