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Little help with a mosfet

Discussion in 'General Electronics Discussion' started by chipperi, Apr 1, 2011.

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  1. chipperi

    chipperi

    9
    0
    Feb 17, 2011
    ok Ill do my best to describe it...
    9v + to a 10ohm resistor to a led to Drain
    Source is grounded to - on the 9v battery

    In theory sending a 5 volt digital signal from my arduino to the Gate should turn the led on...

    Nope nothing but if I touch the Gate to the +side of the battery the light comes on and stays on even after I remove the Gate. I pulled the gate low with a resistor to ground and corrected that. However the problem still remains Why cant I get the Digital signal from the Arduino to close the circuit???

    The Mosfet I'm using is a N type IRF830 I pulled from an old power supply.
     
  2. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    check the datasheet to make sure this particular FET will drop all the way to zero... you may need to pull the gate low with something like -5

    Im pretty sure its the Gate to drain graph that will tell you this.
     
    Last edited: Apr 1, 2011
  3. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    What's the actual output voltage from the Arduino? Could it perhaps be an "open collector" output (sink only) ? You need over 3V to start opening the MOSFET at all.
     
  4. chipperi

    chipperi

    9
    0
    Feb 17, 2011
    The Arduino puts out just a hair over 5v according to my dmm.
     
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    According to the datasheet that should be enough to turn it on well enough for the 0.5A current the LED draws, even if it's not a logic-level MOSFET.
    I'm sorry but I can't come up with an explanation for its failure to conduct at 5V. Try to set up a test potentiometer as a voltage divider from the 9V, feeding the Gate.
    That way you can measure at what gate voltage it actually starts turning on.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Just out of interest, do you still have the resistor between gate and source, and what is it's value?
     
  7. chipperi

    chipperi

    9
    0
    Feb 17, 2011
    the resistor on the gate is going to the - pole of the 9v battery to pull it low. else the led stays on when power is removed. It is a 100k. The only other resistor is a 15ohm before the led to keep it from drawing too much current.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, I too have looked at the datasheet.

    It looks like the gate threshold is typically low enough that 5V Vgs will turn on the device enough to illuminate your LED, however Vgs(th) is one of those parameters which varies considerably from device to device.

    It is possible that you have one which requires just a little more voltage than you can give it. This is certainly supported by the evidence that it turns on with Vgs of 9V. The fact that it stays on with gate drive removed is some evidence (although by no means conclusive) that the gate insulation has not been destroyed.

    You could wire up a transistor so that the gate gets the full 9V (which would use several additional components and require a change in the programming of the pin) or you could get another mosfet with a lower Vgs(th).
     
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