Confused about my LEDs

Using Arduino General Electronics
#1

I was tinkering with the Arduino Starter Kit. It comes with a book with some projects proposals.

The very first "project" has nothing to do with programming nor IO: it is just building a LED circuit using the +5V/GND pins in the board.
In the book they explain that because 5V is too much for the LED, a resistor in series is needed (a 220 Ohm one).

So I did that, and the red LED worked as expected. Then I picked my cheap multimeter and measured V and current. Voltage drop in the resistor was about 3V, and in the diode itself it was 2V. I calculated the expected current in the circuit based on the voltage drop in the resistor, as follows:

I=V[sub]R[/sub]/R = 3V / 220 Ohm = 13,63 mA.

Then put my multimeter in series and measured current. The reading was slightly lower, about 13,1 mA.

So far it looks like a normal LED that works on 2V and the current is less than 20mA, which makes sense.

But then I was curious about whether this resistor in series was really needed, in case I decided to connect the LED directly to the 3V pin in the Arduino board. According to the kit's web page, the datasheet for the red LEDs is this one. I was surprised to read that these LEDs have an internal resistor, and they are able to work at 12V, being 14V the max. Also according to the data-sheet, the max forward current is 11.5 mA, while in my circuit I had measured at least 13mA.

So I'm confused because:

  • Why would a 12V LED with built-in resistor work on only 2V (unless it had a variable resistor)
  • Why my LED was able to work on a current value allegedly out of specs.
  • Why would they include a 12V LED in a kit for prototyping, where most common voltages are 5V and 3V.

I wanted to verify whether the LED had a resistor at all. (To be honest, turning on a LED is no fun, and I had a few more spare LEDs in the bag, so I was planning to blow up one from the very beginning ]:D). I connected it directly to the DC adapter I was using to power the Arduino board (9V, 1A). It worked for a few seconds, but then it got damaged and now it only glows with a very dim light.

So my question is, could such a LED be used as described? Or is it possible that the datasheet link is wrong? I've been checking the links for the other kinds of LEDs in the kit's page, and while the green and yellow LEDs have datasheets that make sense, the link for the blue LED one is also of a 12V one. I've however tested all four and they work the same.

#2

Which "Arduino Starter Kit"? Can you provide a link? There are a -lot- of crappy "starter/beginner" kits out there, just like there are -lots- of crappy "beginner" Arduino books out there.

And you never, NEVER connect an LED directly to a voltage supply without a series current limiting resistor. It sounds like those have them built-in, in an attempt to protect your LEDs and your Arduino in case you connect them directly to an output pin or a power supply.

And PWMing a pin is -not- enough. You should not connect a regular LED without an internal resistor directly to an Arduino pin, and expect the analog output PWM to operate it properly.

#3

buffer_overfly:
So my question is, could such a LED be used as described? Or is it possible that the data-sheet link is wrong?

Option (b) is: They lied about what the LED was - it's not the one in the datasheet.

#4

polymorph:
Which “Arduino Starter Kit”? Can you provide a link? There are a -lot- of crappy “starter/beginner” kits out there, just like there are -lots- of crappy “beginner” Arduino books out there.

No, it’s the official one. The link is included in my original post. And the book I’m talking about is the one included in the kit.

polymorph:
And you never, NEVER connect an LED directly to a voltage supply without a series current limiting resistor. It sounds like those have them built-in, in an attempt to protect your LEDs and your Arduino in case you connect them directly to an output pin or a power supply.

I might be wrong, but if a power supply provides exactly the voltage needed to turn on the LED, and the current is within specs, this should be possible.

polymorph:
And PWMing a pin is -not- enough. You should not connect a regular LED without an internal resistor directly to an Arduino pin, and expect the analog output PWM to operate it properly.

I was not PWMing. The circuit was connected to the +5V and GND pins in the board, not to the I/O pins.

#5

fungus:

buffer_overfly:
So my question is, could such a LED be used as described? Or is it possible that the data-sheet link is wrong?

Option (b) is: They lied about what the LED was - it's not the one in the datasheet.

That seems to be the case. If so, then the link for the blue led datasheet is also wrong.
I think the correct one for the red LED is probably this one. I'll try to report this to the web guys so that they can check the links.

#6

Sorry, missed the inline link.

I have no idea what is in the "official" book, so I cannot comment on its content.

As for LEDs and constant current power supplies, don't trust me. Google for it. LEDs are a forward biased diode junction. A very tiny change in driving voltage -or- temperature can cause a very large change in current. Each color has a different forward bias voltage, and that varies from batch to batch, too.

So that is why LEDs are usually driven with a voltage much higher than the turn-on voltage, with a current limiting resistor, or driven with a constant current source.

#7

So I’m confused because:

Why would a 12V LED with built-in resistor work on only 2V (unless it had a variable resistor)

2v is the bare min (eg, it’s forward voltage) to light up, I don’t believe this 12v LED is a 12v LED just a regular RED one.

Why my LED was able to work on a current value allegedly out of specs.

The 12v LED just a regular RED one.

Why would they include a 12V LED in a kit for prototyping, where most common voltages are 5V and 3V.

To scam more money? or they confused or ran out?

I wanted to verify whether the LED had a resistor at all. (To be honest, turning on a LED is no fun, and I had a few more spare LEDs in the bag, so I was planning to blow up one from the very beginning smiley-twist). I connected it directly to the DC adapter I was using to power the Arduino board (9V, 1A). It worked for a few seconds, but then it got damaged and now it only glows with a very dim light.

That’s what a regular LED does…

So my question is, could such a LED be used as described? Or is it possible that the datasheet link is wrong? I’ve been checking the links for the other kinds of LEDs in the kit’s page, and while the green and yellow LEDs have datasheets that make sense, the link for the blue LED one is also of a 12V one. I’ve however tested all four and they work the same.

If you have any working left, they should handle 20ma

#8

A 12V LED is really just a regular 2V LED with a resistor built-in that is the right value to allow normal operation with 12V connected directly to it. It can work at lower voltages, just not as bright, and even dimmer if you add another series resistor.

A regular, no resistor red LED may be around 1.5V drop, green and yellow around 2V, blue and UV even higher voltages. White LEDs, it depends on which kind it is. Some make white with three separate red, green, and blue LED dies internally, some have a UV LED coated with a phosphor.

#9