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# Why is LED controlled by current?

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#### alphacat

##### New Member
Hey,

In one of the threads in the General Electronics forum I wrote about a problem that I encountered.
I wanted to design a driver circuit for a blue LED which its VF has spreaded out from 3V (min) to 3.8 (max), that is according to datasheet.
The blue LED IF is 20mA.

I was adviced to use a current source, that will provide 20mA for every of these blue LEDs, and was told that both 3V and 3.8V blue LEDs consume 20mA current.

Therefore I wanted to ask, how come 3V and 3.8V LEDs consume the same current in order to illuminate at the same intensity?

Thank you.

Manufacturing tolerances?

I meant, if a 3V blue LED consumes 20mA, wouldnt you expect a 3.8V blue LED to consume more than 20mA, in order to illuminate at the same intensity as the 3V one?

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No, the 3.8V LED should require less current assuming the efficiencies are the same which they won't be.

Thelink below is from the Otherpower Forum and is the 5th installment of the LED masterclass written by a user called commanda.

Just click on the link at the start of the thread to go back to the first one ( just remember to make sure it opens in a new tab or window) and I recommend every Newbie to read what those threads have to show as some very ingenious circuits for driving multiple led's. I have made the circuit up myself using 4 off 550,000mcd white leds in series and for the last 3 years it's been going 24/7 in my shed powered off my nife batteries.

Regards Bryan

No one has mentioned the correlation between LED color and its chemistry. On top of that, throw in what temperature the junction must be at to emit a particular wavelength of light and how much current that takes... and how that in turn relates to its forward voltage… etc.

No one has mentioned the correlation between LED color and its chemistry. On top of that, throw in what temperature the junction must be at to emit a particular wavelength of light and how much current that takes... and how that in turn relates to its forward voltage… etc.

Wikipedia:
This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor.
I will not pretend to know what they mean by energy gap. But I was under the impression that the color is not related to heat as in incandescence.

Thanks guys.

Energy gap in semiconductors is the amount of energy thats needed for an electron in the valence band to pass to the conduction band.

Hi there,

The voltage is something that is a bit harder to control in the manufacturing
process. The way the device reacts to current however is roughly the same
all the time, or varies in a way that doesnt really hurt anything.
The small LEDs take 20ma but their efficiencies can vary, so one LED might
have a different voltage than the other.
This voltage is usually called the "Characteristic Voltage", but sometimes when
the average voltage among many devices is mentioned it is called the
"Nominal Voltage".
It's nice to know though that the intensity curve is drawn with Intensity vs Current,
so when we drive the LED with a certain current we know we can get a certain
intensity. We also, at the same time, know that the LED should be relatively
safe if we stick with the current recommended by the manufacturer, letting
the voltage go to whatever it needs to be for that particular LED.

The only point I was raising was that each different color LED has its own unique chemistry. And in order for a particular chemistry to emit visible light the junction must be raised to a certain temperature range which is done via current passing thru it, which correlates to a certain forward voltage. When a LED is spec’ed at say 20mA, that doesn’t mean you have to have 20mA of current flowing through it to make the LED “light”. That 20mA relates to a certain wavelength/intensity which will cause a certain forward voltage, and vise versa if you will.

Hi,

Yes, i believe that is a very good point too. That's why different colors
have different forward voltages and it's good to have a feel for these
differences when using the various LEDs out there.
There is also difference between the 'regular' LEDs and the 'high brightness'
LEDs.

Thank you guys, i learned a lot from this thread.

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Hi,

Yes, i believe that is a very good point too. That's why different colors
have different forward voltages and it's good to have a feel for these
differences when using the various LEDs out there.
There is also difference between the 'regular' LEDs and the 'high brightness'
LEDs.

Hi.

Being true the fact that different color LED's have different forward voltages, i think it has less to with the current it can be operated upon and more on the material used.Increasing voltage and inherently current causes an LED to turn a dull green(for a white 5mm LED atleast) before it burns which shows that temperature does not have a huge effect on LED color other wise its colors would be going haywire before it burns out.

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Hello again,

Yes the semiconductor material is what causes the different color, except for white
which also uses phosphor. The different material also causes a different characteristic
voltage.

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HI
I learnt a lot from this thread.

Hi guys
I just come here yesterday.Something need your help,I want to ask some questions,but I don't know how to manage to do that?Would you like to tell me?

Hi guys
I just come here yesterday.Something need your help,I want to ask some questions,but I don't know how to manage to do that?Would you like to tell me?

Hi wealth210 and welcome to the forum, as far as your question goes have a look at the forum titles and see which one best fits your question then click on 'New Thread'. i'm sure you'll know the rest from there.

Regards Bryan

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