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MOSFET Help

Discussion in 'General Electronics Discussion' started by iipreca, Feb 26, 2012.

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  1. iipreca

    iipreca

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    0
    Feb 26, 2012
    Hello all, I am new to the forum and seeking some help and in the future hope to provide some help as well.

    My situation is this, I am attempting to build a sunrise simulator using a 555 timer generating a PWM signal to a MOSFET to drive a 60-500w light bulb.

    My problem is in driving the FET I currently have the output of my 555 going through a 200 ohm resistor to an MOC8204 Opto isolator. The mains voltage is full bridge rectified 120VAC I have a 68k ohm from v+ to the collector of the opto isolator and then the gate of the FET connected as well to the collector; the emitter is tied to neutral. After powering up the circuit, It shorted out the FET...it is an N channel FET IXTH30N60P and I am doing low side switching with the Load on the Source side of the FET.

    What am I missing? when the isolator is off the 68k ohm should keep the gate within the 30V of the source which is a requirement of the FET, and when the isolator turns on it grounds it which should turn the FET off correct?

    Any help is greatly appreciated.
    Thanks
     
  2. iipreca

    iipreca

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    0
    Feb 26, 2012
    Drawing

    Here is the Schematic.
     

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  3. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The load is on the drain of the FET.

    The rectifier will give 170V rectified. This is a bit much for the gate of the FET!!!
    If you have a common negative line, there is no need for an optical isolator.

    You could drive the FET directly from the 555 which would limit the gate voltage to 12V. If tou keep the isolator, put a 12V zener diode across the gate/source terminals
     
  4. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    I don't get what you mean by this. According to the datasheet...

    I thought this threshold voltage was the turn on voltage and turn off voltage. The source is connected to ground as duke pointed out. Maybe someone else can straighten it out.
     
    Last edited: Feb 27, 2012
  5. iipreca

    iipreca

    6
    0
    Feb 26, 2012
    Ok, I think what I am confusing is Vgss and Vgs(th) I thought so when I saw the +- 30 for the Vgss I thought that ment the gate had to be 30v max difference from the source. So by pumping 170 into the gate I shorted it out.

    Do I then need a current limiting resistor if I drive the FET directly from the 555?

    Thanks.

    Jesse
     
  6. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    You will probably want a resistor to control the current, but if the current is limited too much the FET may never fully turn on due to the time it takes to charge the gate capacitance. Find out what the max output current is of the 555 and set your resistor to this value or slightly higher. If it doesn't work then you may need to add in a mosfet driver to help with the switching speed of the mosfet.
     
    Last edited: Feb 27, 2012
  7. iipreca

    iipreca

    6
    0
    Feb 26, 2012
    Thanks very much, I'll give it a shot.
     
  8. duke37

    duke37

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    772
    Jan 9, 2011
    The FET does not need current other than to turn on and off. It behaves as a capacitor. A gate resistor is used to stop high frequency oscillation during the transition time, 100 ohm is often used. Too high a resistance will slow the transistion to an unacceptable level.

    To drive a light bulb, you do not need a very high frequency so it will not be necessary to drive the FET on an off very fast.
     
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