Using Mosfet to "switch on" whole circuit, 2 voltage levels

Discussion in 'The Projects Forum' started by KonradIC, Dec 25, 2015.

  1. KonradIC

    Thread Starter New Member

    Oct 26, 2015
    16
    0
    Hello,

    I want to turn on my circuit consisting that uses two different voltage levels (5V for powering MCU and 12V to power other load).

    Normally i would simply use DPST or DPDT switch. Unfortunately, the type i want to use, rocker switches are very big while in this particular case i need to use switch as small as possib. I decided to go with small slider switches but it was no go, the 12V powered load will be taking up to 12A of current which will most likely fry little switch.

    So i figured out i could use single SPDT slider switch and connect it to GND, OUT and 5V where OUT would drive two N-Mosfet transistors where each would be delivering 5V and 12V.

    This is the initial schematic where i still planned to use DPST switch it was no go because of switch size:
    http://i.imgur.com/oVQOKIT.png

    This is my usual schematic when i want to use mosfet as a switch, i deliver 0 or 5V on the gate resistor and when the input comes from MCU i add in extra pull-down resistor :
    http://i.imgur.com/xDWLBeF.png

    Using above schematic as example i tried to rewire my main switch circuit to use single SPDT switch and 2 mosfets to turn on 12V and 5V. The main switch was removed. S1 is SPDT switch, it will deliver either 5V or 0V (GND) to mosfet gate (not sure if i should add pull-down resistor back to schematic):
    [​IMG]


    My question is, is this way in which i connected Mosfets valid? I usually used them just to switch on/off single load and never tried using them to turn off/on a whole circuit.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    Since you are using the FETs as high-side switches, the gate voltages will need to be about 10V volts above the source voltages (not ground) to turn the FETs on fully. So the 5V switch and 12V switch would need gate voltages of about 15V and 22V respectively above ground.
    Try the FETs as low-side switches instead.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    If you need high-side switches, use P-MOSFETs instead of N-MOSFETs.
     
  4. KonradIC

    Thread Starter New Member

    Oct 26, 2015
    16
    0
    I redesigned circuit to use N-Mosfet as low side switch, this is overall schematic, i know its zoomed out too much but just wanted to give idea how it looks, but the point is, both Vin 12V and Vin 5V will be connected directly to circuit that they need to power
    http://i.imgur.com/MZ9VR6G.png

    but then, every single GND point from circuit (12V and 5V share common groun) will be gathered in one place i called MOS_D_OUT (but wont be have direct connection to GND from power source) and MOS_D_OUT will be connected directly to N-Mosfet Drain pin and there a signle N-Mosfet transistor with a single SPDT switch on gate (that will connect to it either 5V from power source of GND from power source) will be switching the MOS_D_OUT (GND points from main circuit) to the GND from power source and so it will be turning whole circuit on (schematic below)

    [​IMG]
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    That should work, provided the 'ground' points for both the 5V circuits and the 12V circuits are in fact the drain of the MOSFET rather than the 0V rail.
     
  6. Picbuster

    Member

    Dec 2, 2013
    374
    50
    enclosed a sample high and in low.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Please attach your file to the forum next time, I already attached the files in #1,#4, if the files missing then the thread will be lose the value of reference.
     
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