Let’s try another approach to explaining this since the previous attempts do not seem to satisfy you.

In the region of operation called the saturation region of a MOSFET, it is passes a specific current that depends in the gate voltage, no matter what the voltage between the source and drain are.

This property is what allows its use as a constant current sink. Set a particular gate voltage, and you will get nearly the same current whether the voltage drop from the drain to the sieve is. 3V or 300V.

Now, we don’t actually know what this gate voltage is for any given current. It will actually vary from one to another of the same model MOSFET. This is why we use negative feedback to set the current. A sense resistor is placed in the current path and it develops a voltage proportional to the current. That voltage is used in a circuit to adjust the gate voltage such that the voltage across the resistor is some constant. You can see all that in the simple circuit posted here. The 0.6V is the base to emitter voltage of a BJT. When it is higher the BJT conducts more current and pulls down the gate voltage. When the sense resistor voltage is lower than 0.6V the BJT conducts less current and the gate voltage rises.

Thanks BobTPH, you cleared my doubt! I was wondering, focusing solely on the LED aspect, how it controls the voltage between a LED or LEDs. According to the “V = IR“ What I understood is:

As long as 'I' is set (according to the LED ratings) constantly, and since 'R' is also constant, the value of 'V' (the voltage between the end of LEDs) is automatically set according to the LED ratings. When more LEDs are being added serially, 'I' will remain the same, and 'R' will be increased accordingly, though. Increasing of “R” will also maintains the “V” between LEDs by increasing the value of voltage accordingly.

Is this Correct ?

 

I am reposting this picture from crutschow but I removed the MOSFET and replaced it with a Pot (variable resistor).
It appears that the LEDs require about 2V/LED to get them to glow.
The Pot adds some amount of resistance to control the amount of current through the LEDs.
Power flows from the supply (Vs) through the LEDs, through the Pot, through R4 or Rs to ground. (R4 is like a current meter)
Q4 looks for the right amount of current flow. (0.6V across a resistor) Q4 "turns" the pot to get the right amount of resistance to get the right amount of current.
View attachment 303919

Thanks ronsimpson

 

Hi @ranatungawk
Check out this link, note how the forward voltage of the LED is a function of the current through the LED, over its specified operating range.

E
http://lednique.com/current-voltage-relationships/iv-curves/
also

 

Hi @ranatungawk
Check out this link, note how the forward voltage of the LED is a function of the current through the LED, over its specified operating range.

E
http://lednique.com/current-voltage-relationships/iv-curves/
also

Thanks

 

Thanks BobTPH, you cleared my doubt! I was wondering, focusing solely on the LED aspect, how it controls the voltage between a LED or LEDs. According to the “V = IR“ What I understood is:

As long as 'I' is set (according to the LED ratings) constantly, and since 'R' is also constant, the value of 'V' (the voltage between the end of LEDs) is automatically set according to the LED ratings. When more LEDs are being added serially, 'I' will remain the same, and 'R' will be increased accordingly, though. Increasing of “R” will also maintains the “V” between LEDs by increasing the value of voltage accordingly.

Is this Correct ?

Nearly correct. At any specific current, the voltage across each LED will be a constant. But the relationship is not linear, as implied by Ohm’s law, because an LED not a resistor. If you double the current in a resistor, the voltage will also double, but if you double the current in an LED, the voltage will only go up by, maybe 10%, depending on where the current starts. This is part of the reason we control current instead of voltage for LEDs.

 

Nearly correct. At any specific current, the voltage across each LED will be a constant. But the relationship is not linear, as implied by Ohm’s law, because an LED not a resistor. If you double the current in a resistor, the voltage will also double, but if you double the current in an LED, the voltage will only go up by, maybe 10%, depending on where the current starts. This is part of the reason we control current instead of voltage for LEDs.

Thanks for the reply ! In other words, If current is fixed according to the LED's specifications, voltage between the LED will be set automatically, Correct ?

 

Correct. But note that the specification will be a range. White LEDs at 20mA might say the voltage is 3.2 to 3.4V.