Use PMOSFET to drive motor

Discussion in 'General Electronics Chat' started by odimachkie, Jan 26, 2014.

  1. odimachkie

    Thread Starter New Member

    Jan 26, 2014
    4
    0
    Hello
    (I AM A NOVICE)

    I've attached the circuit for reference.

    I'm trying to power a 12VDC pump with 6-10V through a p-mosfet.

    i use a summing circuit to get the levels (0-5) from the Arduino, amplify it by 2.4, then use a follower to "equalize" the voltage on the power circuit to the amplified output. At least this is my intention.

    I'm finding that when it's all connected, i always have 12 volts on the power circuit, regardless of input.

    what i expected to see what that the non-inverting input of the opamp would read 'actual' voltage, and inverting input would read 'set' voltage, then the output would change the output to 'equalize' the non-inverting output to the inverting one.

    if i have this all wrong - please correct me.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Here is how I have done it: Most PFets have an allowable gate to source voltage of -12V or less. This circuit limits Vgs to about -10V. This circuit will be much faster than relying on the slew rate of opamps.

    In your circuit, the resistors around the first opamp are way too low...
    How high will the output of your chosen opamps pull when trying to turn off the PFET?
     
    • D151.jpg
      D151.jpg
      File size:
      192.2 KB
      Views:
      24
  3. odimachkie

    Thread Starter New Member

    Jan 26, 2014
    4
    0
    Your circuit looks great. if the method i have chosen is completely unsalvageable, i'll definitely study yours and make it work for me. (does your power circuit voltage match the BJT input*2.4?

    I'm sorry but I don't really understand your question. How high will they pull?

    Would like to make my setup work without changing too much. is that even possible?
    Sorry.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,233
    Are you trying to make the pump voltage variable between 6 and 10 volts or just turn it on and off?
     
  5. odimachkie

    Thread Starter New Member

    Jan 26, 2014
    4
    0
    I need it to be variable.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,233
    I think you were after a source follower. To do that you need to put the load (Pump) in the source side of the FET. Once you do that you will need to invert your input. But you could just use a NFET and leave everything else the same.
     
    Last edited: Jan 26, 2014
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    My circuit is a simple on-off switch as would be used with PWM; not a linear amplifier.

    If you want a linear amplifier, how much current does it take to run the pump flat-out at 12V? You are likely to need a huge heat sink for the PFET.
     
  8. odimachkie

    Thread Starter New Member

    Jan 26, 2014
    4
    0
    3 amps at 13.6V... at 12 a little less.

    i had switched it with a Nmosfet, thinking that should work... little did i know i the Nmosfet was blown.

    got another one on a hunch and bingo. works great!

    Same exact circuit i posted originally with the P-channel swapped with a N-channel.

    thanks so much for your help dudes.
     
Loading...