Model Railroading > Need Help with LED Voltage/Resistor

I need a little help deciphering an electronic schematic for some bipolar LED's I just purchased....

http://www.kingbrightusa.com/images/catalog/SPEC/APBD3224SURKCGKC-F01.pdf

If I'm reading it right, these LEDs operate at about 2v (which I suppose would explain why I got no illumination at all with a 1.5v battery and why the LED essentially vaporized when I then tested it at 3v). If I'm reading it right (2v), what would be a good resistor to use if the power supplied is at 5v?

According to the spec sheet, these are characterized at 20 mA. That means it is the best current to use. At that current, forward voltage (what does forward mean when it is bipolar?... but I digress...) is about 2 volts. Subtract the 2 V from your 5V supply and you must drop 3V at 20 mA. the math is E = I*R, thus R = E/I. You would need a 150 ohm resistor and the power dissipated is P = E*I, so you would have 60 mW, which is no big deal.

The datasheet says you want a forward current of 20 mA, and they operate
at 2 volts. For a 5 volt supply, you will need to "drop" 3 volts in the resistor,
with 20 mA current. Ohm's law says R=E/I; so 3 volts divided by 0.02 amps
gives you 150 ohms. Don't go below 100 ohms on this led or you will exceed
the maximum current rating.

dr_grid
Matt Martin

OK, great!

Thanks, guys!

T-fan

Just to be sure that you understand, these are NOT bi-polar LEDs unless you wire them that way. This is two electrically separate LEDs in one package, one red and one green. Both of them have the positive terminal on the same side of the surface mount strip, so to make this into a true bi-polar (red on one polarity and green when voltage is reversed) you need to tie terminal 1 & 4 for one connection, and tie terminal 2 & 3 for the other. That may not be real easy to do physically with these tiny chips, but will work fine electrically. It may be much easier to use these with terminals 2 & 4 connected to make a common cathode bi-color LED, which is also usually supported by signal driver circuits (I'm assuming that's your end use. VBG) You still need resistors, you ALWAYS need some kind of current limiting with any LED, although sometimes it is incorporated into the driver circuitry.

Barry Draper

Thanks, Barry.

Yes, these are for searchlight signals.

The signal driver I will be using actually requires the LED's to be wired in common anode.

Nonetheless, now that I have the right resistor, I'm pretty sure I can get these wired up correctly.

Thanks!

T-fan

Common Anode is no problem, just connect 1 & 3 for the common anode, and 2 & 4 are your cathodes. Have fun!

Barry Draper