I noticed that no matter how much resistance, the led's voltage does not change.

Thread Starter

babaliaris

Joined Nov 19, 2019
44
I created a circuit with some resistors in series with a led. I tried 1k, 2k, 10k, 20k, 32k but the voltage on the led always stays at 2.20V while of course, the voltages of the resistors change, since its a voltage divider.

Why is that?
 

dl324

Joined Mar 30, 2015
9,332
What voltage is the power supply? It's best to post a schematic when discussing circuits so there's no ambiguity.

LED is an acronym and it's written in all caps.
 

AnalogKid

Joined Aug 1, 2013
8,227
Why is that?
Because the D in LED stands for diode.

One of the fundamental characteristics of a diode is that the forward voltage drop is relatively independent of the forward current (within limitations). Datasheets for signal diodes and many LEDs have a voltage-current plot that shows this.

ak
 

djsfantasi

Joined Apr 11, 2010
5,841
LEDs are NOT a resistive load. The Voltage drop across an LED is a constant. Therefore, a) you don’t have a voltage divider and regardless of any resistor, the LED is going to drop the same amount of voltage (it’s a parameter of an LED, and called forward voltage or Vf.) The resistor sets the LED current.
 

SteveSh

Joined Nov 5, 2019
104
An LED has several PN (or NP) junctions in series. Think of a LED as several diodes in series. Since a diode's forward drop is, at least to a first order approximation, constant with respect to current, the same would apply to an LED.

Go look at a data sheet for an opto-coupler, and look at the forward voltage drop for the input diode (which is an LED). You'll see values somewhere between 1.5V and 2.0V.
 

crutschow

Joined Mar 14, 2008
23,791
An LED has several PN (or NP) junctions in series.
Where did you get that info?

Most LEDs have just one PN junction.

The difference in forward drop is due the different bandgap voltages of the junction material used, and that determines the LED light photon energy and thus the light color.

Gallium arsenide LEDs emit red and infrared light.
Gallium nitride LEDs emit bright blue light.
Yttrium aluminium garnet LEDs emit white light.
Gallium phosphide LEDs emit red, yellow and green light.
Aluminium gallium nitride LEDs emit ultraviolet light.
Aluminum gallium phosphide LEDs emit green light.
 

dl324

Joined Mar 30, 2015
9,332
Here's an IV curve for Lx3341 LED's:
1574888925829.png
If you have the datasheet for the LED you're using, you can determine the typical forward voltage for a specific current. There will be some variance because the graph is for typical.

This is the parameter range for the LEDs mentioned above:
1574889070967.png
1574889050253.png
1574889095821.png
 

Thread Starter

babaliaris

Joined Nov 19, 2019
44
The supply is 5VDC. Ok with the graph that @dl324 posted it now makes sense. Since the resistors were actually changing the current slightly, the multimeter was always measuring about to 2.20V across the LED. And the graph shows that while the current increases the voltage changes slightly until you reach a very high current.
 

bertus

Joined Apr 5, 2008
20,180
Hello,

Repeat your test with resistors of 150, 220, 270,330,390 and 470 Ohms.
Then the current will be higher and you will be higher in voltage accross the led.

Bertus
 

Thread Starter

babaliaris

Joined Nov 19, 2019
44
I know that I'm making stupid questions in this forum recently, but I'm quite new to this stuff. Even If at the university I completed 3 labs about analog and logic circuits, you really don't learn much from them. They just give the schematics without explaining what they do, and we just construct the circuit and do some measurements, that's it.

Btw I skipped class when the professor was talking about diodes :p
 

SteveSh

Joined Nov 5, 2019
104
Where did you get that info?

Most LEDs have just one PN junction.

The difference in forward drop is due the different bandgap voltages of the junction material used, and that determines the LED light photon energy and thus the light color.
You're right, I stand corrected. I think my comment came from 1) the Vf of a LED relative to a standard Si diode, and 2) some of the graphics I've seen that showed more than 2 PN layers, something like this:
Edge-Emitting-LED-Structure.jpg
 

MisterBill2

Joined Jan 23, 2018
4,545
OK, with a regular multi meter you will not notice the difference in the forward voltage as the current is changed. Bout with a digital voltmetr having adequate resolution it will be obvious. Most of the diode voltage drop is due to the "band gap" voltage, which is rather constant with temperature, but there is also a resistive part and that does vary with current.
 
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