Just curious...
Since I'll be incorporating two auto-reversers in my layout, I've been reading with interest the recent postings on these. Especially the situation where metal wheels will potentially cause a short as they bridge the gap between two track sections; leading to the advice to keep the train length shorter than the reversing section.
I have two boosters in my layout dividing it into different power districts. Obviously the same short must occur when metal wheels bridge the gap between the districts. A locomotive with pickups in each wheelset will be picking up power from both districts as it traverese the gap.
What happens? There aren't polarity indications on the boosters. I wired them identically to their power buses; left terminal to red wire, right terminal to white wire. What if they are reversed?
I don't see why a power district should be polarized in such a way as to conflict at gaps wth metal wheels, unless you are running DC? Then it would be a reversible block to get the correct term. If you are running DCC and power two or more districts, everything should be in phase. It is only when there is a true reverse that you need a switch or an auto-reverser.
Or am I not following?
I use metal wheels on everything and have multiple power districts and autoreversers installed. I don't have an issue of metal wheels causing a short as they bridge the gap.
The warning about autoreversing is as you have written "keep the train length shorter than the reversing section". By train this really means the part of the train drawing track power - engines and lighted passenger cars. This warning is so that the train is only on the "normal" section and the reversed section at one time. This is what the autoreverser is made to solve. But if your power-drawing portion of the train is longer than the reversing section, the train could stretch over "normal" track, the reversing section, and back to "normal" track all at the same time. The autoreverser can't respond to multiple needs to reverse power simultanesously so it shorts. Metal wheels are not the issue.
The same problem doesn't occur with boosters because engines won't run from one district to another if the polarity is reversed - they will stop at the boundary. If engine pickup wheels span both sections, you will get a short. If you push the engine beyond the short without rolling down the throttle, it should run again, but now in reverse so it will immediately short again when it backs across the gap. The solution is simple - just to reverse the Track A and Track B wires into the booster. Again, metal wheels aren't the issue.
Mat
ocrr wrote: Since I'll be incorporating two auto-reversers in my layout, I've been reading with interest the recent postings on these. Especially the situation where metal wheels will potentially cause a short as they bridge the gap between two track sections; leading to the advice to keep the train length shorter than the reversing section. The warning about autoreversing is as you have written "keep the train length shorter than the reversing section". By train this really means the part of the train drawing track power - engines and lighted passenger cars. This warning is so that the train is only on the "normal" section and the reversed section at one time. This is what the autoreverser is made to solve. But if your power-drawing portion of the train is longer than the reversing section, the train could stretch over "normal" track, the reversing section, and back to "normal" track all at the same time. The autoreverser can't respond to multiple needs to reverse power simultanesously so it shorts. Metal wheels are not the issue.The same problem doesn't occur with boosters because engines won't run from one district to another if the polarity is reversed - they will stop at the boundary. If engine pickup wheels span both sections, you will get a short. If you push the engine beyond the short without rolling down the throttle, it should run again, but now in reverse so it will immediately short again when it backs across the gap. The solution is simple - just to reverse the Track A and Track B wires into the booster. Again, metal wheels aren't the issue.Mat
I think you are misunderstanding what is happening.
With DCC, direction is reversed inside the decoder, not by the "polarity" on the track (I'll use the term polarity even though it's not strictly accurate for the complex DCC track signal).
When a reversing section exists, on either side of one set of the gaps both rails will be of opposite polarity to their mate on the other side of the gap. You will have not only a "+/-" across the track (required for a train to be powered), but also a +/- at each rail gap. Just as a piece of metal bridging the two rails on normal track causes a short circuit, so a metal wheel bridging the gap of a reversing section causes a short circuit, too. That is why we have reversing section gaps in the first place, DC or DCC.
The autoreverser senses the short circuit and very quickly swaps the polarity of the reversing section so that the reversing section and the main match polairty across the gap. Since the polarity swap does not change the direction of a DCC locomotive, all is now well.
Note that the powering of the train has nothing to with the functioning of the autoreverser. It is in fact the metal wheels crossing the rail gap that causes the autoreverser to function. If metal wheels or a locomotive or car that links wheel sets are spanning both sets of reversing section gaps simultaneously, the autoreverser gets confused.
back to the OP....
A booster simply provides extra power for the command station. Think of a booster as a power amplifier - it only adds power to the signal fed to it. A single command station generates all the DCC signals for the entire layout. The single command station is linked to each and every booster, which in turn provides the power to the power district. Metal wheels crossing the power district boundaries do allow power from the boosters to temporarily "merge". This is not a problem as long as both boosters are of the same polarity.
If you reverse one booster, you will set up a short circuit when you span the power district gaps. Which is why common rail is not recommended for DCC. By only gapping one rail between power districts, you create a permanent short circuit should the polarity of one booster somehow get reversed. Hence, the conservative and manufacturer recommendations are to gap both rails between power districts.
yours in wiring
Fred W
The booster should just supply power and signals to the section it is hooked up to. All commands come from the command station. It is possible the booster is out of phase with the other booster/command station. Try swapping Rail A and Rail B at the booster, to see if it clears up.
We had a similar problem at the club, when we installed some PM42s. The layout was DC, and we hooked up each portion on a PM42 to an existing block. When going over the block boundaries, sometimes things weren't happy. Some wire swapping cleared things up.
Mike WSOR engineer | HO scale since 1988 | Visit our club www.WCGandyDancers.com
fwright wrote:Metal wheels crossing the power district boundaries do allow power from the boosters to temporarily "merge". This is not a problem as long as both boosters are of the same polarity. If you reverse one booster, you will set up a short circuit when you span the power district gaps.
Metal wheels crossing the power district boundaries do allow power from the boosters to temporarily "merge". This is not a problem as long as both boosters are of the same polarity.
If you reverse one booster, you will set up a short circuit when you span the power district gaps.
Thanks for taking the time to satisfy my curiosity. It seems that I was lucky in wiring the second booster. Defying Murphy, I managed to make the correct choice for the track connections.