Using 2n7000 as switch with 3.3V MCU

Discussion in 'General Electronics Chat' started by aamirali, Sep 6, 2014.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    can we use 3.3V Vgs to turn on/off 2n7000 mosfet. I am using mosfet as a switch.

    1. datasheet shows that vgs(th) of 2n7000 is 2.1V min.

    2. I had given 3.3V from MCU to gate & source is grounded but mosfet is not turning on.

    3. Circuit is to drive a relay as we normally use npn transistor . But I had replaced with 2n7000.
    No success till now
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    In the datasheet is given that the Vgs should be at least 4.5 Volts for switching the fet on:

    [​IMG]

    Bertus
     
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi aamirali,
    As you say the d/s indicates a threshold of approx 2Vgs, which will give a Drain/Source current of approx 100mA at 3.3Vgs.
    What is the relay coil specification.? and do you have other 2N7000 on the bench that you could select from,?
    E
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If the Vcc for relay is 5V then you still got the chance to drive the mosfet using 3.3V.
    The idea as below, you can try it :
    3.3V → (-)Diode(+)(-)Diode(+) → Vgs (2N7000)
    Vgs adding a 2.2K ~4.7K to +5V.
    The diodes are using 2 1N4148 in series.
    If it doesn't work then please describe the status.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    Despite the stated gate threshold figure, you probably need about 5V to guarantee the full headline drain current/RDSon.

    With a bipolar output, you could also be worried about Vo-low.

    A common trick is to tie an external resistor either from Vdd to output or from output to Vss, depending whether the output doesn't go high enough or doesn't go low enough.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    Do you have any reason when the Vds is 5V, the Vgs must need 5V?
    The typical Vgs = 4.5V.
    When mcu output a 3.3V then Vgs = 3.3V+(0.65V*2) = 4.6V, so 4.6V>4.5V, that will be enough for Vgs.

    The typical Vgs threshold voltage is 2.1V.
    A mcu output 3.3V then I will assuming that is CMOS type, the output low level close to 0V, two diodes voltage equal to 0.65V*2 = 1.3V, so 1.3V<2.1V, and the OP mentioned that when the mcu ouptut a 3.3V still can't turn on the Rds, so I think 1.3V as a low level is safe for the Vgs(th).

    Have you ever saw any load as relay was used like that way?
     
    Last edited: Sep 7, 2014
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    You have to look carefully at the datasheet to see exactly what RDSon and Id you get at just the VGSthr and no more.

    If the CMOS Vo-low goes down to 0V, you should have no trouble turning the MOSFET off - I wouldn't rule out maybe needing to assist V0-high with an external pull up resistor.

    I never mentioned the relay at all - only the interface between the chip and the MOSFET.
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    I was used 74HC02 to simulate the mcu on breadboard, and to output a 3.3V to drive the Vgs of 2N7000, and it worked fine, the R2 could worked for a big range, I was suggested the R2 use 2.2K ~4.7K on 4#, and it also in the range, you may also to try it, I was used a led and resistor connected to the N.O and C of 5V relay, used led as a indicator when the relay is on then the led will be lighting up.

    [​IMG]
     
  9. NorthGuy

    Active Member

    Jun 28, 2014
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    I used 2N7000 to switch directly from 3.3V MCU in several projects, and it works fine.

    It won't work if you need higher current.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    That's pretty much what I was saying - you don't necessarily get the headline RDSon unless you drive the gate a little bit better than VGSthr.
     
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