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I have scratchbuilt two lamp posts to light the front steps of an N-scale mansion commercial structure. Each is lit by a surface-mount LED (very small ones.) I thought the two LEDS were identical, but I absolutely cannot get both to light at the same time! The LEDs are wired in parallel, with a 62 ohm resistor in series with the positive feed to both LEDs. Either LED will light just fine on it's own, but they will not both light. I'm testing these with a 3 volt flat round disc battery. Switching batteries does not help. Either LED will light with, or without, the resistor in the circuit. The only thing I can see is different between the two is that one appears substantially brighter than the other, although either produces ample light. What I very much don't want to do is scratchbuild another tiny brass lamp post and solder tiny magnet wire to another tiny ( approx.2 x 3 mm) LED and superglue the LED, a plastic washer, and a BB-size glass bead together. I have not tried a higher voltage because I really, really, don't want to burn out one, or both, LEDs, and have to make new lamp posts.

Any help will be appreciated.

Traction Fan :(
 

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What is your plan for a power supply? 3 volts is fine for testing but not operation. Once you determine that, you add resistance then it should work. Now you are under voltage. I would guess with that resistance you are using only two volts.
 

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LED's are current mode devices, and differences in manufacturing makes parallel operation an iffy proposition. If you add a small resistor in series with each of them, that will balance the load and they'll light. They also need a little voltage headroom above the operating voltage. For white LED's I suggest at least 5V, for red, you can get away with as little as 3V. Again, you need a current limiting resistor.
 

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John, I thought he said he had a 62 ohm resistor, that ought to do it, although maybe he needs more voltage or did not describe it correctly.
 

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And if you do use a pair of button cells, make sure you recalculate the resistor for the new voltage! 5v wall-worts are a dime a dozen these days, it's probably a lot easier to just get one of those to power the lights, then you'll have plenty of voltage AND current to drive a street full of lamps.
 

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FYI, on the GE SIlverliners and Budd cars (Multiple Unit passenger) the large diode banks which converted AC to DC for traction motors, each diode had it's own resistor. Same principle applies to LED's. For example, you have ten diodes or LED's in parallel. As voltage increases, one diode conducts current at 1.4999 volts. If the other diodes need 1.5000 volts, they never turn on (conduct current). So a resistor in series with the first diode than creates a larger voltage drop; 1.4999 + current X resistance.

OP, if you built the lamps with two LED's 'up top' and two wires coming out the bottom, you may need to rebuild.
I could not tell from you post how you design/built.

Pictures would help :)
 

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TF, I have read that typical Vf for a white LED is 3.3/3.5V. Or are they not white?
If white then maybe no resistor required?
ANY appreciable load will reduce the available battery voltage.
If you run 'em off your 12 v supply, try wiring them in series with one 300ohm resistor.
or in parallel with two 470 ohm.
Or a different supply would need a diferrent resistor.
I'm just used to always having 12 or 24vdc available, it colors my thinking! 😆
 

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Traction fan, heed what GTR says. You need a resistor per LED, makes no difference which lead on the LED has the resistor. You proved its necessary. Hopefully you can remove the single resistor that is in common and replace it with seperate resistors on the other 2 leads.
 

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Discussion Starter #12
LED's are current mode devices, and differences in manufacturing makes parallel operation an iffy proposition. If you add a small resistor in series with each of them, that will balance the load and they'll light. They also need a little voltage headroom above the operating voltage. For white LED's I suggest at least 5V, for red, you can get away with as little as 3V. Again, you need a current limiting resistor.
Thanks to all who responded. Special thanks to Gunrunner John for his tip about separate resistors. That did the trick. Both lamp posts are now working!

Traction Fan
 

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Yep, balancing the currents lets all the LED's loose. Glad it was an easy fix.

TF, I have read that typical Vf for a white LED is 3.3/3.5V. Or are they not white?
If white then maybe no resistor required?
LED's operate in a fairly narrow voltage band, they're current mode devices. Unless you specifically match the power voltage to the LED characteristics, you need current limiting. Even so, unless parallel LED's are perfectly matched, the brightness will vary between two parallel LED's.

In a typical 20ma LED, note that at 3V the LED current is under 5ma, but at 3.5V the current has already risen to 20ma, the curve steepens from there. At 4V, you're way over current at almost 40ma, and the life of your LED is less than an incandescent bulb.

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This is why the LED's for any structures on my layout will be powered by the 3V terminal on my power supply.
The PowerCab will be run by the power supply from NCE, it's not getting hooked up to this.
 
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