Yes, you change the loop polarity. What do you think the PSX-AR does? With DCC, you change the polarity under the running train, NOT the main line like you do with DC. Polarity of the DCC signal has no bearing on the direction of the train. Forward is forward, reverse is reverse - relative only to the loco, not to the track power. This is also why autoreversers don’t work for DC - they change the loop polarity, so the train would approach the mismatched gaps, short - the reverse would trip, and the train would instantly move the opposite direction.
You CAN still do it the old way, and reverse the rest of the layout, but it’s pretty pointless - plus you need to switch potentially a lot more current if you reverse the entire rest of the layout - on a big layout you could have dozens of trains running. The loop, especially a simple one, can only hold one train, maybe two.
Yeah, you do. Whatever the polarity was as the train entered the reverse loop, the polarity needs to be reversed upon exiting the loop to avoid a dead short. The Tortoise has two SPDT switches, one of which can be used to reverse polarity.
Well, you need both to actually change polarity - but one set is enough to trigger a relay to do the actual reversing. The Tortoise contacts tend to not like switching high currents, plus they can make conact with the opposite side before they have physically move the points off the prior stock rail. And there is a bit of a dead spot in the middle, which can cause a stutter, or a sound dropout.
Well, yes, I understand the DPDT contacts on the Tortoise. I was wondering what Randy was refering to when he first mentioned using them. I mean, you have the Tortoise that operates the points on the main loop switch.
So, you enter the loop depending on that switch set for, lets say the diverging left hand route. Rail A is happy with + and + on both sides of the gap and rail B is happy with - and - on both sides of the gap. The train continues forward.
The end of the train clears the switch and you have to throw it to the straight route. As the points move the Tortoise contacts change rail A to - and rail B to +
So what happens to the train as this polarity changes on the rails under the engines’ wheels?
That’s the part I’m a little corn fused about…
The polarity you really want to change is on the remainder of the layout AS the train is within the loop. At least that’s the way it used to be done back in DC days.
Maybe rail A and B can be quickly swapped and the decoder won’t know the difference?
I’m just askin’
One of my reversing sections is part of an engine treminal and I have a dozen or so engines sitting there. When I move through there the PSX-AR switches and I see a quick blink of the engine lights but I’ve never had any of them moving at the time.
Nothing happens to the train. It keeps moving as the polarity changes on the rails that it is riding on. As the polarity changes, each rail simultaneously acquires a changed polarity.
The reverser is making changes that a DC person would call reversing polarity, but the decoder doesn’t see it that way. In fact, the decoder wouldn’t care if the reverser kept changing polarity every second under the locomotive, provided the right reversal takes place while a metal tire is bridging a gap. That bridging is when the short takes place due to the conflict in phase between two meeting rails.
The reverser senses the conflict. The conflict might be as the train enters the loop, but it might be when it leaves the loop…depends on what the reverser did to the rails last time it switched them. It doesn’t switch unless there is a conflict, and that takes place wherever the conflict is. So, if a train entering the loop does not cause a short/conflict, the PSX senses no fault. The train trundles around the loop, but momentarily causes the short at the far end gaps, whereupon the PSX reverses the rails.
Later, a train comes along and enters the loop in the reverse direction as the previous train. The PSX corrected the conflict at that time between those rails, so there will be no conflict in the reverse direction for this second train. However, there must be the inevitable conflict when the new train crosses the far side gaps because there is where a conflict must be, logically.
Yup, absolutely nothing. You can flip the polarity back and forth under the running train all you want and nothing happens. Go ahead and try it - hook a piece of flex track to your DCC system with clip leads. Start the train going forward. Unhook the wires (don;t touch the throttle or anything). Clip them back on the opposite way. As soon as power is applied, the train will start moving again - in the same physical direction as before. If it was moving left to right, it will still move left to right.
This is one of the keys to understanding DCC. It is the commands sent to the decoder that tell it what to do, not the polarity of the track. In DC, direction is controlled by the track polarity - which is why if you take two F units and put them on DC track back to back, they work together and move the same direction - one running ‘backwards’ as in, away from the cab. However, if you put two (independent, let’s ignore consisting and so forth) F units on DCC track, back to back, and tell them both to fo in the forward direction, they will move apart, away from each other, becuse ‘forward’ is the direction of the cab. Unless wired incorrectly (there is an NMRA standard for DC loco wiring), you can;t have a cornfield meet with DC locos, they’ll never run towards each other (in the same block). But since the polarity of the track power means nothing in DCC, it’s quite easy to play Gomez Addams and smash trains together if you don’t pay attention.
It greatly simplieifres reverse loops, since you can change the polarity under the moving train. No more synchronizing the cab and mainline reversing switches to keep the train moving smoothly. Once the entire train, or at least all power pickups (locos, lighted cars) and metal wheels are past the gaps, you can flip the polarity under the running train without a blink and run it out the exit of the loop with no hesitation (obviously you need to line the switch).
Depends on how long the loop is. In my case, it’s long enough and the trains are short enough that I can stack up to six trains, three on each track. But this is a case where if you are switching power with the Tortoise you probably want a relay to handle the actual switching to avoid hi currents across the Tortoise contacts. Many of my narrowgauge locos have sound, so there could be more draw than just the loco crossing the gap imposes. I think this is also where using the PSX-AR may be just as easy and almost as cheap as trying to use the Tortoise contacts.
I would (and plan to) use a relay in all cases, just to make the switching faster and avoid any dropouts. $3.38 on ebay gets me TWO DPDT relays with 12V coils that draw less than 40ma, and have contacts rated at 10A ($1.38 for the relays and $2 shipping). So less than $2 per reverse loop. And a PSX-AR is how much?
There may be complex situations where it’s easier to just let some fancy device figure it out, but I prefer to avoid the short, not correct it. Same with frogs. EASIEST way to power frogs is with something like the Tam Valley Frog Juicer. DCC in, and hook a wire to the frog. It figures everything else out. But at $80 for 6 frogs. The relay they sell for the various servo controllers is $32 for 8. $4 per frog vs $13+. OK, I have to figure out the proper polarity for the bus wires to connect to the relay. It’s a 50/50 shot. and they do use screw terminals, so if it’s bckwards the first time, it only takes a few seconds to swap. Depending on how I mount the servos, it may be even cheaper to just use a microswitch, but $4 per is pretty cheap.
PSX-ARs were ~$40ish when I bought mine but it’s been a few years.
Me, too. But electricity and plumbing are instances when I often find the technical solution saves enough time over the DIYing to make buying out the frustration factor and the cost of my time level the playing field. In this case, the numbers may not quite support that conclusion, but close enough for me.
Also, what happens if you accidentally cross the gaps with contact-driven reversing? I suppose whatever circuitbreaker it’s on shuts it down? But could also sit and fry a bit if the quarter test wasn’t applied to verify wiring suitability. Same thing applies to the auto-reverser of course, but once again it depends on your comfort level with the particular solution.
I do wonder about the contact driven reversing not suffering from the small dead spot in the middle of the two sets of contacts on the Tortoise. Doesn’t that carry over to the relay, too? I’ve never noticed any such stutter from my PSX-AR sections, so to me it seems sorta a win for the PSX-AR on that, although no direct experience with the contact-driven method to hear what happens with sound locos passing through it.
The whole premise of DCC is to automate all this stuff and relieve us from all the DIY tedium - “operate the trains, not the track”. Your scenario of using the tortoise contacts to reverse the loop still leaves you with the job of having to manually operate the switch. If you get distracted, you get a real short that takes out a hunk of the layout. My reverse loops, being quite unprototypical for a mainline, are hidden. When I send a train into the loop, I just want it to turn around and come back. I’ll gladly pay the $54 for the PSX-AR for that convenience.
Just because it’s possible to DIY it doesn’t mean that you should. How much other progress on the layout is not happening while you design, shop, assemble, test and debug these little inventions? It’s all about the value of your time and the aspects of the hobby you find interesting. Do you want to be a model railroader or an electronics tinkerer? Either choice is fine, but I’d rather spend the cash and have the time to do something I find more interesting.
Besides, having something that has been a pain to deal with since the invention of model railroads work without intervention is really cool! [:)]
Just for the record, my Chama staging loop still uses the manual turnout switches. I know I could use the PSX-AR to automate the turnouts, but I prefer manual control. I can monitor everything via CCTV, so I can throw them manually just like I do elsewhere on visible track.
Ultimately, for control systems for hidden track, you should choose the one that works best for your particular application. There is no single solution that fits all circumstances, so knowing your options in a case like this is always good.
You have to throw the switch ANYWAY to get the train out of the loop - otherwise you will derail on the misaligned points! There’s no more manual operation involved than there is if you hook up an autoreverser.
As for doprouts - no, that’s another reason to have the contacts operate a relay. As soon as the Tortoise contacts leave the section of the circuit board inside that is connected to the terminal energizing the relay, it will drop out and chance the phase. Even before the contact crosses the dead spot and touches the other side. And when throwing the switch the other way, the relay will not flip until the contacts touch inside the Tortoise, after they cross the dead spot. In either case, instant contact change from one state to another, with no delays.
As for goofing up and forgetting - yes, it will short, and you’ll also likely be on the ground. That’s why you have power districts, and if you aren’t making your wiring robust enough for the quarter test to work, an autoreverser isn’t going to work reliably either.
This is pretty simple stuff, no more complex than the fancy electronic solution. Now, if you want a complex solution to a simple problem - that would be like wiring the layout for MZL with DC, so you can just drive the train and not the track. All thouse relays and contacts just so you cna concetrate on the throttle only - connecting a DCC system, even with feeders every 3 feet, is much simpler. The advantage of the relay, both for loops and frogs (or just the contacts, in the case of frogs - it’s not like 10 locos can all be sitting on one frog, creating a need for high current contacts) is that in both cases you eliminate the short, not just correct it. An autoreverser waits for the short, then tries to fix it by switching phase. Hopefully it does, otherwise, time for the circuit breaker to work. And so do the Frog Juicers - they wait until a wheel touches the frog, it if’s a short, it flips the frog phase in hope
All of which makes me wonder how the OP plans to use the loop with the siding inside of it.
Since it is a single track coming into the loop, an exiting train is obviously headed in the other direction. What does the rest of the layout look like?
Will an entering train move into the siding so that another train may pass through, perhaps from the other side of the controlling turnout?
I’m not disparging it, I’m just saying I can do darn near the same thing for a $1 relay and NEVER get a short instead of using a $70 device that waits until there is a short and fixes it. And I really wish they would quit claiming they have feedback, they do NOT, you need some sort of input device for your DCC system to get feedback - none of the PSX devices have Loconet or ExpressNet or NCE Cab Bus.
I know all about Tony’s products and how they are SO superior to the Digitrax relay devices (gets posted here ALL THE TIME) yet my PM42 breaker works JUST FINE with sound locos - including articifically loading up one section with ALL of my sound locos at the same time and deliberately shorting it. And it connects to Loconet so I can control each section manually and/or view the status even if the board is up under the layout and I can’t see the LEDs.
On a complex piece of trackage, where trying to figure out all the possible combinations of some puzzle trackwork like a multiple double slips - forget about it, use the electronic device. For a simple loop or wye, totally not needed.
In fact it is not a ‘‘loop with the siding inside of it’’. It is a double track siding located at the North end of a End to End layout that I use as a reversing loop. Either track can be used as the reversing loop.
As it is a End to End layout, no train is allowed to go thrue the loop without stopping. I only use the reversing loop to reverse the trains between operating sessions. So the need of the PSX-AR is not mandatory. As noted by Randy, the use of the turtoise to change polarity is all I really need as the trains are still when I change polarity. The PSX-AR would just be a more convenient way to manage the traffic through the loop, I think.