New LED Lighting Question

DC. Looks like no resistor: 20ma @ 3v, which is pretty much a standard white led.

I think most LEDs have to be DC powered to light up.   They only light with power flowing one way.    

You need a resistor on LEDs to be safe.     The useful range is very wide, for 1k ohm to 5k ohm and 1/4 watt should work.   then you can run them off any reasonable model train voltage from as low as 5 to about 16-20.  

I have found packages of 70 bright white Christmas light LEDs on sale at Menards after Christmas for $5.    I remove them from the wiring and mount them as I need for train stuff.    I built light strips for 4-6 passenger cars this way.   

Also, if you put a capacitor in parallel in the circuit with the LEDs, it will remove a lot of flicker while rolling down the track.   The capacitor also has to be DC and correct polarity.

First off, these are bright white, so they'll have a blue cast to them, many folks prefer warm white for model RR applications.  As mentioned, you really want current limiting for LED's, below is a graph that should explain why.

The nominal full intensity voltage for the LED depicted in the graph below is around 3.3 volts.  However, if the voltage increases just .1 volts to 3.4 volts, the current doubles!  The life of the LED will be very short at those currents!  The graph is a plot of both a blue and white LED, they're similar chemistries, so they have a similar plot.

Oh, and I forgot to mention, they're DC devices.  LED's have a fairly low reverse voltage tolerance, so it's best to have a blocking diode and the current limiting resistor for AC use.

I also think you're "just" getting LEDs with wires attached.  So you're responsible for coming up with a suitable DC voltage/current to drive them.  I figure the "LED only" label attached to each is a kind-of warning to that effect. 

As stated on the product description, it's meant to be used with their Light Genie system which apparently includes a pack of resistors as shown in photo.  What's curious for something being sold as a "system" is why doesn't it include a power source!  I see 2 batteries for the remote, but where's the wall-wart or AC-to-DC adapter?

Track power Stan.

You're right.  I found a PDF manual here.  User supplies AC or DC power up to 18V max.  Or you can buy a 1 Amp supply from MRC for $24.98 (MSRP).

It appears they have a 4-diode bridge on-board but I see so little capacitance that LEDs will be pulsing at 120 Hz when powered by Accessory AC voltage...not that it will be noticeable.  The board does have some useful layout lighting features built-in such as ability to assign 3 LEDs (red, yellow, green) to automatically sequence like a traffic light.

WOW!   They're sure proud of that power supply!  That's pretty pricey!

This is interesting, apparently those LED's do have resistors, or at least they sell them with resistors.

I found another photo of the MRC pre-wired LED and what I thought was a label is apparently a thin, square circuit board on which there's a surface-mount LED plus a 470-ohm surface-mount resistor.  I can't tell if it comes with 2 or 3 wires but if only 2 wires then I guess you plug in the soldering-iron to move the one wire before/after the resistor as the case may be.

So you get 4 LED+resistor pairs pre-wired for $10 (MSRP)... $2.50 per LED; I realize MSRP is frequently discounted by dealers.  But  I notice on eBay you can get similar pre-wired surface mount LEDs with resistor for 65 cents per LED (free shipping from Asia):

The outputs appears to be 12V DC so presumably that's why resistors are involved.  What I find curious is, given the 12V DC output, why they don't offer (I didn't see) LED strips as an option.  LED strips are warm or bright white, run off 12V DC, are surface mount LEDs with resistor, and run about 2-3 cents per LED.  

It's kind of confusing that the pre-wired LEDs have resistors and yet the system comes with separate loose resistors too.  Comments in the manual touch on when to use the resistors to configure LEDs for series or parallel operation and reference the MRC website for further info but I couldn't find any.  Easy for me to throw peanuts from the gallery but I think their manual could use a re-write. 

Maybe they intended the separate resistors for bare LED's in custom installations?

After poking around their site, I agree with Stan, they don't do a real good job of describing things and the instructions do need some help. Looks like a neat idea and product, but more information is definitely needed.