Ignition coil supply switching with MOSFET (is it acceptable)?

Discussion in 'The Projects Forum' started by dimkipa, Jan 26, 2016.

  1. dimkipa

    Thread Starter New Member

    Jan 26, 2016
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    Hello everybody,
    newbie in this great forum! my name is Dimitrios and I have started a project involving ignition cutoff.
    The way to go is using a MOSFET as a switch to prevent the pulse signal from reaching the coil.

    In my case the signals feed are two wires, but in different cases the number of wires can increase, leading to many wires in and out the circuit.
    So this leads to my question.
    What will happen if I cut off the 12v supply (or the ground) of the ignition coil? What are the effects?
    Will that be acceptable? Will it wear the coil down long term?

    Thank you in advance

    ps. I have an ignition coil in my bench and I will make the experiment but I believe that any responses would be very helpful.
     
  2. shortbus

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    That is exactly what happens in a automobile ignition. Usually the ground side is interrupted. The coil is basically a stepup transformer, so for it to work one side of the input (+ or - ) must be interrupted to 'collapse' the magnetic field.
     
  3. MaxHeadRoom

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  4. dimkipa

    Thread Starter New Member

    Jan 26, 2016
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    Thank you very much for your responses.
    If the magnetic field collapses, then the spark plug will fire..what i need is the exact opposite!

    So i guess that this rules out the possibility of cutting off ignition by interrupting the (+ or -) supply :/
     
  5. shortbus

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    double post, sorry.
     
  6. shortbus

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    Don't understand what your after here. When you turn off the ignition switch in a car the coil stops 'firing'. After that last field collapse it's shut off, that's the end until the next time it's started. The field collapse is how an ignition coil makes the high voltage, whether by 'points' or an electronic ignition. Maybe you could tell us what your trying/wanting to happen.
     
  7. dimkipa

    Thread Starter New Member

    Jan 26, 2016
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    Hello again,
    My goal is the following.
    I want to "interrupt" ignition at a specific time (when I have a specific trigger from an inductive sensor). I want to create a "flat-foot' shifting circuit.
    The ignition coil that I have has got a 4-wire connector.
    1--> + 12v
    2--> ground
    3--> firing pulse (1/4 cylinders)
    4--> firing pulse (2/3 cylinders)
    The way to go is to pass the pulses through two mosfets (for each wire) and stop 'firing'.
    If I isolate/interrupt the supply (or ground) the outcome will be the same BUT what are the effects on the coil? Will it be affected long term?

    Thank in advance for your time!
     
    Last edited: Feb 9, 2016
  8. ian field

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    Oct 27, 2012
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    As purely a step up transformer - it wouldn't do anything particularly interesting.

    It works on the flyback principle - the dwell time while the points are closed, builds up lines of magnetic flux around the coil. When the current is interrupted, the lines of flux collapse rapidly and induce a much higher voltage than was originally applied.

    With the old Kettering system, the points capacitor was mainly to slow down the back emf rise time so the points parted fast enough to quench the arc.

    The peak voltage on the LT winding was usually about 200 - 300V, early transistors needed a high voltage zener type device to prevent breakdown. Years ago I used a 9A 900V MOSFET salvaged from the PSU in a 19" monitor, I didn't use any voltage clam and didn't have any failures.
     
  9. dimkipa

    Thread Starter New Member

    Jan 26, 2016
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    Thanks a lot!This is very promising!
    I will try by isolating the ground and start from there!
    I want to use a mosfet (due to very low switching time) such as the FQP30N06L .

    Hopefully everything will operate fine!
     
  10. ian field

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    My ignition circuit used the MOSFET in grounded gate configuration (*SERIOUSLY FAST*) Gate goes to +12V drain to bottom of LT winding and source to the points - the big deal with that is; the grounded gate setup passed the same current through the points as the coil, it was on a small motorcycle with a puny generator that couldn't support a separate supply of "wetting current" to keep the points clean.

    If you don't have any points - you can drive the source with a low voltage high current MOSFET, it should never see any higher voltage than the 12V the gate is tied to. If needed - there should be no difficulty getting a logic level MOSFET to drive the source of the high voltage one.
     
  11. shortbus

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    I know this, just trying to simplify it for the TS.
     
  12. shortbus

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    While that was part of the reason for the 'condenser', it also stores the high (~300V) flyback for the next time the points close. A points ignition without the 'condenser' will not respond to the throttle as well, causes back firing due to not enough spark voltage to fire the plugs correctly.
     
  13. dimkipa

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    Jan 26, 2016
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    This is the ignition coil that I use.
    Regarding about what shortbus said about the flyback, is there a chance that there wont be enough energy stored in order to fire (at the instance that the 12v are connected).

    I think I will go with the configuration of the MOSFET as a switch, switching the supply circuit based on the 5v logic from the microcontroller.
    mosfet circuit.gif
     
  14. MaxHeadRoom

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    As per the online schematic examples and post #4 you need a cap across the mosfet.
    Max.
     
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  15. shortbus

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    That coil looks like it's for an electronic ignition. If your working with an electronic or capacitive discharge ignition, the 'flyback' is taken care of internally in the ignition circuit. What I was talking about is the old points style ignition.

    You should really give the type of vehicle and ignition system information, giving information in bits and pieces only drags things out. And makes things confusing.
     
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  16. ian field

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    Nothing much happens when the points in a Kettering ignition close - all that any remaining charge in the capacitor does; is sputter very tiny bits of metal off the point faces.

    The main contribution the capacitor makes to spark energy is resonating the inductance so it rings a few times - but the vast majority of any stored energy is consumed by the first spark breakover.

    That should be apparent from the fact that failing to quench the arc at the points early enough, saps so much of the energy that the engine may not even fire.
     
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  17. dimkipa

    Thread Starter New Member

    Jan 26, 2016
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    I wasn't trying to make things complicated (or more complicated as they are in mind)! My apologies!
    The ignition coil is from a peugeot 106 1.6 (as the ignition coil I stated above).
    I think that I explained it on a previous post that there are two wires carrying the pulses to trigger the spark plugs (coming from the ECU)!

    Because I see it as a complicated matter, in the beginning I started off with "interrupting" the two wires carrying the pulse but I wanted to find out If i could make things simpler by "interrupting" the supply or ground.

    Any help would be appreciated..
     
  18. shortbus

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    Do you have a wiring diagram of the engine? Interrupting the "pulse" seems like it would be the best bet. The coil supply would be a higher amperage and voltage than the pulse from the ECM. Interrupting the negative lead from the coil to the ECM would be a "low side switch" and easy to do.
     
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  19. dimkipa

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    Jan 26, 2016
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    This was my initial thought, interrupting the pulse but I want to investigate the rest of the options.
    How high is the amperage?
    I attach the wiring.

    wiring.jpg
    110 & 111 are the pulses (as i understand) and the other two (ot top) are the supply and ground.
     
  20. shortbus

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    Was doing some thinking about this. If you just interrupt the ignition, the fuel injectors will still be dumping fuel, and when the ignition starts again there will be a backfire in the exhaust pipe. And over time (unknown) it will damage the catalytic converter, not just from the backfire but also the excess fuel.

    Back when I was a teenager we would shut off the key for a second or so, with the throttle about half open, then turn it back on to make the loud backfire, so I'm pretty sure it will happen with EFI too.

    Maybe interrupting the wire from the key switch to the ECM would be a better approach. Is this something for racing? So you don't need to use the clutch to shift gears?
     
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