Am I correct (after reading a post saying that rail polarity in DCC is a misnomer, it should be rail phasing)…that there is no such thing as a positive and negative rail in DCC? Is that why my test with an LED and resistor was inconclusive…it lit up no matter which lead was touched to either rail?
I had thought that I could determine which way I had wired the origninal setup with the LED, but I imagine the current is swithing back and forth because of the way the LED lit.
Now, my question is, does it matter which way I connect all the drop feeders to my new expansion on the layout? I had been careful to keep left (-) and right track (+) drop feeders together on the original section of layout. Perhaps now I can just connect any feeder to any controller terminal?
Strictly speaking I guess it isn’t polarity, but you won’t go wrong if you think of it as polarity. At any point in time one of the rails has track voltage (say 12V) and the other 0, then they switch. That’s why your led was always lit (half the time, but that looks about the same as always). But, you still need to keep your feeders consistent, you can call them A and B, rather than + and -, but if A and B short, it is still a short.
You could use your LED to tell you which feeder goes to a rail before you hook it up though. Put one end on the rail, and the other to the loose feeder. If the LED lights that is the wrong feeder for that rail. If it does not light, it is the one you want.
If you get just one feeder crossed your DCC booster will shut down. It is very important to keep your A rail and B rail correctly connected to the power bus. It is not always easy to tell, so I have a simple circuit made from a 9V battery, a buzzer and a pair of alligator clips in series. I connect this across my 2 rails. As I am soldering feeders to the bus under the layout I get an audible alarm if I cross the feeders.
What a cool idea, Simon. I assume this is the same process that lets a 12v auto tail light bulb do its thing when we use it to detect and to manage shorts? I think the buzzer would be better for startling effect.
It is the same in the sense that with a short the current flows through the circuit. Don’t try and use a buzzer in place of the tail light bulb because it would fry as soon as you turned on the DCC system. The little rig I use is simply for alerting me to shorts when soldering, without the DCC system connected to the bus. I can’t in any way take credit for this idea, I think I read about it on the wiring for DCC web site. Anyway, it is a simple device and has saved me from several errors that would have been time consuming to trouble shoot.
I made one of those boxes, there’s a picture on my web site. I also added a switch to change from the battery/buzzer to a bi-color LED and reistor, to check for presence of DCC power int he rails.
It’s lived in my toolbox since I finished putting it together.
I used the same two wire colors for my feeders as the bus wires. I also was consistent on which rail the red feeder went to and which rail the white feeder went to all around the layout (red on outside, white toward center of the ‘donut’). Underneath is was a simple matter of connecting red to red and white to white. Why red and white? Because in the darkness under the layout, red and black look an awful lot alike. (I am NOT color blind - those who are might want to be sure to pick colors that contrast enough for them to see. Actual colors used don’t matter, just be CONSISTENT).
Oh and even though it’s been a while, I still remember which color is on which rail. However, it IS written down in my book because maybe next time I won’t remember - another key tip, keep a manual with your wiring information - this is especially critical for any ‘odd’ or complex areas, because a year from now when something starts acting up will you remember that in this corner you ran out of black wire and had to splice in a piece of grey?
DCC uses 14v AC on the track at all times (well OK some systems use a little more or less voltage). AC power cycles back and forth very quickly back and forth rather than just power always going from + to - like in 12v DC systems.
Think of AC “polarity” this way…say a pitcher A and catcher B are going to play catch. The pitcher throws the ball to the catcher, the catcher throws it back to the pitcher, the pitcher throws it back to the catcher etc. etc. so far, so good.
However, if you wire things up with a “short”, both the pitcher and catcher will try to throw the ball at the same time - the balls will hit halfway or the players will get hit on the head with all the baseballs flying around or something.
You’re OK if the ball (power) is going from A to B to A, or B to A to B, but it can’t do both at the same time.
Yes, it is correct to think of as phasing. This becomes especially important when you have multiple boosters. Within a power district you don’t want to cross the left track and right track (to use your terms), otherwise a short will occur. When you go across boosters then you want the rights to line up and the lefts to line up. If not you can get wierd things and booster shutdowns when locomotives cross boundaries betwen boosters. I don’t know how all vendors do it but Digitrax has a sync signal on loconet which keeps all of the boosters in phase. I have three boosters and without a sync signal funny things happen when locomotives cross boosters and the DCC pulses are out of phase (voice of experience here )…
I don’t have much use for measuring rail power. I find that just being able to measure continuity between the track and the relevent bus is sufficient. I find use for a continuity checker most often when setting which rail my switch frogs are powered by, and to ensure that I have continuity to the rail from the bus after I install a new section.
While some folks may not have ever needed it… some kind of buzzer/light simple continuity checker is invaluable when you do have a problem. The last time I needed it to troubleshoot was when a switch was not activating the frog properly. I was able to observe that the frog had continuity to the wrong rail rather then a dead connection and was able to troubleshoot from there.
I ended up buying a simple little thing with a probe and a gator clip at home depot for $3. Well worth it although a home grown unit would be just fine.
You can still use the LED as well. If your system is capable of 00 addressing then you can call that up set it to wide open throttle and it will allow the LED to stay at 1 colour and let you know how your polarities are.