LED Directional Polarity Question


KB02

KB02

Well-Known Member
LED's are polarity dependent to emit light. But will current still flow through them if the polarity is going the wrong way?
 
not until it's over the breakdown voltage, somewheres around four volts for a single led
 
Just thinking about adding lights to some passenger cars and wondering what would happen if they crossed into a section of track isolated for a reverser.
 
Current will not flow through them if the polarity is reversed. However the LED can be damaged if the peak reverse voltage is exceeded. You can get LED's that will be one color and change color when the polarity is reversed ( green to red ) .
 
KB - kinda a can-o-worms. Depends on what you want. Am sure others will chime in on this. If you are pulling power from track DCC on one wheel ya might think about adding a full wave bridge and regulator with some value of cap after the regulator. Assuming that you have a series string of LED's to power you need to find a data sheet to see what the forward voltage [Vf] at the current [If] you want to run the LED's at. Or, seat of the pants it - Use some standard power supply that puts out a known voltage, say 12VDC [measured] and add some resistor value in series with the LED you are testing with; probably start at 800 to 1000 Ohm and raise or lower the resistance to get the LED brightness you wish. Now you can measure [Vf] across the LED and [If] by measuring the voltage across the resistor then divide by the resistance. If the LED's are from the same manufacture they should have pretty close to the same rating. For instance, say your LED [Vf] is 1.2V and you have 4 of them in a string you would have a 4.8V drop across the LED string. The regulator you choose needs to waste ( Output bridge voltage - 4.8V ) as heat. If DCC voltage is 14V, the bridge should drop about 1.4V, then the regulator needs to drop 8ish volts. At 0.015 [If], it probably will not matter and would not heat up much and you wouldn't need some sort of heat sink for it. LED's would be happy as would the regulator. If the regulator does heat up, you just need to calculate a resistor to insert into the LED string. Using the above, 8V/0.015[If] is about 530 Ohms. Power is about .119 W so 1/4 watt resistor should work. Regulator specs usually have some voltage drop input to output that the thing can withstand without heat sink which you can incorporate into your design. Cap on the output of the regulator probably could be 100uF @ 25V which should help keep the LED's on when the wheel is going across a dead frog or the like; think of it something like a 'stay alive'.
Ok, says you - I will grab DCC+ from a wheel on one end of the car, DCC- from a wheel on the other end. That should solve the dead frog problem, but not when there is a polarity change cuz of a reversing loop. Simple to solve, another bridge and regulator.
 
KB02 said:
Just thinking about adding lights to some passenger cars and wondering what would happen if they crossed into a section of track isolated for a reverser.
Click to expand...
Have you thought of fitting battery powered LED's with a reed switch, or motion sensor, won't matter what the polarity is on the track then.
 
Basically, I have one reversing section on my track and one really long passenger consist that I run occasionally. The reversing section is not quite long enough to hold the entire consist, so my worry is that if I power the lights on my end car, and it crosses into the reversed section, will it trip the system into thinking there is a short? They're older Athearn car with pick up from each rail at either side of the car.

CTCLIBBY - That is a wonderful, and thoroughly thought out answer that I understood maybe 1/10th of. 😁 I know just enough about electricity to get myself electrocuted... I mean KEEP myself from getting electrocuted... It's one of the two...
 
Thanks, sorry it did not help. I do have a question though. If your reversing section is "not quite long enough" how are you going to reverse that long consist? I assume the reversing section is a return loop or something of the sort?
 
If you use a bridge rectifier ( 4 diodes) then your LED will work with any polarity , even AC. Connect the LED through a resistor to the + - terminals of the bridge and the track power to the AC terminals. At 12 volts using a 470 ohm resistor , your LED will draw 20 ma. With a 1k ohm resistor it will be about 10 ma , 10k around 1 ma.
 
KB02 said:
Basically, I have one reversing section on my track and one really long passenger consist that I run occasionally. The reversing section is not quite long enough to hold the entire consist, so my worry is that if I power the lights on my end car, and it crosses into the reversed section, will it trip the system into thinking there is a short?
Click to expand...
Ok, so this is the real question.

That is not the system thinking there is a short, there is a real short. That is how the automatic reversing units work. They sense the short circuit and flip the polarity. The problem with too long a train is that the lead locomotive that caused short circuit can be trying to flip the polarity at the same time the rear car is trying to flip the polarity back. Not a good situation. Could cause the auto reverse unit to go into oscillation, burn out, or worse cause the whole system to short (hopefully the DCC would then self shut down).

I would shorten the train to fit into the reversing section, remove power (track lighting) from the rear cars and install plastic wheels on them so that all powered stuff fits into the reversing section, or make the reversing section longer.
 
if you are looking at a new LED, the [marginally] longer lead is the positive lead. there also should be a flat spot on the lower part of the housing, that flat indicates the negative. you can also look at the LED diode directly, and the upper [and smaller], portion is the positive . diodes don't have to be run at say the full 20ma, that's a guideline for maximum brightness, they do last a little longer at 10ma, or even 1ma, they usually won't light below that current level. and in packs of 500 they usually cost less than a cent each [shipping included from china], lol
 
My problem.
Built a turntable with a small control house on it. Put a LED inside, direct wired to the TT track, glued it all up.
When my layout was DC, the LED worked fine, indicating track direction, LED on, pull loco on to TT. LED off, loco goes other way.

Now I'm DCC, and the LED flashes :mad:
 
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