Arduino to 74HC595 using MOSFETs

Discussion in 'The Projects Forum' started by allenpitts, Sep 6, 2016.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Working on a project that would sequence 320 LEDs
    using an Arduino Uno and shift register 74HC595
    based on

    Have run into problems with the max current that the Arduino Uno
    can handle.
    Note figure says "18 more SprkFun BOB10860" but the design is now 38 more.

    So it was reasoned that a BJT transistor could be used with a separate, higher voltage source to reduce the amps
    required by the 75HC595s.
    But even with the reduced current required to saturate
    the transistors the microcontrollers max was still
    getting in the way.

    Then I ran onto MOSFETS. Cool. The transistor is controlled by voltage instead of
    current so the current hassle should go away.

    So looking to acquire 320 MOSFETs research was done at DigiKey and Mouser. The Arduino can supply either
    3 or 5 volts. A MOSFET seen mentioned in several articles and tutorials is the Fairchild 2N7000.
    ( I have read everything I could find and understand including Mr. Kiem's excellent Understanding MOSFET On-State Drain-to-Source Resistance
    but am still getting my head wrapped around it.)
    It seems an important spec for the MOSFETs is the Gate Threshold Voltage (Vgs). The Vgs for
    the 2N7000 is .8 3 volts min. and 3 v max. So it would seem that even using the Arduino's lower power output would
    be bumping the max of the MOSFET. (I believe the Vgs is the point at which the MOSFET would allow the higher voltage source to
    flow to the LEDs, right?)

    If that is so, then using even the lower Arduino power source
    would be constantly bumping the 2N7000 against it's max 3Vgs.
    Another MOSFET is the ZVP2106A which, if the data sheet is
    read correctly has a 1.5 min Vgs and max of 3.5 v.

    I believe this is the MOSFET to get but would welcome
    input before I send Digikey my hard earned dollars.


    Allen Pitts, Dallas Texas
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    That Vgs(th) specification does not express the maximum gate voltage the 2N7000 can tolerate; when the data sheet lists Vgs(th) minimum and maximum, it means the range of gate-to-source voltage to begin turning the 2N7000 on. In other words, it says the part will begin to turn on at a gate-to-source voltage of somewhere between 0.8V and 3V. Any more voltage than that, merely assures the MOSFET will achieve a lower Rds(on).

    The maximum gate-to-source voltage the part will tolerate is something else entirely; it is given in the Absolute Maximum Ratings section of the data sheet, as ±20V.

    So no problem.
    ronv likes this.