Questions about LEDs

The resistor is built in when the led requires no external resistor. Assuming no built in resistor, different colors require different voltages== quantum physics at work here.

Al W.

Tom,

I'm sure those LEDs have a resistor somewhere in the mix.

Dave

> I see Al already has your answer.

Here is a picture of the LED.  The leads are naked; could the resistor be built into the LED?

Built-in  resistors are part of the "die"not part of the lead.

Al W.

Can you point to the actual listing?  That picture appears to be a bog standard LED from the looks of the internal construction.

Here is a link to the listing:

http://www.ebay.com/itm/15-pcs...;hash=item23450c81ff

The lead picture for this listing shows only a very short length of the wires connected to the LED.  Could this be concealing the resistor out of frame?

Also learned a new phrase, "bog standard".  Never heard that before...

It doesn't look like there's a resistor, but I've never heard of those.

I went over to Digikey, and apparently there are indeed 12V LED's that don't have any resistor, how about that?

Here's even one cheaper than the eBay listing: http://www.allelectronics.com/...ed-5mm-t1-3/4/1.html

If there is no resistor either internal or external, what limits the current so the led

doesn't get destroyed by excessive current?

Al W.

The details for the LED you linked to at Digikey also say it is for DC.  Apparently no AC for LED's that operate at this high (for an LED) voltage.

It is 5 mm (about 0.2 inches); would that be very bright at 12 volts? If you put a resistor in series (say 1000 ohms), would it be even brighter?  Could it still operate at 12v DC?

I also noticed that one wire lead on the LED is longer than the other; is that how you distinguish between the anode and the cathode?

The long lead is positive (anode).  In one listing, an internal resistor was mentioned, so I suspect it's a small chip resistor hidden inside the component.

I am not saying it's never done but there is no need for a separate chip resistor.

The resistor would be made at the same time as the led on the wafer. ---- and next

to the led on the die.

Al W.

You do have to think about power dissipation in the package.  If you're running the LED at 20ma and applying 12V, a white LED would be dissipating about .18 watts in the resistor, that's a consideration as far as packaging and getting rid of the heat.  It doesn't sound like much until you put it in that small a space.

LED-switch.com has a large assortment, also.