MOSFET as switch instead of a transistor

Thread Starter

ZwareValk

Joined Aug 14, 2019
3
We were given the circuit in the image below. With the following data:
Vin = 0V or 5V
Vcc = 12V
β = 50
The desired current through the LED is 50mA and the voltage over that LED is 2.7V in that case.

Then we were asked what would happen if the npn-transistor was replaced by an n-channel MOSFET and if a Vin of 5V would still be sufficient. This is the part where I'm having trouble: How can I tell if 5V would have the desired effect without a datasheet or anything given about the MOSFET?
What I know:
With Vds is the voltage over drain-source, Vgs is the voltage over gate-source and Vt is the voltage threshold of the MOSFET.
Of course Vgs is where the 5 Volts would be applied.
  • Saturation occurs when Vds >= Vgs - Vt
  • ID = K(Vgs - Vt
However, this doesn't seems insufficient to solve this problem.
Where could I start to solve this?



upload_2019-8-14_15-40-8.png
 

danadak

Joined Mar 10, 2018
3,682
You have to have the datasheet for the MOSFET to tell if it is
a logic level MOSFET and whats its Rdson is as well to do
the design.


Regards, Dana.
 

Audioguru

Joined Dec 20, 2007
11,251
You were told that the hFE of the original transistor is 50. But hFE is used when the transistor is a linear amplifier with plenty of C to E voltage. To turn on the LED then the transistor must saturate with a very low C to E voltage when hFE is not used. The datasheet for most little transistors shows that they saturate well when the base current is 1/10th the collector current.

Most Mosfets conduct well when the gate to source voltage is 10V. But some Mosfets are called "logic level" and conduct well when the G to S voltage is 4.5V or 5V.
 
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