LED direction??

[u18b]

Hi guys.

A find a great source for LEDs is to buy Christmas lights marked down 75% after Christmas.

Done it many times.

Well, last week, picked up 100 light strand.  LEDs, warm white.  Perfect.

But when I installed it with a limiting resistor in my N scale loco.......
the light comes on in BOTH directions.

Huh?   

Never had that happen before.

So, anode, cathode, plus, negative, doesn't matter.
And I'm not overdriving the LED.  Not even warm.

Are some LEDs bipolar (is that the right word?).

What's going on?

[nickelplate759]

can you test one with a battery?  I suppose it's possible that there are two LEDs in one package, one in each direction, or one LED with a diode rectifier, but I don't know why they would be made that way.    Just to be sure they can't be installed "backwards"?

[peteski]

Yes, since Christmas lights are operating from 120V AC they have now included 2 LED dies (wired opposite) in each LED package. That way they will light on either cycle of the AC.

This is similar to the bi-color (red/green) LEDs with only 2 leads except here both LEDs are blue (with phosphor coating which converts the blue light to white). Neat, eh?

To make things more interesting, some early LED Christmas lights used standard LEDs which were actually on only 50% of the time (at 60 times a second). The dual-chip LED thing you found is a recent development.

[u18b]

I'll try to post photos later tonight of packaging.

Brand was Ecosmart sold at Home Depot.

[peteski]

I'll try to post photos later tonight of packaging.

Brand was Ecosmart sold at Home Depot.

Few years ago I purchased (during after Christmas sale) a similar 50-ligth LED string of warm white 3mm LEDs (same brand and same store) and at that time they were standard single-die LEDs which only lit in one direction. Like I mentioned, it seems that this is a fairly recent development.

I also just purchased another 100-LED EcoSmart string of multicolor LEDs just a week ago. I should check to see if those are also dual-die LED.

[u18b]

Well, the bottom line implications are that it is great for DCC, since you no longer have to pay attention to which wire goes where.

But on analog, it would mean that I would have to add a diode to make them come one only in one direction.

right now, the light (front and back) are always on.  Forward and reverse.

[peteski]

But the bright side  is that if one burns out, you can flip the LED around and use the other side. 

As far as using direction diode, that is a good idea anyway because White LEDs are quite sensitive to reverse voltage (and current).  I also don't often use those large LEDs anymore - with the proliferation of very inexpensive warm white tiny SMD LEDs on eBay I almost exclusively use them in my N scale models.

I did check the 100-LED light sting I got few weeks back and just like yours, the LED lights up with either polarity. Then I grabbed my X20 magnifier and examined one of the blue LEDs. Just as suspected, there are 2 LED dies inside wired in reverse parallel.

[u18b]

This is what I bought.

http://www.ebay.com/itm/100-Light-LED-Warm-White-M5-Light-Set-Ecosmart-Christmas-Lights-/272515139262?hash=item3f732b06be:g:tGYAAOSw5cNYSv5Y

But I paid 6 bucks.   


I'll never pay about 1 dollar per LED ever again.   Those days are gone.  There are too many options if you look for them.   

[peteski]

This is what I bought.

http://www.ebay.com/itm/100-Light-LED-Warm-White-M5-Light-Set-Ecosmart-Christmas-Lights-/272515139262?hash=item3f732b06be:g:tGYAAOSw5cNYSv5Y

But I paid 6 bucks.   


I'll never pay about 1 dollar per LED ever again.   Those days are gone.  There are too many options if you look for them.   

I agree about the dollar-LED days begin long gone.  I thingk I paid about $12 each for my first 5mm white and blue LED (when they first bacame available for sale).  But back then that was a really exciting item.

But 3mm LEDs seem so bulky for use as N scale headlights or ditch lights. Plus those Christmas light LEDs have a special concave cone shaped lens which actually scatters (instead of focusing) the light.  That is because they are supposed to imitate the 360 degree light emission of incandescent bulbs.

I now go with Surface Mount Device (SMD) warm white (or other color) LEDs.  I usually buy sizes like 0805, 0603, or 0402.  You can find them very inexpensive on eBay.  Just search eBay for something like "0603 SMD warm white LED" and you'll find a range of LEDs and prices.

For example here is a thousand of warm white 0603 LEDs for $18: http://www.ebay.com/itm/360601770480
But when you start shopping for LEDs on eBay I recommend that you first get a smaller quantity (like 10 or 50) to make sure that you like the color. Not all warm white LEDs have a color good for model train headlights.

[drgw0579]

Does anyone have a favorite Digikey part number for a SMD LED for locomotive headlights?  I know they can be had cheaper on ebay, but I'm ordering other stuff so the shipping is already paid for.

Bill Kepner

[peteski]

Does anyone have a favorite Digikey part number for a SMD LED for locomotive headlights?  I know they can be had cheaper on ebay, but I'm ordering other stuff so the shipping is already paid for.

Bill Kepner

I usually use 0603 size (sometimes 0804 or 0402) warm white LEDs and surprisingly Digikey does not carry many warm white LEDs in general. So even thought I buy lots of components from Digikey, warm white SMD LEDs are not it.