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Thread: About Using P-Ch Mosfet/Diode to Switch between USB power and Battery Power

  1. #1

    About Using P-Ch Mosfet/Diode to Switch between USB power and Battery Power

    Hey guys,

    I'm wondering, is there a major practical difference between these two methods of switching from battery to usb power:

    P-ch MOSFET with Shottky diode:
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    Just Schottky diode:
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    I know circuit 2 drops the battery's voltage, but for my project that's not an issue. My only goal is that no power can flow out of the battery
    when usb power is attached, which (I think) should be the case in circuit 2, since Vbat will always be at a lower voltage than Vusb.
    Am I correct on that? Are there any possible issues with just using a single diode to switch between battery and usb power?

    Thanks!

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by Electric Potato View Post
    Are there any possible issues with just using a single diode to switch between battery and usb power?
    Usually 2 diodes would be used.

    With only 1 diode, when the USB cable is plugged into a computer with the power turned off, the battery can try to deliver current back into the powered-down computer.

  3. #3
    Ah that makes sense, thank you!

  4. #4
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    Quote Originally Posted by PaulStoffregen View Post
    Usually 2 diodes would be used.

    With only 1 diode, when the USB cable is plugged into a computer with the power turned off, the battery can try to deliver current back into the powered-down computer.
    Hi Guys, currently working on a joint hardware/software project. I ended up choosing the Teensy 3.6 due to some features. I am having some difficulty understanding the Vusb and Vin.

    Question 1: If I cut the track between Vusb and Vin, can I put a SP3T switch so in Position 1, it connects Vusb and Vin (Powered by the microUSB), Position 2 is off(not connected) and Position 3 is connected to a battery (not directly but through a Linear Charger)? Please see the attached image for details.

    Question 2: If this is possible, then I know the controller is limited to 250 mA but can Vusb supply more current? The orange arrow in the image indicates a connection to the charger. Would this be possible? Thanks in advance!

    Click image for larger version. 

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