Help with MOSFET switch circuit

Discussion in 'The Projects Forum' started by FlexedCookie, Oct 1, 2014.

  1. FlexedCookie

    Thread Starter New Member

    Oct 1, 2014
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    Hi guys,

    Needing some help in getting my MOSFET switch circuit to work.
    Currently im using IRFZ34NPBF power mosfet, and im trying to use it to switch a DC motor on and off with PWM from an arduino.

    My current circuit is attached below.
    At the moment, I've got it wired up as shown, and am just trying to control the gate voltage with a power supply outputting 3V DC.

    My problem is that whenever I do this, the mosfet never turns on. I checked the voltage between the resistors and the gate, and it seems like there is no voltage going to the gate of the mosfet.

    Also interestingly, i can take out the wire leading to the gate from the resistors, and then just put a resistor from the gate of the mosfet to ground, and all of a sudden the motor works. However, i can do this nonstop until i turn off the power supply, which makes me think the mosfet isnt turning off.

    Any help is greatly appreciated. Cheers MOSFET circuit.PNG
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You need what is called a logic level FET. The one you have may need as much as 4 volts to just begin to turn on. You will also need to remove R1 even with a logic level FET.
     
  3. FlexedCookie

    Thread Starter New Member

    Oct 1, 2014
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    Ahh ok, so can i also control the speed of the motor with PWM with a logic level mosfet as well? I'm new to mosfets and am currently still trying to understand the difference.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Yes, they work the same, just have a gate threshold voltage around 1 volt. You could add a transistor to your circuit and then use the 12 volts to turn on your FET as well. I'll add a circuit in a minute.
     
  5. FlexedCookie

    Thread Starter New Member

    Oct 1, 2014
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    Excellent. I'd prefer to stick with just one transistor since space is an issue for me. Is the circuit for logic level mosfets the same? Thanks again :)
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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  7. wayneh

    Expert

    Sep 9, 2010
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    I'd say, remove R2, not the pull-down R1.

    If it was me, I'd use a small transistor to switch the 12V to the gate of the MOSFET. This would allow you to use a regular MOSFET (which I have on hand).
     
  8. FlexedCookie

    Thread Starter New Member

    Oct 1, 2014
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    I'm sorry to be a pain, but what should the circuit for something like that look like?
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Can you live with reversed logic, so that 3V is fan off?

    Similar to your diagram: 3V signal connects to base pin of transistor through 10K resistor (or 47K). Emitter to ground. Collector to gate pin of MOSFET. A resistor, say 10K, connects gate to +12V.

    So when the 3V comes on, the transistor conducts and pulls the gate down and turns off the MOSFET. When the 3V goes low, the transistor quits conducting and the gate is pulled high by the 10K, turning on the MOSFET and fan.

    But if you have to buy a MOSFET anyway, you might as well get the logic-level one.
     
  10. FlexedCookie

    Thread Starter New Member

    Oct 1, 2014
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    Seems pretty simple.
    Since space is an issue, I'm investigating how a logic level mosfet should be wired up. I'm struggling to get the right resistor values and how it should actually look.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    It would be like your drawing, minus R2 or minus R1, either one or both. Just don't use both together as drawn. That cuts the gate voltage in half and that could be too low for even a logic-level MOSFET.
     
  12. FlexedCookie

    Thread Starter New Member

    Oct 1, 2014
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    Ok. Excellent mate, thank you and thank you rony for your help!
     
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