I have a question regarding switching track polarity for return loops in a DCC layout. I know that you need to switch polarity or there will be a short circuit when the wheels cross the gaps, just like for DC operation. I understand that it’s okay to switch the polarity of the track under a moving locomotive, i.e. as it’s travelling around the loop, as DCC doesn’t depend on track polarity, and that this doesn’t hurt the locomotive. I am told that DCC operated polarity changers do it so fast that the receiver doesn’t see any interruption. However, what about using a toggle switch or relay to change the polarity? There would be an interruption of a few milliseconds as the contacts transfer over. Is this going to bother the receiver? I need to do the polarity change manually, so if a switch or relay would upset the receiver, is there such a thing as a DCC switch which can take a toggle switch input as well? Pardon me if my terminology isn’t right as I’m not very familiar with DCC.
Seamonster,
Why do you need to reverse polarity in a manual method? The auto-reversers out on the market are as easy as pie to install and take all of the worries out of the equation. Perhaps you need to expand your reasoning behind your question so forum readers can provide the information you’re seeking.
Don Z.
Bob,
I guess the first question I would have to ask is: Why do you need to manually reverse polarity rather than letting an auto-switch do it for you?
Tom
Yes, you can just use a toggle switch, but it’s not the best option. Somewhere in each decoder is some sort of capacitive storage that allows it to “coast” for a short while while these flips happen. I’d imagine that it’s different for each decoder.
I’ve got a couple of turnout/locomotive combinations that cause momentary “stalls” as the engine passes over. In one case, I think I’ve got a bad contact that I need to look into between wheels and trucks, and in the other there’s a traction tire I can’t do much about. If the engine hits the turnout slow enough, it may stall out completely. Fast enough, and I barely notice it. In between, there’s that gray zone where I hear the whole sound sequence reset, and, since I’ve set the acceleration and deceleration parameters in CV’s 3 and 4, the engine will slowly regain speed from zero. If your toggle is too slow, that’s what you’re going to see.
I’d give it a try. If you’re happy with the performance with a toggle switch, fine. If not, then spring for a reverser and never think about it again. I’ve got a couple of Tony’s PS-Rev units, by the way, and I think they were a great investment. Not only do they reverse the polarity, but they also provide localized circuit-breaker protection for the track in question, so a short in your reverse loop won’t shut down your main line.
Drop-outs of a few milliseconds are not problematic, but you will take longer. If I can go by my newest SW8 that kept shutting down and cycling on again as it traversed joints in the rails due to rubber traction tires preventing some of the needed contact, even short gaps in power can cause decoders to cycle. When my Walthers indexed turntable gets to the split in the power rings, sometimes my QS1 decoders cycle, sometimes they don’t skip a beat.
I have a Tony’s Train Eschange PSX-AR, the new version of the old PS-Rev. It is seamless. You can’t possibly detect when it reverses the circuit to prevent a short. Not even a light flicker.
You need a toggle switch or an automatic reversing module. With DCC you have to switch the phase, not the polarity, and the auto reverse can do it automatically without needing to stop. With a toggle switch, it is still best to stop the train as you change over to avoid damaging the decoder because you cannot change the phase manually anywhere near as fast as the automatic module.
Auto reverse modules are available from MRC and Tony’s Train Exchange. The MRC module uses a mechanical relay and Tony’s is electronic.
A toggle switch will work fine, and there’s no need to stop. With the toggle switch you don’t need to wait until you are crossing the gap to change the polarity (we can ague about whether it is polarity or phase, but I really don’t want to, we all understand polarity, and for practical purposes that’s good enough), just change it any time while you are in the reversing section (or change the main anytime you are in the reversing section for that matter). The only thing that happen if you cross the gap with the tracks not matching is a short, which should be detected by the system, which should shut down. The decoder should not be damaged by this, but it will be annoying.
Just to confirm,
A Toggle (DPDT) switch will work fine with DCC and a reversing section. I started out with one, then later, replaced it with a Auto Reversing Module from MRC.
I used a DPDT toggle switch for a few months until I had more of the layout built and my PM42 was ready and installed.
I wanted to run trains and the old toggle switch worked great. I would just flip the switch quickly when the train was completely inside the reverse loop. The train would just keep running.
I had the toggle mounted on a board so I could keep moving it as I completed track in different reversing areas.
BOB H – Clarion, PA
A toggle will indeed be fine. And unlike DC, you connect the toggle to the loop itself, and can flip it at any time the train is in the loop, without stopping first. The ‘polarity’ of the signal on the rails does not control the direction the loco travels in. If the decoder is commanded to go forward, it will go forward no matter what. If you want to demonstrate this, start a loco moving forward, then disconenct and flip the track wires at your command station and notice the loco will still go the same direction.
An auto-reverse device is better in the long run, if even to eliminate the task of flipping the toggle. Like DC, if you forget to throw the toggle switch, there will be a short either entering or leaving the loop, and the train will stop. With an auto-reverser, all you need to worry about is making sure the turnout it set correctly - and some of them can even operate the turnout for you!
–Randy
Thanks for all the replies. It looks like it won’t hurt a receiver to have the polarity reversed by a relay.
I guess I over-simplified my question when I posted it by using the term “manual.” I have a perfectly good auto-reversing unit designed for DC operation which reverses track polarity with a relay when triggered by an IR sensor near the exit of the loop. Of course with DC, you have to reverse the mainline polarity, but I want to use it with DCC and reverse the loop polarity as the train goes around the loop. I was concerned that the few milliseconds that it takes for the relay contacts to move would upset a receiver but it sounds like that won’t be the case. Thanks again, everyone.
Actually, many of the auto-reversers on the market are relays. Tony’s PS-REV is solid-state, but others use relays to flip the polarity. I’m sure yours will be fine.