# MOSFET as switch instead of a transistor

#### 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?

Joined Mar 10, 2018
4,057
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.

#### ZwareValk

Joined Aug 14, 2019
3
Thank you, that is what I thought as well.
Good to know I'm not going crazy.

#### crutschow

Joined Mar 14, 2008
31,112
A small nit, but a MOSFET is also a transistor.

#### ZwareValk

Joined Aug 14, 2019
3
A small nit, but a MOSFET is also a transistor.
Woops, that's right.

#### Audioguru

Joined Dec 20, 2007
11,248
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.