On/off Mosfet. Voltage Control.

Discussion in 'Digital Circuit Design' started by jgreene44, May 1, 2017.

  1. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    Hello,
    Thanks in advance! My understanding this far. BJT, current controls the output. Mosfet,
    voltage controlled. Ok, am trying to simply turn on and off a mosfet with my Arduino's
    output. I have 28VDC @ the drain, source reading 3V. Verified I have 5V @ the gate.
    So If I am using 5V to the gate, mosfet is voltage controlled... Am I getting about what
    I am supposed to @ the source? Seems like maybe so. If that be the case. How would
    I unleash the full 28V with this setup? This video:



    Makes it appear that its really simply.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    6,420
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    How did you measure the voltage?
    Does the source connected to ground directly?
    How did you connect the wires?
    +28V → x → D → S → x → ground?
     
  3. EM Fields

    Active Member

    Jun 8, 2016
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    154
    You can't unleash the full 28 volts that way; you must connect the source to the battery negative, the drain to one side of the load, the other side of the load to battery positive, the gate to the Arduino output, and battery negative to the Arduino ground.

    That is, if your MOSFET is an N-channel enhancement mode device and if it can be driven into saturation with the Arduino's 5 volt output on its gate, like this:
    NMOSFET common source connection.png



    What's the MOSFET's part number or can you post a link to its datasheet, please ?
     
    Last edited: May 1, 2017
  4. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    Ok EM. The Mosfet is: IRF640.

    It is hooked up proper. As you say.

    Also. It is starting to sound again, like im getting what I should. Perhaps some kind of voltage loss when using such a
    mosfet is why im getting 3V out @ the source?
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    IRF640 is not a logic level MOSFET and you cannot fully turn ON this MOSFET with 5V gate voltage.
    You need to increase gate voltage to around 15V to turn it ON fully.
    You need additional components to do so.
     
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  6. jgreene44

    Thread Starter Member

    Dec 8, 2016
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    Ahhhh ok ok. Now I know the problem and also how to fix it. R!f@@ Thank you sir!
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I like this TS. TS called me "Sir". :D

    Well ! I was a "Sir" once. :p
     
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  8. EM Fields

    Active Member

    Jun 8, 2016
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    You _are_ getting what you should, but the reason isn't because of the vagaries of the MOSFET you're using, it's just physics and it's because, as you've explained earlier, you've connected the transistor as a source follower.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    My MOSFET turns on fully driving 80V motor from 5V
    MOSFET.png

    MOSFET's are IRF540
     
    Last edited: May 1, 2017
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  10. HW-nut

    Member

    May 12, 2016
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    TR1 is connecting MOSFET gates direct to 80 V. You need to reduce the gate drive voltage or you will kill the MOSFET's.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Did you not see zener ZD1?
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Sorry to disappoint you but circuit is running without a problem.
     
  13. jgreene44

    Thread Starter Member

    Dec 8, 2016
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    Ok EM, I am still with you! I am thinking the simplest and best solution is to go with a logic level mostfet? Or would a change
    in configuration make more since?

    On that diagram. The item listed PC817, I do not understand the symbol.
    What is that?
     
  14. EM Fields

    Active Member

    Jun 8, 2016
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    Using a logic level N MOSFET driven directly by the an Arduino I/O would, in my opinion, make the most sense since one could get the job done with only two parts: the MOSFET and a catch diode across the motor.

    It's an optocoupler and its data sheet is at:
    http://www.sharp-world.com/products/device-china/lineup/data/pdf/datasheet/pc817xnnsz_e.pdf
     
    Last edited: May 1, 2017
  15. jgreene44

    Thread Starter Member

    Dec 8, 2016
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    Sounds like I got it all answered thanks to EM Fields and R!f@@. !!. Glad you both where here. Thank you.
     
  16. HW-nut

    Member

    May 12, 2016
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    My mistake, I usually run the devices in saturation for level translators. Assuming the 80v supply is reasonably regulated, you could probably get by with a divider network and dump the zener regulator.

    Also, you may want to check the rest state of the MPU to make sure the motor is off during reset.
     
  17. R!f@@

    AAC Fanatic!

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    My 80V is not regulated hence the zener diode.

    I did not have a logic level MOSFET that could drive the motor so I compromised.
     
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