Hello everyone!
Happy New Year!
I have a question:
What could i use to switch +12(not the low side, thats important) from car battery to RBPi power supply using arduino?
Right now i am using a relay module, but i am trying to build a dedicated board for my project and need it to take as little space as possible
I was thinking about p chanel mosfet but i always had troubles with them
Any input would be appreciated
It would be extremely helpful to tell us what those problems were. Otherwise we'll tell you to use a P channel MOSFET and you'll have all the same problems...
Circuit F1 or H1 can HIGH side switch 12v
Thank you for your responses!
I guess the problems was my negligence regarding how to use them properly...
So far i come with this design, could you tell me if its good or maybe i should change anything? Maybe the resistance should be different?
RpiMainPwr is the arduino output pin and 12vToPiPsu is where the Rpi buck converter would be connected and should have 12v if RpiMainPwr is High
Thank you!
What is that?
"Better" DC/DC converters have an enable pin to switch the converter on/off,
so you don't need a high-side mosfet switch.
Leo..
The one i will use(Meanwell NSD15-12S5) actually have the control pin, however used this way, it do consume 20ma in standby mode... thats why i need to cut the power completely to it
You need to flip the P channel MOSFET.
Follow the schematic F1 offered to you in post #3 !
Thank you Larry!
Was not sure at first what F1 or H1 mean.. sorry.. but now i do
What about the resistance? Should i use the same as in F1 or i have to calculate it somehow?
By the way, IRF9540N is a bit big for my application, is there any sot-23 you could suggest?
Thank you!
See these logic level MOSFETs
Since you are using an Arduino to turn on a P channel MOSFET, the value of the BJT resistors are not too critical.
The important thing is to measure the Source to Drain voltage when the MOSFET is turned on.
This voltage should be very low, probably less than 0.1Volts.
Thank you Larry, AO3401 should do, however not sure i do understand the part of 0.1v...
In examples like this, we want the driver transistor to be fully turned ON when switching power to the load.
Fully turned ON means the transistor should have almost zero volts across the device.
For a BJT we want the transistor to have a very small Vce voltage (voltage collector to the emitter).
For a MOSFET we want the transistor to have a very small Vds voltage (voltage drain to source).
The smaller the voltage across the transistor, the less power it consumes, the less heat it gives off.
Lets say you have a Vds of 6 volts in this 12v power supply project; lets say also that the current flowing in the transistor is 1 amp.
The power consumed by the MOSFET would be P = V * A = 6V * 1A = 6 watts OUCH!
If this same MOSFET only had 0.1 or 0.01 volts the power dissipated would be 0.1 and 0.01 watts respectively.
With the AO3401 the maximum Gate-Source Voltage is +-12V.
Since your 12v supply is at the top of the maximum range for this MOSFET, the circuit below would be recommended.
Note: you can play with the resistor values to reduce power consumption if it is an issue.
ALWAYs measure the Vds of the AO3401 to make sure it is saturated/fully turned ON.
If this voltage is too high, you may have to adjust component values.
Thank you Larry! Still a bit confusing but i am slowly getting there
Will go with your suggestion, make my board and see from there
By the way, you are mantioning that my 12v supply is at the top of the maximum range for this MOSFET, how ever it wont be power from a battery but from the car witch can have 11.6-13.4v
Does that mean i am better to go with AO4409?
What a question! You should never exceed the rated specifications of any electronic device. If they were not important, no manufacturer would bother to measure or publish them.
I am just not sure if its something i would be able to manage using some resistors or not, i am still a bit confused about transistors 
A very good reason to fully explain what you are trying to do...
The project goes in a car to be connected to the car multimedia to enable android auto on the stock car screen
The project consiste of arduino with 4 canbus interfaces + raspberry pi
Raspberry pi does provide phone connection + all the video/audio streaming
Arduino does handle canbus camunication between car and pi and manage the power for the pi based on the canbus activity
Right now its is builded from modules but i am trying to make a proper one piece board for it to make it cleaner and more reliable
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