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I have 9 Lionel Fastrack lighted bumpers, none have working bulbs. I found wired 18 V RED LED lighting bulbs. I was told they only work with DC, from what I have been able to find Lionel transformers work on AC. I am running both Lionel TMCC and MTH DCS. I have both a ZW and KW from the 50's and 3  Lionel 180V bricks attached to the tracks. Am I right to think I can't use these LED bulbs into my Bumpers

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You just need to follow basic rules around using LEDs with AC track power.

Let me see if I can explain this in a simple comparison. A diode is like a check valve in an air or water system. it allows current to flow one way and blocks the other way. An LED- Light Emitting Diode- again, is still a diode and has simple electrical characteristics. Those key points are, you need to limit current externally using a resistor or other method to control the current to around 10-20mA. The second part is that the LED like any other diode has a peak inverse voltage (PIV) and that is the maximum voltage (pressure equivalent to the check valve comparison) that is typically low. So when you buy an off the shelf DC prewired LED that likely has a resistor in series rated for that voltage range to control the current- what it is lacking is a second plain non light emitting diode to deal with the reverse voltage presented when using AC. Again, the point of adding the diode to your existing LED is, this extra normal rectifier diode blocks that full track voltage AC (again, think of it as maximum back pressure against a check valve) because that diode is rated much higher back voltage than the LED.

Again, in a nutshell, you just add a plain jane 1n400X (X being the voltage rating of the diode) in series with the prewired LED and resistor you already have.

Excellent post by Stan2004 explains and has a diagram

@stan2004 posted:

Never a teacher...but of the school-of-thought that a picture is worth a thousand words.

case 4

Right. It is the connection from the LED case to common that completes the circuit allowing the train transformer AC to power the LED.  And the diode insures that the LED "+" terminal only sees positive pulses.   A positive pulse or voltage is "positive" with respect to, or relative to, a reference.  In other words the voltage represents the difference between two points in the circuit; the difference can be positive or negative.  In this case that reference point is the common bus or outer-rail.  It is expedient for analysis purposes to think of the common as being 0 Volts or the horizontal line on the graph.

It is indeed confusing that the reference connection might be called "common" or "ground" or "neutral" or "return" or "black" or "zero" or "outer-rail".   

Keep asking questions!

BTW. I have been consciously avoiding "current" or Amps in the discussion.  Voltage and current are as thick-as-thieves as it gets!  But I believe the issue at hand (your dying LED bulbs) can be satisfactorily explained using voltage only. 

Last edited by Vernon Barry
@John H posted:

the problem was that the bumper frame was used to complete the  circuit from the track to the led, and connectivity would be lost at a joint.. I added a diode and hard wired my own 2mm leds to the track tabs.

John and GRJ,

Constant PIA trying to keep these lit. Even brand new ones with new track. Thanks for the suggestions. I def need to spruce these up because they're great when they work.

Just an off-hand question: how many real-world bumpers are lit?? I can't say I ever saw one, except for may be subways.


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