Yes, I "measured" it in LTspice.

 

How have you measured the VLED???

12-R4*Ic-V(Vcol2,Vem2)-R5*(Ic+Ib)=Vled

 

Yes, I "measured" it in LTspice.

I was asking where have you placed the probes to measure it or if you have calculated it.

12-R4*Ic-V(Vcol2,Vem2)-R5*(Ic+Ib)=Vled

Thanks mate!


I tried to place 1st probe before the LED and the 2nd probe after the LED to measure the voltage drop but It says 0V.

 

You can click on one node and drag the mouse to another node. You will see the red voltage probe at the first node and a black probe on the second. This allows you to differentially plot voltages.

Also why you using transient analysis to do the DC operation point ?

https://www.youtube.com/watch?v=FEGT5dUpdrc

 

Ok, I don't know why I have tried that and it gave me 0V. Now I managed to do it in precise the same way and it gave the correct Vdrop in that LED.

Edited;

Is there any big difference between Ie = ((β+1)/β)Ic and Ie = Ic + Ib???

I'm asking this because I'm trying to get the VLED of 2V as I have assumed before but I can't and I'm not using Ie = Ic + Ib but Ie = ((β+1)/β)Ic.

Forget it, I got it!

 

Is there any big difference between Ie = ((β+1)/β)Ic and Ie = Ic + Ib???

You don't need to know Ib to find Ie.
But for this circuit we can find Ie from inspection:
Vz - Vbe - Ie*Re = 0 ----> Ie = (Vz - Vbe)/Re

I'm asking this because I'm trying to get the VLED of 2V as I have assumed before but I can't and I'm not using Ie = Ic + Ib but Ie = ((β+1)/β)Ic.

But what this Ie = ((β+1)/β)Ic has something to do with VLED ?

 

You don't need to know Ib to find Ie.
But for this circuit we can find Ie from inspection:
Vz - Vbe - Ie*Re = 0 ----> Ie = (Vz - Vbe)/Re

Indeed... thanks for the tip!

But what this Ie = ((β+1)/β)Ic has something to do with VLED ?

I was using the "output mesh" to calculate VLED and I need Ie there!

But I got it, thanks...

 

I was using the "output mesh" to calculate VLED and I need Ie there!

But I got it, thanks...

But you know Ie, because the circuit act just like a constant current source.
But I still don't see how can you make VLED = 2V? You need to change a diode LTspice model or replace diode with 2V voltage source.

 

But you know Ie, because the circuit act just like a constant current source.
But I still don't see how can you make VLED = 2V? You need to change a diode LTspice model or replace diode with 2V voltage source.

Well, we assumed 2V because the exercise didn't provided that info and as we know it can go from 0.7V up to 5V or more we picked up a value within that range!

 

Ok I see, but the LED forward voltage depend on the LED color.
And it's start from 1.6V for IR or red and end at 4V or so.

 

Morning...

Thanks to all that helped...

I did the exam yesterday and after all, no math was needed. It was only asked to setup a given circuit and write down some measured voltages and currents!

Psy