Mosfet basic circuit

[e44-72]

Hello

I don't know much at all about mosfets and wanted to ask a question about the following N-channel mosfet circuits.


If the load is after the transistor should gate be help at ground like this circuit:



Or held at source like this circuit:



which of the two circuits is correct?

thank you for reading and any replies

 

[jpanhalt]

Neither is correct. Remember that the voltage drop across the mosfet is low when it is fully turned on. Thus, the entire voltage drop is across the load (R5 or R1 in your examples). In both circuits, that would make the Vs (source voltage) approximately equal to Vcc or 9V. Thus, a Vg of 9V relative to ground will not turn the N-mosfet on.

John

 

[crutschow]

Both circuits will work so both are "correct". The lower one has the advantage that the current through R6 goes through the LED and is not wasted.

Not that these circuits are source-followers and thus the voltage at the source output when the transistor is ON will be below the battery voltage by the gate-source voltage required to turn on the transistors. If you want the transistor to be fully on as a switch, you either need to put the transistor in the ground side of the LED or use a P-MOSFET.

 

[2001dragon]

Neither is correct. Remember that the voltage drop across the mosfet is low when it is fully turned on. Thus, the entire voltage drop is across the load (R5 or R1 in your examples). In both circuits, that would make the Vs (source voltage) approximately equal to Vcc or 9V. Thus, a Vg of 9V relative to ground will not turn the N-mosfet on.

John

Why is N-mosfet not on if Vg is 9V relative to GND?

Both circuits will work so both are "correct". The lower one has the advantage that the current through R6 goes through the LED and is not wasted.

Not that these circuits are source-followers and thus the voltage at the source output when the transistor is ON will be below the battery voltage by the gate-source voltage required to turn on the transistors. If you want the transistor to be fully on as a switch, you either need to put the transistor in the ground side of the LED or use a P-MOSFET.

What transitor are you talking about? I don't see it in the diagram

 

[jpanhalt]

Why is N-mosfet not on if Vg is 9V relative to GND?

The mosfet doesn't care what ground potential is for the purpose of turning on or off. What it sees is the the difference between the gate and source (Vgs). Of course, when the source is connected directly to ground, then Vgs is the same as the potential difference between gate and ground.

Of course, my assumption in the circuits above is that the mosfet is intended to act as a switch. That is, be almost fully on or off and not have significant (i.e, Vds >Vth) voltage drop across it.

If yow want to source current with a mosfet switch, it is often easier to use a P-channel mosfet as pointed out in post #3. You can also use a gate driver to make the gate voltage above the drain voltage.

John

 

[crutschow]

.........................

What transitor are you talking about? I don't see it in the diagram

I am referring to the MOSFETs (Metal Oxide Silicon Field Effect Transistor).

 

[audioguru]

Every Mosfet has a part number but yours has no part number so we don't know which one you are talking about.

Most ordinary N-channel Mosfets are completely turned on when the gate is 10V higher than the source voltage. So in your circuit for the source to be at +9V then the gate must be at +19V.
Since your gate can go only as high as +9V then the Mosfet might have a source voltage of only a few volts. Then the LED might or might not light.

 

[e44-72]

thanks for the replies and helpful information