Using Ardunio and MOSFET to drive 5V to 12V

Hello,

I am working on a project which involves using an Arduino Mega, and I am trying to use an NMOSFET (BUZ101S) and an NPN BJT(2N3904) to drive the 5V output from the MCU back and forth between 0V and 12V across "SOL".

At the moment, the problem I have is that Vgs always stays high, thus driving SOL on all the time, and I cant figure out why this occurs.

Here is my circuit diagram:

https://i.imgur.com/ls9l9oa.jpg


Any help is appreciated. Thanks!

Okay one problem is that you have the NMOS instead of a PMOS. BUZ101S is an enhancement mode NMOS transistor. So if a voltage higher than the threshold (3V) is applied at the gate it turn on or creates a connection. You have a voltage divider between the 5K and 100K/50K. This will produce a voltage larger than 3V assuming SOL has no voltage drop.

What is SOL? What is the desired current flowing through SOL?

I think you need a PMOS or to move SOL above the NMOS transistor. The reason for this that an gate input voltage of 3 volts higher than the voltage drop across SOL. If you want to pull a larger current you will need more. Depending on what SOL is this will cause a larger voltage drop on SOL. So you may not be able to control it. Depending on what SOL is, moving it above the NMOS will not cause any problems.

In your circuit configuration the NMOS will always be on normally because you are using an NMOS with a voltage divider. If you use a PMOS it will be normally off.

Your Mosfet is connected as a follower so when its gate voltage is +4.5V then its source voltage is 0V to about +1V.
I see nothing in your schematic to make the output high unless the D and S of the Mosfet are connected backwards.

If you want the Mosfet to switch the load on and off then connect the S to ground and connect the load between the D and +12V.

Using your config and above suggested solutions, you could drive the current across the SOL in only one direction. If you wish to control the direction of current aswell, you could use the H bridge config, where 4 MOSFETs are required to drive one load..