What is the Resistor to use for 2 Green LED in series connected to 6V ?

[Bracer]

Awww, a test from someone that has taken this much time to teach me.

LT E6SG-V2AB-36-1-Z.......3.4v 30MA[0.03Amp]

Resistors in series = Combine all the resistance up....in this case their complete voltage drop.....3.4 X 3 = 10.2V

Ok....so the overall voltage drop is 10.2v.

Which means we only have to deal with 1.8 Volts.

We have 1.8 Volts, Current Required is 0.03Amp.

Ohm's Law:
Resistor = 1.8/0.03 = 60 Ohms.

60 Ohms ?

[I have another question based on this, Kirchoff's law says that the current throughout a single closed circuit is the same...but we all know from playing with light bulbs in series that the first one will be the brightest and the last one will be the dimmest....whats....what's up with that ? Don't they all get the same current ?]

 

[carbonzit]

Congratulations; you get an A on that.

Resistors in parallel = Combine all the resistance up...

You meant to write "Resistors in series" there.

Regarding the light bulbs in series thing, I think you're wrong about that. Have you seen this, where the first one is actually brighter? That would be only be possible if the bulbs were different. There's nothing about the electricity going through the first bulb that's any different from that going through the other bulbs. In other words, nothing different about a series circuit that would be different depending on position, assuming all devices are equal.

And yes, trust me, the current is the same through all parts of a series circuit. Has to be. Think of the plumbing analogy: current is like water volume. Even if the pressure (voltage) in a series-plumbed system is different at different points, the total volume of water going in and coming out has to be the same (well, assuming no leaks!).

 

[Bracer]

Sorry sorry sorry, I mean series ! ;p

Hmm...You are right....but I always get confused when people match water pipes with wires....because physically...for the speed to be faster, you would shut the pipe holder a little, we all know that from spraying at kids when we were a child, now if we were to chop up the cross section of the pipe [hereby volume, hereby current...] the resistor part of the pipe [The part where you squeeze] would have lesser volume! [smaller current!] but outside it would be the same in volume [current], which means for the duration of the path travel by the water, the resistor part actually have less current! So it's not all the same!

But then again...electricity is not water. Sorry, just expressing my confusion when people use water pipe as analogy.

carbonzit, I am very grateful for you.

 

[audioguru]

LT E6SG-V2AB-36-1-Z.......3.4v 30MA[0.03Amp]

No they aren't. They might be 3.4V.
They are anywhere from 2.9V to 3.8V.

Your 60 ohm resistor will overload the 2.9V LEDs and cause the 3.8v LEDs to be dim.

Try it again with only two LEDs in series with another resistor.

 

[carbonzit]

No they aren't. They might be 3.4V.
They are anywhere from 2.9V to 3.8V.

Your 60 ohm resistor will overload the 2.9V LEDs and cause the 3.8v LEDs to be dim.

Try it again with only two LEDs in series with another resistor.

[This reply is directed to Bracer, not audioguru]

Once again, the big, bad, grouchy Audio McDuck is being very unhelpful and trying to muddy the waters here. For the purposes of our little lesson here, you can safely ignore everything he wrote here.

(Not that he's wrong, mind you: he's raising the point that the voltage given for a LED might not be exact; his drawing showed a voltage marked by "(typ)" for "typical". However, when the Mouser catalog page, f'rinstance, gives 3.4 volts as the "operating voltage" for this device, then I think we can take them at their word. At least, as I said, for the purposes of this discussion.)

In other words, don't sweat it; you done good.

[to Audio McDuck:]

Where did you even get those specs? Are they for this device? If so, mind doing us the courtesy of posting a link to the source?

OK, I see it's from the datasheet. But a couple questions for you: do you even know what "Durchlassspannung" means. I don't. (Have to look it up.)

And under the note for that parameter, they say:

Forward voltages are tested at a current pulse duration of 1 ms and a tolerance of ±0.1 V.

Not exactly normal operating conditions for a LED which we're going to illuminate with DC, yes?

OK, further research reveals that Durchlassspannung means "forward bias", "conducting state voltage" or "forward voltage" according to this online dictionary. But the datasheet lists the smaller voltage (2.9V) as "min" and the higher as "typ". Now, this could mean any of several things:

1. That the manufacturer is so uncertain about the characteristics of this device that Vf can vary from 2.9 to 3.4 volts, which I would agree is significant, or

2. That the "min" figure is the minimum voltage to get any usable light out of the damned thing, while "typ" is the typical operating voltage, with a much smaller variation in Vf between individuals.

Take your pick. (Maybe someone who knows better can tell us what these specs really mean ...)

 

[Bracer]

Yeah...I am a happy little boy
I've done good said the Teacher

 

[audioguru]

Mouser doesn't make the LED. I looked in Google for it and the first link is Digikey where they have Osram's datasheet that I copied.
https://media.digikey.com/pdf/Data%20Sheets/Osram%20PDFs/LB_LT_E6SG_Pb_free[1].pdf

When you buy an LED you don't know if the manufacturer had low voltage ones or high voltage ones for the last production run.
So the datasheet shows the range of the low and high voltage limits.
Try to order a "typical" LED. The electronic parts distributor will be laughing for weeks.

 

[audioguru]

The forward voltage for an LED is pulsed for only 1ms at the manufacturer so that it doesn't heat up and change. It also makes the test very fast.

A low forward voltage is its voltage, not its brightness which depends on the current. Maybe the actual forward voltage is also the colour. A yellowish green might have a lower voltage than a bluish green.

Most LEDs have a fairly wide range of forward voltages, a range of 2.9V to 3.8V is normal. I wonder how many of these LEDs are less than 2.9V or more than 3.8V that they throw away?

 

[carbonzit]

A low forward voltage is its voltage, not its brightness which depends on the current. Maybe the actual forward voltage is also the colour. A yellowish green might have a lower voltage than a bluish green.

What? That doesn't even make sense. Re-read what you wrote and tell me what it means: "A low forward voltage is its voltage, not its brightness ..."???

Most LEDs have a fairly wide range of forward voltages, a range of 2.9V to 3.8V is normal. I wonder how many of these LEDs are less than 2.9V or more than 3.8V that they throw away?

So you still haven't answered the question I raised above, which is what those datasheet figures actually mean. Do you know, or are you just pulling stuff out of your butt as usual?

 

[audioguru]

The colour of an LED and the semiconductor material it is made with determines its forward voltage.
The current fed to the LED and the semiconductor material it is made with determines its brightness.

Old red LEDs were brighter than old green LEDs but now new green LEDs have a higher forward voltage than before and are the brightest colour now.

 

[carbonzit]

The colour of an LED and the semiconductor material it is made with determines its forward voltage.
The current fed to the LED and the semiconductor material it is made with determines its brightness.

Old red LEDs were brighter than old green LEDs but now new green LEDs have a higher forward voltage than before and are the brightest colour now.

You still haven't answered the question.

Keep on dancing ...

 

[audioguru]

CarbonZit,
Your 1st question was asking about the datasheet for the LED. I posted its link.
Your 2nd question was about the translation of Forward Voltage that is already in the datasheet.
Your 3rd question was about what (do) those datasheet figures actually mean. I explained.
Now what are you asking about? I don't manufacture LEDs, I simply read the datasheet and use them.

 

[killivolt]

(Post Deleted)

 

[audioguru]

(Post Deleted)

Umm, I agree.

 

[killivolt]

(Post deleted)

 

[carbonzit]

Your 3rd question was about what (do) those datasheet figures actually mean. I explained.

No, you didn't. But since you asked, I'll try again. In the datasheet, there are two values for forward voltage (you posted a snippet of it in a picture up there):

Durchlassspannung (min) | Vf: 2.9V
Forward voltage (typ) | Vf: 3.4V

So we know that "Durchlassspannung" just means "forward voltage". My question (which you didn't answer) is, what is the difference between these two values given in the datasheet (2.9V vs 3.4V)?

I proposed that the first figure is the minimum forward voltage for the LED to emit any light, and "typ" is the typical operating voltage. I don't know this is the case, but it certainly seems plausible.

So what is your explanation of the difference between these two figures? Remember, this is for one particular LED, a green one.

 

[audioguru]

So we know that "Durchlassspannung" just means "forward voltage". My question (which you didn't answer) is, what is the difference between these two values given in the datasheet (2.9V vs 3.4V)?

I proposed that the first figure is the minimum forward voltage for the LED to emit any light, and "typ" is the typical operating voltage. I don't know this is the case, but it certainly seems plausible.

No.
The forward voltage is a range of voltages. One LED can have its voltage that is within the range. The voltage range for this LED is from 2.9V to 3.8V when its current is 30mA.
3.4V is the "typical" voltage which is close to the middle of the voltage range. This LED is sold in 3 sub-groups of forward voltage range.
Since the current is 30mA when the forward voltage is measured then the LED is shining very brightly, not barely emitting any light.