Subject:
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Re: Reversing Loop without Insulated Tracks
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Newsgroups:
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lugnet.trains
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Date:
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Wed, 11 Aug 2004 04:26:43 GMT
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Viewed:
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1652 times
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In lugnet.trains, David Laswell wrote:
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In lugnet.trains, Ross Crawford wrote:
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Yes, that is correct. The electrical contact is not made by the switching
section of rail. I doubt you would cause any damage to the motors - any
damage would be to the controller. They seem to be pretty tolerant
electrically, but as Lar said, continued shorting, even if momentary, may
eventually cause damage.
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I’m not a big Train-head, so I’m trying to figure out where the problem is
from various bits of this discussion. Is the problem that one pair of wheels
is connected to power flowing in one direction and the other is connected to
power flowing in the other direction, or is it that one pair of wheels is
under power while the other is not? If it’s the former, the short leg
between the A and B points shouldn’t be charged at all until you manually
switch the A point, right? If it’s the latter, maybe you could get away with
setting the B point to curved and building a manually activated gate that
will switch it to straight after the entire motor bogey has passed through
it, thereby cutting the power to both wheel pairs at the same time.
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I suggest you read up on reversing loops for some background. There are plenty
of references out there if you use Google to look. But, failing that, and also
for the benefit of the rest of the readership...
Consider an ordinary reversing loop without the extra switch, first. Visualise
it in your head please. (suppose for the sake of discussion it’s arranged just
as the loop under discussion is, with the input on the left, and the loop on the
right) In this loop you have a continuous run of rail (along the outer rail of
the loop) that starts on one rail of the input (the lower one, say) proceeds
anticlockwise around the loop and ends up at the other rail of the input (the
higher one in that case) That’s a short. Wihthout something to counteract it,
it’s always there.
There’s a short on the inner rails too, but it depends on how the turnout routes
power. If the turnout is branch wired (supplies power to both legs at all times)
it’s bad. If the turnout is power steering (supplies power to just the way it’s
lined and not to the direction in opposition) it can still be bad depending on
how the frog is wired.
The normal way around this is you put an insulating section in. If you somewhere
along the loop insulate both rails, no short. At least not statically. If you
have just one pair of insulators at one place, though, you still have a problem.
Any conducting wheelset will cause a momentary short as it spans the gap (unless
your insulator causes it to rise up, as the most common brand of HO insulators
used to do). But worse, any multiple axle motor with more than one axle pickup
will also cause a short because the motor’s two wheels, connected together,
conduct across the short. That’s a bit longer than momentary, especially if the
motor stalls out and stays put, which it might well do when it loses power.
So the normal way to handle this in regular Model Railroading is to have TWO
breaks. One break is at the start and one is at the end of the loop, and you
arrange affairs so that the polarity never is different across the particular
insulation that you are traversing. You switch polarity of the loop section to
match the incoming train on the direction it’s coming in, and while it’s on the
loop, you switch polarity on the rest of the layout so that it matches on the
way out (really! Atlas even made a special set of switches specifically for this
task, the most elaborate one was called the Atlas Controller and could handle a
double track reversing loop with internal and external crossovers, IIRC)
This innovation under discussion takes advantage of the fact that LEGO turnouts
are power steering (for the inner rails that go through the frog, but not for
the outer rails) and uses a second turnout as, essentially, an insulating
section.
I contend there’s still a short potential when any wheelset traverses B because
at least momentarily, you’re aligning the turnout (although it is spring loaded
so it springs back as soon as the wheels go back) to steer power onto the loop
instead of the dead stub end. That’s a short. The lower rail of turnout A is not
steered.
I also think this is neat, but think that saving the cost of 4 slips of paper
(for insulators) and an extra power lead (which, granted are hard to come by)
and an electrical polarity switch, at the cost of a turnout and the requirement
to stop the train... I think that might be false economy.
I prefer the more conventional approach. Polarity switch controlled reversing
loop segment to handle short avoidance.
But that’s just me. I STILL think it’s a very clever idea, and working through
why it works (despite momentary shorts) is a great way to understand how LEGO
turnouts do power routing.
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Message has 3 Replies:
 | | Re: Reversing Loop without Insulated Tracks
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| (...) Well yes, but not because the short switched track segment is displaced - the electrical routing is not done through that, but via contacts hidden inside the switch, which don't get displaced. The short is caused by the electrical pickup in (...) (20 years ago, 11-Aug-04, to lugnet.trains, FTX)
| | | Re: Reversing Loop without Insulated Tracks
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| In lugnet.trains, Larry Pieniazek wrote: SNIP (...) Hi Folks, For what it's worth, this is the layout w/ reversing loop I was playing with. As always, I'm limited in size to something that fits on the big kitchen table I use for train layouts. I (...) (20 years ago, 11-Aug-04, to lugnet.trains, FTX)
| | | Re: Reversing Loop without Insulated Tracks
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| (...) Eh, I'm more interested in the electrical puzzle than the trains running on it. Besides, I was considering the possibility that a motor might be set up with two input leads and two output leads that would cause skipped poles if only one pair (...) (20 years ago, 11-Aug-04, to lugnet.trains)
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Message is in Reply To:
| | Re: Reversing Loop without Insulated Tracks
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| (...) I'm not a big Train-head, so I'm trying to figure out where the problem is from various bits of this discussion. Is the problem that one pair of wheels is connected to power flowing in one direction and the other is connected to power flowing (...) (20 years ago, 11-Aug-04, to lugnet.trains, FTX)
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