Attached circuit - why transistor not saturated_Vce = 1.9V

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mishra87

Joined Jan 17, 2016
1,034
Hi ,

Kindly find attached schematic and let me know where i am doing mistakes.

VF_led = 1.8V
Rc= 100 ohm
IF_led = 15mA
Transistor = MUN5211DW1
Vb= 3.3V from MCU

Tested the circuit and Vce = 1.9V

Please guide me

Thanks !!!
 

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Jony130

Joined Feb 17, 2009
5,484
Well, I think that your transistor is saturated. The Vce is equal to 1.9V when the BJT is cut-off. And this voltage does not reach 3.3V due to the LED Vf voltage drop (1.4V) when a very small current is flowing due to scope probe loading.
 

danadak

Joined Mar 10, 2018
4,057
The datasheet says .3 mA base current for Ic = 10 mA.

The current thru the input R = (3.3 - Vbe) / 10k =~ 270 uA

The current thru the internal pull down is Vbe/10k =~ 60 uA

So Ibase = 270 - 60 = 210 uA, and thats not worst case, just nominal values in above
calculations.

So transistor is not in sat as you have observed.

"Normal" rule of thumb for saturation is Ib = Ic/10

This part with internal Rs on a 3.3V processor not a good choice.


Regards, Dana.
 
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