Is it OK to PWM logic level MOSFET straight from μC?

Discussion in 'The Projects Forum' started by urb-nurd, Mar 13, 2015.

  1. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    3
    As the title says - I can see a-lot of people doing this on forums etc.

    This being: the omission of galvanic separation.

    I need to PWM a 24V DC fan fan from my micro-controller .

    I have already sorted the power supply for the fan, but i was digging through my box and found a suitable MOSFET that will be more than open enough for my use at 4V.

    Though, if I hook the PWM output up to the gate terminal with a suitable series resistance and resistor to source - then i will need to connect the source of the MOSFET controlling the fan to VEE of my micro-controller board.
    This means that one of the terminals of my power supply will be connected directly to my micro-controller, which has me questioning weather or not i will need an opto-coupler.

    The schematic below shows my circuit configuration. (two zener diodes are 12V to give 24V)
    Circuitwithconverterandfan.PNG

    If i were to use an opto-isolator, then I would need to generate another DC level at 5V for gate driving of the logic level FET. Which is of course MORE work, though i would prefer that to a blown Micro-controller.
    A voltage divider across the 24V DC would suffice for providing 5V at <1mA that the FET needs, this has just occurred to me.

    Any insight on the matter would be more than helpful.
    Thanks!
     
    Last edited: Mar 13, 2015
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,565
    2,379
    It seems to me you have sufficient isolation, just the the DC fan supply should be shown as a separate supply (24v)?
    Although you could use two supplies with commons connected?
    Max.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,067
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    @urb-nurd
    You are way overthinking the problem. It is no problem to add the fan to the same common (0V) as the micro. Just add a reverse-biased diode across the fan motor terminals.
    Add a nice 7805 for the 5V supply to the micro and PWM to your heart's content.
     
  4. #12

    Expert

    Nov 30, 2010
    16,343
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    The omission of galvanic separation is the rule, not the exception.
     
  5. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    Why would i use more than one supply if i can derive all the power levels required from one? (genuine question)
    A diode is already present on the schematic, unless you are talking about adding a second one.
    The Microcontroller is actually a ready for PIC board from MIKROELEKTRONIKA, so it already has a supply regulator.
    I am currently powering the MCU board via USB as i am sending variables and measurements over serial for graphing my PID performance.
    Thanks for the info!
    I see - wise words as always.
    Thanks all!
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,565
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    Fine, but whether you use opto isolators or not, the commons will still be the same.
    This is often done with AC & DC loads and much higher voltages.
    Max.
     
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