Should MOSFETs be open all the way?

Discussion in 'General Electronics Chat' started by stevenmahoney, Mar 11, 2013.

  1. stevenmahoney

    Thread Starter New Member

    Jan 25, 2013
    10
    0
    I have a question, perhaps a simple one.

    In the following schematics driver send a signal to the gates of three MOSFETs. I expected a square signal in the output, but it is straight DC. It looks like one or all MOSFETs is open.

    I replaced the driver IC, driver transistors, all MOSFETs, but result is the same.

    What am I missing?

    [​IMG]
     
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  2. panic mode

    Senior Member

    Oct 10, 2011
    1,319
    304
    can you change duty cycle of your pulse?

    what is your output? two wires with arrows pointing down?(P3,X8 and P5,Y2)
    what is the connected load? is there any capacitance in the load?

    your signal is close to 100% duty cycle so even small capacitance on the output or no load will give you impression that output is constant.
     
  3. stevenmahoney

    Thread Starter New Member

    Jan 25, 2013
    10
    0
    The service manual indicated that signal is as it should be (see below) and I cannot (probably should not change the duty). The output is an HMI light connected to the output (arrows pointing down). I am not sure what the capacitance of the load, as signal goes to ignitor first. Yes, I checked the signal with no load and it is probably why I could not see the gap in the output signal. Thanks a lot!

    [​IMG]
     
  4. panic mode

    Senior Member

    Oct 10, 2011
    1,319
    304
    you may want to connect some dummy load (even if it only draws 10% of current) then you'll be able to see what goes on.
     
  5. stevenmahoney

    Thread Starter New Member

    Jan 25, 2013
    10
    0
    That is what I think I am missing... Thanks!
     
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