# Find LED current in switching circuit

Discussion in 'Homework Help' started by ShowerOf, Nov 17, 2009.

1. ### ShowerOf Thread Starter Member

May 4, 2009
13
0
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 ?

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
Yes, you can assume that Ic=β/(β+1)*Ie
So Ie≈Ic in active region.

3. ### ShowerOf Thread Starter Member

May 4, 2009
13
0
To find the power dissipated by the transistor.
using calculation based on another thread

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 ?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
Yes, look ok for me.

5. ### ShowerOf Thread Starter Member

May 4, 2009
13
0
Thank you Jony130, I am grateful to have somewhere to get help and

Cheers

6. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
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.