Does anyone know if there is any type (optical) of sensor that could define the track polarity (direction) for DCC voltages? Thanks, Jim
Jim, DCC is using AC current on the track. That means You don't really have a positive and a negative or "common" rail. The rail polarity doesn't control train direction of travel. Or did I misunderstand Your question? Art
No direction (polarity) information exists on the track. The direction is defined by the decoder and how the motor is wired to it. The direction of the decoderless locomotive is determined by the basestation/throttle and can be determined at the track.
Polarity isn't really the right word to describe what I think it is you are trying to find out. Athought the current on the track is not DC - you still have to watch out for 'shorts' caused by return loops or crossed wire while wiring. The only way I know of how to do this (there may be other ways) is to disconnet the DCC system and plug in DC power and go to town Or - even better - hook up a continuity tester with a REALLY loud buzzer and clip it across the rails - as you solder jumpers, if you cross one, you will know right a way by the loud buzzing sound
Hi guys, Thanks for your inputs. I realize that DCC isn't a fixed level polarity but instead a pulsed system. Measuring with a multimeter shows higher voltages in the AC mode rather than the DC mode. The DC voltages read a polarity but the reading is 9 volts (HO) showing polarity (+ or -) while the AC mode shows (22 or 5 volts depending on the track direction. I have a large layout, and have numerous reversing switches (15 circuits on 5 levels). It takes a lot of concentration to avoid shorts unless there were/are reminders (LED's) to recommend changing polarity (like at ramps or reverse loops). Auto reversing modules would avoid the problem but are expensive plus it still would be 'neat' to have track signals (dual red/green LED's) at turnout locations. I still think there must be a rectifier circuit that would show polarity. The multimeter in the DC mode does accurately define polarity. Then again the scheme would need to be pretty simple to be low cost. Take care, Jim
I would think you could use bicolor LED's to indicate polarity. As far as the expense for the auto reverser units, perhaps you wouldn't need to buy 15 of them, as long as you only run into or out of one at a time, one would do the job. This may not be acceptable, but you may be able to get by with say 5 or 6. Gary
Hi Gary & Co., Thanks for your inputs. LED's on DCC circuits emit either full color or partial color depending on the polarity. Dual LED's will have one chip 'on' while the other is partially 'on'. Ad 5.6K ohm load is a reasonable comprise for cutting back on one color to favor the other if the LED lens is perpendicular to the observer. So,,,, by putting a bi-color LED across tracks that are on different circuits the LED will behave as follows: when the tracks are out of sync (track polarity will cause a shortr circuit) their is no polarity difference (but when they are in phase the LED will emit a red or green depending on the polarity. This solves my problem because I'll use the green to denote 'going up' an incline while red will denote 'going down an incline. So when I throw a turnout to change the trains route I can then look to make sure the voltage is corrent by how the LED appears. I'll use the same scheme when coming out of a reverse loop. I'll also add LED indicators at the track where one LED will denote the turnout route and the second LED (lower one) will denote the polarity between the circuits. Take care, Jim
Jim as I understand out of your post you try to find a way to avoid shorts. I always power up my layout before I start wiring. As soon as I shortout the tracks my Digitrax systems starts buzzing. I then reverse the wire and if it's still buzzing I'll go and get a beer or two... Once I accidentally placed a beer can on my tracks and tried to find the short in my wiring for quite some time.... must have been something wrong with that beer.
Jim, If you are going to place the LED across the gaps, you could use a lamp instead, when polarities match, it will not light, when mismatched they will. I was thinking of two LED's, across the rails on both sides of the gap. -----------gap---------- / / led led / / -----------gap---------- My thought was the led's would light in different colors when mismatched. The led's could be in a signal head. Gary
Before I submitted the last post, there was one led on each side of the gap. When I hit the add reply, one moved over! I think you'll know what I meant tho. Gary
Hi Guys, I place the LED diagonally across the gap. The reason for one bi-color LED is that the color will tell me the polarity (direction). The purpose for the LED isn't to diagnose track shorts but instead to 'warn' me that when I intend to move a train from one level to another (or come out of a reverse loop) that the intended track is at the same polarity as the track section that is in use; otherwise the loco will short as it moves across the gaps. Sorry for the confusion. Jim
Hello, I thought, maybe you could connect an LED and resister across the gap in one of the rails? Polarity unimportant. And then, if it's lit, you have mismatched polarity. If it isn't lit, you don't. Cheapest solution I could think of.
The problem with LED's and DCC voltage is the same whether it's bi-color or mono-color (single or doual chip). The diode doesn't shut off with reverse polarity but partially turns-on. For a single chip LED the light would be either bright or dim. On a dual color, both chips emit light but one predominates. By selecting the load resistance (varies by doide characteristic) the back current could probably be reduced to get a dominant color by the forward biased diode. If anyone has experience with diodes that have very little back bias current please let us know. Thanks, Jim
My point was that single color LEDs are cheaper. But I also meant that the LED could be put across the gap in one rail. That way, if the two sections were of like polarity, then there would be no potential across the LED at all, so no light. Only when there was a mis-match would there be current flowing through the LED. Make the LED red as a warning, flip polarity switches until the light goes out. How do you post pictures / illistrations here anyway?
WP Russell, You'd still need a dual led, two led's or a rectifier as you would never know which side of the gap has the greater potential but I still think you've got the best idea. As for the other idea's you could probably add a capacitor parallel with the LED to filter out the AC portion of the signal so that you just get the DC average
With DCC, unless you are using pulse stretching to run a DC locomotive, the potential is one way half the time, and the other way for the other half. The LED should be as bright either way you wire it. Unless you are using pulse stretching. Depending on the reverse breakdown voltage of the LED, you might need a series diode, but those are only pennies, even in small quantities. Something like a 1N914. And a series resister (Lessee, 22 volts, about 10 mA, diode drop(0.6 volt) and LED drop(1.2 volt) would be 2020 Ohms), use a 2K resister. Oops, one quarter Watt! Use a half Watt resistor, at least. Wattage burned in the resistor is 0.2 Watt or 200 mW. That would make a three part circuit. A bulb would be one part, but like I said, I can't find bulbs cheaper than this, even when including all three parts. Though if you can't find the resister surplus, you might find the bulb answer cheaper!
Model Railroader did this in the WC project layout to indicate turnout position. Very simple circuit too.
Hi jimnrose, I just ran across your june post asking about polarity indication systems. I do not use DCC, just straight DC and I have a switchyard that has so many reverse loops in it I have not really been able to determine how many there are. So to solve my problem I designed a polarity monitoring system ( diode matrix) that uses scale two light boards to tell you if you have matched polarity at each end of a block or reverse loop. It also does other things I didn't know it would do, like monitoring the position of the turnout (through or on spur) at the end of the reverse loop. So I modified the diode matrix and now have turnout indicating lights on all my mainline switches. That is REALLY nice when the switch is 14 ft away and you can't remember which way it is thrown. This signaling system I developed makes for more fun in the operating mode. I use Kato Unitrack and have buried the diode matrix under the unitrack roadbed, so if you want to move the turnout or the block end, it moves with the unitrack turnout or track section. If Trainboard had a the system that would allow us guys that don't have websites, to post pictures, I could show the system to you.
Hi WP, What an easy simple way to spot potential shortouts, I have been racking my brain over this problem. I had almost given up, figuring a reversing module would be the best solution. The polarity on my reversing loop/wye combo is controlled by switches and relays attached to my turnouts. Everything worked when running a train over the whole of the loop & wye in either direction. Problems started I tryed to run two trains in opposite directions, I run into shortouts. I have lights that indicate turnout state but no way of knowing the polarity state of the track. A search of the DCC forum for reversing module brought this thread to light. So a big "thank you "to you WP Russell, and Trainboard in general. Cheers, Ken.
