I want to wire a dpdt switch to reverse track polarity on my dcc powered layouts turntable. I have the switch which has 6 connections so I need a diagram (or written explanation) on how to wire the switch for this purpose. If I remember right I need to ‘crosswire’ the connections in the four corners, but I’m not sure where to go from there. Anyone know of a diagram for this?
Thanks,
JaRRell
Take little pieces of wire and wire the end terninals together in a X. left side on one end to right side on other and right side to left.
Put the power into either end and take the power out of the center.
Thanks for the help fellas. For anyone else, here’s the diagram from the NMRA website.
JaRRell
It costs a bit more ($45), but an automatic reverser, such as the PS/Rev from Tony’s, does it automatically – you’ll quickly forget it’s even there! Here’s the scoop: http://www.tonystrains.com/technews/powershield_icb.htm
and in the end it’s probably what I’ll end up doing!
Thanks,
JaRRell
It’s what I ended up doing. I have had the switch for nearly two years now, but relented and installed a PS Rev…actually they’ll send you the new version, a PSX-AR. Believe me, it is silent and lightning fast.
A ‘Reversing Switch’ for DC is a simple DPDT toggle on a section of track - Cost $5.00 - wheras a ‘Automated Reversing Circuit’ is $45.00
I think it is important to know HOW they work: Unlike 3 rail, OUR 2 RAIL models get their direction of travel from POLARITY (+ /-). To go forward or backward one just reverses the polarity.
The simpler, cheaper, DPDT switch controls a section of track a trainlegth (or better) long. This DPDTpolarizes just this track section - so that one sets it for the Direction of travel, and the train enters same, YOU reverse the power pack, for the train to EXIT (reverse) on the track it came from…
Metal wheels crossing a reverse polarity can cause electrical spikes - which is why one wants it longer than the train.
DCC has a higher voltage and current, and Reversing Circuits use these ‘spikes’ to reverse itself. Again, the length of the reversing track should be longer than the train, to avoid multiple spikes. (Cars with plastic wheels are excepted).
How long these circuits can avoid damage depends how good they are. $$
On this post alone one contributor has said “2 years” without problems. Yesterday, a different poster was having 'intermittant problems - sometimes yes, sometimes no. Who knows?
I know that electric arcs eventualy burn out electronic circuits. It’s a question of how much, and how often?
And I thought DCC wiring was supposed to be simpler. Obviously it’s not cheaper.
Besides, I thought polarity of the track wasn’t an issue with DCC, the pulse train coded to the locomotive was supposed to determine the direction the locomotive would run. The loop should just need to be electrically isolated like it would for straight DC.
Tom, polarity doesn’t matter in DCC. The motors are still DC motors, but they get an ersatz DC signal from the decoders, which get a sequential digital signal from the controller to tell it what it is to do with the motor…converted to DC current.
A short is a problem in either case. If you had household 60 cycle 120v current to your rails and threw a pair of pliers across them, you now what would happen. So with a steady 16v of AC to the DCC powered rails, the same problem applies. Polarity is irrelevant…power finding a "short"er path is not good.
You have to remember to use a DPDT, and that means every one of us will miss it at least once in our lifetimes. A well used PSX-AR or equivalent might also miss it once, but that is a chance we take with all electronics. In the meantime, I can run my trains across gaps in my reversing loop with something like 99.999% impunity. If my reverser fails, I also have the taillight bulbs as a failsafe.
Even DPDT’s can fail.
I understand the use of a power reversing DPDT switch in DC wiring for a reverse loop. But what seems contradictory is that you say above that polarity doesn’t matter to DCC, then recommend an automatic polarity reversing unit. If it doesn’t matter, why would you need one?
As I stated above, insulating railjoints as have been traditionally applied to reverse loops (all four rails as you come out of the switch) would electrically isolate it, and it should be preventing a short.
The reason I keep asking these type questions is that my club is making the move
Okay, Tom, I understand…sorry about the mini-lecture. When the locomotive crosses a gap, whether or not it has insulating joiners, the drivers are either going to straddle negative or positive charge in alignment from rail to rail, or they will get a reverse charge, just like if you crossed wires in your wall socket. This only happens where the gap separates the 180 completion of the loop, not at the other side of the loop where it is also gapped. So, it is what I believe Joe and Randy call a “hard” short, and that is just bad news anywhere you have sensitive electronics in the electron path…and that includes expensive decoders. So the Digitrax DB 150 I have senses this, not knowing where it is, just that it is there, and will open its circuit breaker and keep it open until dweeb corrects the fault. It detects the correction, and will happily restore track power automatically…you do nothing but correct the fault wherever it is.
The AR elbows its way into the millisecond line-up in front of the DB 150 before the latter can get to the short detection counter (if you can stand this metaphor), and it switches the two rails so that they are aligned, and you now don’t have the crossed wires of the wall socket…power can safely enter the decoder for distribution to the loco’s innards.
If I am missing your meaning, I apologize, and hope that maybe Randy or someone else will assist both of us with a better explanation.
The DCC supply on the rail suffers from shorts just like DC, and the ends of the reverse loop must be gapped just like DC. The DCC supply is an alternating square wave. At any instant, there will be a voltage of a particular polarity between the two rails. What happens at a reversing gap is that the polarity can be in the opposite direction on the other side of the gaps. For example, with the switch thrown against this gap:
+V gap -V
=============== ================
=============== ================
-V +V
If the loco wheels get across the gap in this situation, +V will short to -V through the loco wheels and frame - just like DC. The only thing special about DCC is that the polarity reverses at the DCC clock frequency, kind of like AC house current.
I don’t have one, but I gather that the auto-reversers detect the short occuring and very rapidly reverse the polarity of the reversing section to eliminate it. If you operate it manually with a DPDT switch (like I do), you need to set the switch for the gap you are going to cross before the loco gets there.
By the way, the magic that happens inside the engine on the decoder circuit takes the alternating DCC square wave, decodes the command signals, and also rectifies and conditions the square wave to create a DC power supply to run the decoder electronics, lamps (or LED’s, which require DC), and motor power.
Hope this helps.
Jim
Me, too. Thanks for chiming in, Jim.
No Selector, you got my question answered. I’ve been an electronics technician for 30+ years, but had no interest in DCC, mainly because of the cost and the fact that the early systems weren’t compatable with each other. In the January issue of Scale Rails/NMRA Bulletin, I finally got to see the waveform that makes up the standard DCC signal. Fortunately, I’m not directly involved in the club’s conversion to DCC, so I’m not doing any of the designs or making any of the decisions. I’m sure that I’m probably going to have to convert some of my locos to DCC to run on the layout in the near future, which is why I’m trying to get an understanding of how it works, in more detail than “plug in the board and hope it works.”
Tom it is easier atleast for me when I started out the only thing that was slightly hard for m was the loconet wiring figured it out in 3 seconds. I ended up buying an AR-1 from digitrax to do the reverese polarity but everything else is simple. Like you said it is expensive but I would not want to do blocks and have a million DPDT switches on the layout I got enough with the Tortoise machines.
Martin