For the switching circuit shown:
Vcc = 6V
Base supply voltage (when ON) is 3.6V
The LED has a forward voltage drop of 2V
The transistor has a Vbe = 0.7V

Question: Calculate the current through the LED when the switch is on.
My calculations:
Ve = Vbb - Vbe
Ve = 3.6 - 0.7
Ve = 2.9V

Ie = Ve / Re
Ie = 2.9 / 180
Ie = 16.11mA

So can I assume that current through the LED is approximately equal to Ie ?

 

Yes, you can assume that Ic=β/(β+1)*Ie
So Ie≈Ic in active region.

 

To find the power dissipated by the transistor.
using calculation based on another thread

http://forum.allaboutcircuits.com/showthread.php?t=7426

Vc = Vcc - VLED
Vc = 6 - 2
Vc = 4V

Vce = Vc - Ve
Vce = 4 - 2.9
Vce = 1.1V

P = Vce * Ie
P = 1.1 * 16.11mA
P = 17.72 mW

Is this correct ?

 

Yes, look ok for me.

 

Thank you Jony130, I am grateful to have somewhere to get help and
get my answers checked.

Cheers

 

If you have 3.6V on the base of this transistor you will have 16.7ma through the collector. This configuration is a constant current source (BJT excel at those).

If the power supply were 2V greater you could use any color LED, as it will still be a constant current of 16.7ma.

Figure 3.6V (base) - 0.6 (BE drop) = 3.0V at the emitter
3.0V/180Ω = 16.7ma.