I know that this issue has come up before in various forms, but I thought that I would start fresh.
How do wire a 2-leg bi-polar LED, including resistors and diodes, to detect reverse polarity on a DCC layout ?
Thanks.
Rich
Alton Junction
I built one and just put a 1000 ohm resister in series with one leg of the LED.
I then added wires and alligator clips so I could just clip the tester to the rails.
I put some heat shrink around the leads coming off the LED so that there would be no stress on the LEDs lead wires.
That was about the only problem I found in building the tester!
BOB H - Clarion, PA
Good question ! .... I used the three legged bi-polar LED on mine.
I'm assuming this is for a lineside signal installation. (?)
The common leg was wired to one rail of the mainline. The red leg was attached to the other side of the insulated rail joiner of the reversing section of the same rail. The green leg was attached to the insulated rail of the reversing section opposite to the red and common side.
When the polarity matches, the green lights, and when the polarity doesn't match, the red lights. Your resistor can be attached to the common leg as only one color will be lit at a time. A value between 1000 and 2000 ohms works fine for DCC track voltage
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Mark R. Good question ! .... I used the three legged bi-polar LED on mine. I'm assuming this is for a lineside signal installation. (?) The common leg was wired to one rail of the mainline. The red leg was attached to the other side of the insulated rail joiner of the reversing section of the same rail. The green leg was attached to the insulated rail of the reversing section opposite to the red and common side. When the polarity matches, the green lights, and when the polarity doesn't match, the red lights. Your resistor can be attached to the common leg as only one color will be lit at a time. A value between 1000 and 2000 ohms works fine for DCC track voltage Mark.
Is there no need for diodes in that wiring arrangement?
LED stands for Light Emitting Diode, so the diode is built in. However, you can add a couple of diodes as a safety so the LED may last longer. I guess it depends on how you want to use it. If it is something that is going to be connected to the track permanently, a couple of diodes would be in order. One on each leg but not the common center one.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
gandydancer19 LED stands for Light Emitting Diode, so the diode is built in. However, you can add a couple of diodes as a safety so the LED may last longer. I guess it depends on how you want to use it. If it is something that is going to be connected to the track permanently, a couple of diodes would be in order. One on each leg but not the common center one.
Thanks, Elmer (and Mark),
That's what I did, and I added the diodes, just to be on the safe side.
And, it works.
cmrproducts Rich I built one and just put a 1000 ohm resister in series with one leg of the LED. I then added wires and alligator clips so I could just clip the tester to the rails. I put some heat shrink around the leads coming off the LED so that there would be no stress on the LEDs lead wires. That was about the only problem I found in building the tester! BOB H - Clarion, PA
Thanks for that info, Bob.
I did the same thing, added alligator clips to make it portable.
i'm a bit puzzled
you presumable want to know if you've wired up ajoining track with the correct polarity. you need to do this because there are no rail joiners or there are insulators. is this correct?
so you would use your led gadget connected to the left (or right) rails of both the existing and the newly wired track. The polarity would be reversed if the LED(s) light. It's ok it they remain dark.
if you connected the led gadget across the left and right rails of powered track, they would light indicating that power is present.
if this is correct, wouldn't an LED (or bipolar LED) and a resistor (1k sounds like enough) be all you need?
greg
greg - Philadelphia & Reading / Reading
greg,
I just got back a repaired auto-reverser, and instead of listening for the click when the polarity reverses, I wanted a visual representation. So, I wired up a bi-polar LED.
If the reversing section, in this case a short section of track leading to the turntable, has the same polarity as the main line, the LED lights green, If it flips to match the turntable track, the LED lights red.
I used a 3-leg bi-polar LED with a resistor on the center leg and a diode on each outer leg.
From what I am being told here, I didn't need the diodes but, supposedly, the diodes prolong the life of the LED.
A couple of questions on the use of the LEDs. An LED used on a DC layout will light with positive polarity connected to the Anode and negitive connected to the Cathode. Reversing the polarity will not light the Diode. But DCC is AC and the Diode will light either way. Is there something else that has to be added to the circuit?.
A bi-color 2-lead LED is two LEDs back to back (anode of one to cathod of the other, and vice-versa). So all it needs to connect to DCC track power is a dropping resistor. On one half of the DCC waveform, it iwll be green, oth the other half, red (assuming a red/green LED - they do come in other combos). Essentially - orange as long as the waveform exactly balances out.
If you clip it across the gap in one rail at the junction of two power districts, it should not light up if the two boosters are in phase - there should be no potential across the gap. If one is wired inverse of the other, then you have the full DCC track voltage across the gap, and the LED will light.
If you are using a system that can run a DC loco on DCC using address 00, if address 00 is in use, the LED will glow more green or more red depending on the direction, and how high the throttle is set.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
To take it a step further, you could use the bi-polar LED connected to a 12 volt DPDT relay. Connect the operating contacts of the relay across the gap in the rail. If the polarity matches, the relay is off, if there's a mis-match in the polarity, the relay latches.
Connect the bi-polar LED to the DPDT contacts wired like a reversing switch. The feed to the DPDT contacts to power the LED can be any DC source you have available. Don't forget to add the appropriate resistor to one of the legs (doesn't matter which) of the LED.
When the relay is on, the LED will be one color, when the relay is off, it will be the other color.
richhotrainI just got back a repaired auto-reverser, and instead of listening for the click when the polarity reverses, I wanted a visual representation. So, I wired up a bi-polar LED. If the reversing section, in this case a short section of track leading to the turntable, has the same polarity as the main line, the LED lights green, If it flips to match the turntable track, the LED lights red. I used a 3-leg bi-polar LED with a resistor on the center leg and a diode on each outer leg. From what I am being told here, I didn't need the diodes but, supposedly, the diodes prolong the life of the LED.
LEDs are often used power lamps, all those red lights on things that are lit when they are powered on. They are just an LED a current limiting resistor. I don't understand why you would need an extra diode, especially if it's a 2 lead bi-polar LED that is red with one polarity and green with the opposite polarity.
how exactly do you intend to wire this circuit with your reversing section? do you want it to indicate the polarity of the reversing section, or whether the reversing section is the same polarity as the non-reversing section or the turntable?
also, why isn't the reversing section the turntable?
I just put some red/green two lead LED's across my NCE Power Cab leads and both the green and red LED are lit. Switch the LED leads and the same thing. AC will do that. The green seems to be the dominant color, though you can see the red dot. I used the 1k resistor.
This diode issue for protecting the LED has been discussed in different forums and I have found some sites with diagrams about this issue but the NCE Power Cab has a red 3mm LED with a 1k resistor directly across the DCC power leads in the panel for plugging in the cab and connecting to the track. I traced out the circuit.
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
gregc how exactly do you intend to wire this circuit with your reversing section? do you want it to indicate the polarity of the reversing section, or whether the reversing section is the same polarity as the non-reversing section or the turntable? also, why isn't the reversing section the turntable?
It is a 3-leg bi-polar LED and I already wired it. It is a temporary set up while I test the auto-reverser which had to be sent back twice for repairs. Since the auto-reverser has failed in the past, I want to use the LED to make it easier to observe whether the auto-reverser is doing its job. Incidentally, both the auto-reverser and the LED wiring are working properly.
As far as the turntable is concerned, that's a whole different story and not really relevant to this issue, since my main concern is just to make sure that the auto-reverser is working properly. But, here is the story. It is a Walthers indexed 130' turntable, the older non-DCC version. It shouldn't need an auto-reverser since it is designed to reverse polarity as it rotates. However, because of the position of one of the approach tracks, I needed to install an auto reverser and an isolated section of track. I could relocate the approach track so that I wouldn't need an auto-reverser, but it has been in place for a long time and so has the auto-reverser, so I am not inclined to change it now.
Incidentally, when I first installed the turntable, I was shocked to realize that I was getting shorts when a loco attempted to exit the turntable track onto that particular approach track. I would up installing an auto-reverser, and that ended up solving the problem, but I never understood what caused the problem until one day I posted the following thread and got my answer.
http://cs.trains.com/mrr/f/744/t/181396.aspx?sort=ASC&pi314=1