PWM Grounding Issue

Discussion in 'The Projects Forum' started by RedandBlack, Jan 18, 2015.

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  1. RedandBlack

    Thread Starter New Member

    Jan 18, 2015
    Hi all!
    I'm trying to make a pulse width modulated heating element, and I've run into some issues. You'll notice there are two rectifiers, a low voltage and wall voltage rectifier. My plan was to have the 555 power the gate on a 200v, logic level gate mosfet, which would switch the 170 volts DC to the load.

    The 555 PWM works fine, but when I connect the ground returns for each rectifier, the 555 square wave disappears with respect to either ground. It's very strange.

    I also tried to set up an opto isolator between the 555 and the gate of the mosfet, but I didn't have any luck getting it work. I may ask more specific things about that later. As of now, any ideas would be helpful!


  2. MaxHeadRoom


    Jul 18, 2013
    With this set up you would have to ensure that the circuit common does not ever contact earth ground or you will see a spectacular display from at least the 120v bridge.
    It is a bit misleading as you have used the earth ground symbol throughout instead of circuit common.
    And one of the 120v RMS conductors is going to be at earth ground potential regardless. although not shown on your drawing.
  3. ScottWang


    Aug 23, 2012
    1. Please check all the wiring using multimeter to do the measure.
    2. If all the wiring are ok then you could do these:
    a) Cut off the wire from g of mosfet.
    b. Cut off the wire form pin 3 to the pin 2 of VR100K.
    c) Connecting pin 3 to the g of mosfet.
    d) Connecting pin 7 and 1K to the pin 2 of VR100K.
    Try again.
  4. bertus


    Apr 5, 2008

    I am closing this thread as it violates AAC policy and/or safety issues.
    The circuit has parts that are directly connected to the mains.
    This is not allowed on our site.

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