NPN exercise - math is not matching the LTSpice simulation

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
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!
 
Last edited:

Jony130

Joined Feb 17, 2009
5,487
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 ?
 

Jony130

Joined Feb 17, 2009
5,487
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.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
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!
 

Jony130

Joined Feb 17, 2009
5,487
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.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
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
 
Top