problem with transistor substitution

Discussion in 'General Electronics Chat' started by lokeycmos, Jun 21, 2010.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    i am trying to make an ignition coil driver based on the attached schematic. The picture shows the wrong polarity. i am trying to substitute a mosfet for a bipolar transisor in the hope of getting a little more output because of lower internal resistance. SgtWookie kindly showed me how to calculate the gate resistor. http://forum.allaboutcircuits.com/showthread.php?t=39677

    im having no luck with this. when i use an irfp250 i get next to no output. am i doing something wrong or do i just have the wrong idea to begin with?? thank you much!! :)
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    MOSFET OR BIPOLAR...the transistor will get fried if there is no protection diode across the coil.
    Ur circuit is in complete, where did you get it.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    and by the way I checked the link, I think Sgt must have missed the diode part.
    Now I really have to wait for Sgt to see this thread..
    You shud have continued on that thread being the same topic
     
  4. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    i found this circuit online, i dont remember which site. the circuit works as advertised with a 2n3055. just hoping more with a mosfet. (I learn best by experimenting like this with hands on) ill try a diode, i have lots of high voltage ones. Srry bout starting new post.
     
  5. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi,

    The back EMF is what produces the high voltage output on the secondary side, so killing the primary back EMF with a diode will also kill the high voltage.

    If the driver switches off fast and clean, then there will be an EMF of 250 to 350 Volts. The value is usually limited by the inter-winding capacitance of the coil windings and by the width of the spark gap, which limits the high voltage. Just ensure you use a driver with a PIV of at least 350V and don't operate without the spark jumping to ground.

    Example: if the turns ratio is 1:70 then an EMF spike of 350V will become 24,500 Volts on the secondary.

    Drivers with 100V PIV can survive for awhile, but are constantly being pounded with spikes of limited energy. This will limit the output voltage as well as shorten the driver life, especially at higher ambient temperatures.

    A MOSFET should work also as a driver, but ensure you turn off the gate (which is much more capacitive than a BJT) with a fast, low Z drive signal.

    Regards,
    Ifixit
     
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