Turning on/off a part of the circuit (with 12V and 2A current) using MOSFET.

[pd123]

I am working on a project which requires me to start and stop a supply of 12V, 2A (source available) to a part of the circuit using Power MOSFET-IRLZ24 and micro-controller. I am a bit too confused about how to design the circuit. I am using PIC18F46J11 micro-controller and am planning to control the gate voltage directly using a pin on the micro-controller. I saw a few circuits on the internet, but I could not understand their working.

Can anyone please suggest any simple circuit, or tell me how to get started. Apart from the MOSFET will I need another BJT or MOSFET like the circuit shown in the link below?

https://www.edaboard.com/threads/145111/

Can anyone please explain why was it used?

PIC18F46J11 works on 3.3V, and the sink and source current on portB and portC is 25mA.

 

[barry]

That circuit uses a P-channel mosfet; the IRLZ24 is an N-channel and will not work in that application. The P-channel device turns on when the gate is at a lower voltage than the source (when the BJT is on). When the BJT is off, the gate is pulled to source potential by R197. You might need a smaller value for R197, depending on leakage of the BJT, etc.

 

[alexan_e]

Your mosfet wan work fine with 3v3, assuming that you are not interested in fast switching (pwm etc) you can drive the mosfet directly.
Use a 100 ohm resistor from the pic to the gate , connect a resistor of 10k from the gate to the source, connect the source to the gnd and the drain as output to the load.
This will operate as a low side switch, your load will be connected to the positive supply and the mosfet will control the ground side of the load supply.

Alex

 

[barry]

Yes, this will work as long as the load is not ground-referenced.

 

[pd123]

I came across this schematic in one of the threads. I do not understand why they have used two MOSFETs if switching can be done using one as has been described in this thread. Can anyone please explain?

And please tell me what does "ground referenced" load mean?

 

[alexan_e]

And please tell me what does "ground referenced" load mean?

The low side switch I have described uses a load that is connected to the positive supply directly and to the ground through the mosfet that will operate as a switch, this is a ground referenced load , the load supply is connected (referenced) to ground.

About the two mosfets in the design I don't know why exactly they were used, maybe it has to do with the output characteristics of the specific mcu or the driver mosfet is used to provide a higher current and turn on the output mosfet faster.
Another reason may be that the mcu supply will be slightly less than the mosfet supply (because of the diode and then the mosfet Vgs drop) and maybe this would create problems.

Alex