batteriesgpiomosfetpowerswitches
I'd like to use 3v microcontroller to activate a small fan after the microcontroller wakes from sleep. I'm not quite clear on the placement of the load (i.e. fan) and the LiPo battery in a MOSEFT schematic. My understanding is that I can use an N-chanel MOSFET to create a closed circuit between the LiPo and fan by supplying voltage via GPIO to the gate. Am I correctly understanding this? Below is my circuit diagram. Thanks for any feedback.
I'm taking a guess that the B772 (PNP) collector is connected to the +12V terminal of the fan and that the circuit switches this on and off as a PWM control of speed.
To replace the NPN transistor use a P Channel MOSFET. This needs to be switched ON by grounding the gate so an inverting transistor is needed.
When the B772 is turned ON a small current flows through R1 and R2 and Q1 is turned ON pulling the gate voltage to about 0.1V. This turns the MOSFET ON this supplies current for the fan. There are lots of MOSFETs out there that would be suitable.
Be careful you don't overload the power supply of the controller
That LED being in the emitter - to - ground connection of Q3A is going to give you some grief since you've only got 3.3V (less actually because of the R7 R8 voltage divider on Q3A's base) to drive Q3A with.
Fix it by connecting Q3A emitter directly to ground, lose R8 and drive the base through R7, and connect the LED from the source of Q1 to ground through a suitable ballast resistor.
From your question:
"OUTPUT4 is the high power load output, which is connected to the Battery negative terminal through the system's common ground." might be better worded to indicate that one end of the load - not Q1's source - will be connected to battery minus.
Best Answer
Your general idea is correct. I would like to clarify a few points:
The FAN connector is the other way round (the top pin is connected to the "+" of the battery, thus should be labelled "+"), while the bottom pin will be brought to ground when the MOSFET closes.
By "3v microcontroller" I suppose you mean "3.3v". In any case, the MOSFET you choose must have a threshold voltage lower than 3v. For example, the FDN338P MOSFET has a 2.5v threshold voltage. EDIT: apart from the threshold voltage, you have to make sure that the MOSFET will be able to handle the current that goes through your fan (this one has 1.6A maximum continous drain current, which should be fine for a small fan), and should also have a low conducting resistance (Rds) while being driven from 3.3v. This one has 155mΩ at 2.5v, which is great. Thanks to Spehro Pefhany for pointing this out.
The idea that you're powering a fan from a separate power source (battery) is a bit strange: if your microcontroller is powered by another battery, but you're doing this because the fan requires another voltage, it would be a better idea to use a 3.3v LDO to bring stabilised power to the microcontroller from the same battery. This way, you will have exactly 3.3v powering the microcontroller, instead of the fluctuating voltage it gets by being powered directly from a battery.
EDIT: To expand on my second point, I would like to add that a MOSFET might not be necessary in your circuit, and could be replaced by a regular NPN bipolar transistor (BJT), since a MOSFET with such a low threshold voltage might be hard to find.
simulate this circuit – Schematic created using CircuitLab
Since the BJT, unlike the MOSFET, "opens" when there's a current between the base and the emitter, we use a resistor (R1) to limit the current.
EDIT: Another valid point (thanks to Icy) is that it would be a good idea to add a flyback diode across the fan - when the motor (or any inductive load) is turned off, it becomes a generator for a short time, because the magnetic field induces a current back into the coil, causing a huge momentary spike. The diode will suppress those spikes.