Switch for board modules (easy)

Discussion in 'General Electronics Chat' started by brianmay27, Mar 29, 2014.

  1. brianmay27

    Thread Starter New Member

    Oct 7, 2011
    7
    0
    Hey all,

    I have a simple question which I think my current design will work but I wanted to check.

    Im designing a PCB and have a bunch of modules on the board (Bluetooth, RFID etc) To save power I want to be able to turn them off. Im using a pic uC to control everything. The pic runs 3.3v and the modules 5V

    My current design is to have a NPN with C connected to 5V, b connected to the 3.3 logic and e to a resistor. The resistor is connected to a pmos gate, the source is 5V and drain is connected to the Vdd of the device.

    Is this a proper way to design a mos switch? Or do I not need the transistor? Again this should be easy but I want to make sure before I send the PCB to be printed.

    Thanks!
     
  2. #12

    Expert

    Nov 30, 2010
    16,295
    6,806
    Your words seem different from this drawing.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    I'd do it like this:

    [​IMG]

    You also need to insure one section does not power the other thru signal lines via the EDS diodes most all IC pins have.
     
  4. brianmay27

    Thread Starter New Member

    Oct 7, 2011
    7
    0
    Thank you both for your reply. I think I will go with Ernie's method, I worried using just mos gates as I was unsure about the 5V to 3.3 turn on but using the resistors in there look like it will do just fine.

    Thanks for the help!
     
  5. #12

    Expert

    Nov 30, 2010
    16,295
    6,806
    You can get a very good result with 2.7 volts on these mosfet gates.
    Surprising how far they have improved in a few years!
     
  6. brianmay27

    Thread Starter New Member

    Oct 7, 2011
    7
    0
    Good to know. I just had in my mind that you needed some voltage close to the source to power on. Good to hear from others who have more EE experience than my CpE major taught. That's why I thought a transistor would get around that. I guess reinstalling a spice program would really help to simulate before realizing its all wrong :)
     
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