I’ve been working on some handlaid turnouts - just getting the mechanical issues down before wiring them, etc.
How wide to gaps have to be where the polarity changes? (this applies to any polarity difference situation - like a return loop for example).
For example, say a gap is .050" wide. As the wheel crosses the gap, it will short the rails on either side of the gap, right?
So is it best to have a really wide gap, then fill it with styrene? It seems if you don’t fill the gap, the wheel will slightly drop until it spans the gap, and short.
With DC, you need to change the polarity of the loop BEFORE the loco enters it or you will get a short.
With DCC, all you need is an autoreversing unit. The gap should be as small as possible, because you WANT the short to be a quick one (to trip the aru)
I found some very small gaps in my flextrack only when the locos found them. They would stop dead and the throttle display would read 00 for speed. These were not polarity problems, just a thin gap that broke continuity. If it works for something like power districts, it will work for polarity’s sake.
To keep tracks separated for the sake of altering polarity, the first thing is to gap the rails for a suitable distance representing the longest train you’ll want in it. Gap each end, and the gap could be as little as 1/1000" (0.001), provided rail expansion never becomes an issue, or rail creep due to poor fixing on its roadbed. I would say that 1/32" is lots, and you could plug that with styrene if you wish.
You will only get the premature short that you worry about in your question if the rails on the far side of the gap are not matched in polarity to those from which the locomotive motor and decoder are currently drawing power. So, match them, make sure the entire metal axle component of your train is in between the gaps that you cut at each end of the switched section, and then match the power in that section of gapped rail to what the locomotive is about to encounter beyond the next gap.
Polarity never changes where the locomotive is – it changes before the locomotive reaches it. If you are not using insulated rail joiners, are you cutting the rail after you have it in place? If so, the thickness of a Dremel disk is enough gap. But you still need to change the polarity before the locomotive reached the joint. Say you have a 16" section of track insulated for a return loop. Polarity matches the track where the locomotive enters it. You switch polarity AND direction [at the same time] before you reach the point where it enters the next section of track. This is for DC operation; I know nothing about DCC operation.
Also, the insulated section should be as long as the longest train you will run IF they are collecting electricity from the rails [for example, for lighting units].
how do switch the polarity ? Why would you switch it? as you have allready figure out I am new to this hobby. I bought most of my stock years ago but due to health problems had to put it on hold. Now wiyh my grandson I am starting it up again.Thanks for the help.
Pat from n.c.
You’d want to switch polarity if you have a reverse loop. A reverse loop, or balloon track, is created when your trackplan sends a train back in the opposite direction on a section of track, without changing the direction of the train. For example, if the locomotive is still moving forward and you end up going back on the same track but in the opposite direction the train was going when it first traveled that track, this is a reverse loop. Of course, if you had no gaps in the rails, as soon as you applied power to it, the circuit breaker in your power pack would have tripped (hopefully), because you’d have a short circuit.
To reverse the polarity, you insulate both ends of a section and feed power to it with a DPDT switch. Incoming power is attached to the center poles, and outgoing power is attached to the end poles on one end. Then, the end poles are attached also to the other end of the switch with the wires crossed over to reverse polarity. That way, flipping the switch will reverse the polarity to the track. Remember, when you change the polarity [in DC], you must also change direction on the power source for the train to continue in the same direction.
you don’t want to cross the gap when polarity changes. (even with DCC, the concept is the same)
how solid the track is says if you need to stablelize it or not. Fill it with epoxy if needed or whatever suitable method.
In answer to the original question, I use a BUSINESS CARD to set the gap when laying track. I fill the gaps with styrene and CA (usually 0.020" but sometimes 0.040").
I think it depends on the season when track is laid in my NY basement (minimal temperature controls). Sometimes the gap gets wider even though benchwork is painted due to rail exapansion and contraction.
Pat, in DC, if I have it right, the motor’s direction of rotation is dependent on the polarity of the current it receives. So, if you want the loco to move in direction A, provide the polarity for that. If you have a gapped rail section on which the polarity is other than what the approaching loco is currently receiving, the loco will buck to a stop at that point and want to reverse its direction abruptly in keeping with the current change. This is undesirable from the point of view of aesthetics, at the least, but is also hard on your locomotive and your couplers.
In DCC, the decoder wants non-shorted current to deal with, and adjustst to the polarity, regardless of what it is, to orient the locomotive to the “world.” So, no matter what the polarity is to the rails on which it sits, the decoder will change the direction of the loco as you direct it to do. Believe it or not. The decoder does the reversing, not you and the polarity switch on your rectifier, as happens in DC. You input a direction change on your control paddle, yes, but the decoder does the rest.
So, in DCC, when you run a locomotive across two polarities, the controller senses a short, and will cut power to the track immediately…hundredths of a second. It does not want a direction change because you have not asked it to do so. So the altered power is seen as a problem. Therefore, you must either mechanically orient all approaching track to what the locomotive is currently receiving, or you must use one of the new electronic versions that are able to do it seamlessly, so to speak, while the locomotive runs across the gaps at either end of the l
To be safe, the section really should be as long as the whole train, regardless of whether cars are picking up electricity from the rails. Even if most of your cars have plastic wheels, some may be metal (and sooner or later you’ll probably convert to metal wheels). A metal wheel can contact the rail on both sides of the gap momentarily and cause a short. Worse, if the truck sideframes are metal, they may conduct from one axle to the other.
True enough for DC, but if you’re in DCC with an auto-reverse unit, yes, the polarity may change while the train is in the insulated section (actually, it changes when a short occurs just as the train enters or leaves). And if you’re doing it manually, with a DPDT switch, you must change it while the train is in the insulated section.
Any gap that isn’t bridged by something conductive is enough to interrupt the rather insignificant voltage used for powering model locomotives. In the course of normal operation on a DC (not DCC) powered layout the wheels should NEVER bridge a gap between positive and negative, but there’s no problem with bridging a gap between either and neutral (aka dead, disconnected or switched off.)
If the gaps are planned beforehand, they can be kept open with insulated (plastic) rail joiners. If the gaps are cut after the rails are laid, they can be kept open with a sliver of any kind of insulating material, glued into the gap and trimmed to eliminate interference with wheel tracking. Simply leaving them open is an invitation to trouble if there is even the slightest possibility of rail creep.
Chuck (who is planning about a thousand insulating gaps in the new layout)