Spark Plug circuit

Discussion in 'The Projects Forum' started by ronv, Jan 6, 2016.

  1. ronv

    Thread Starter AAC Fanatic!

    Nov 12, 2008
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    I'm working with @jgrupczy on a project to simulate biigg guns firing. See here: http://forum.allaboutcircuits.com/threads/propane-oxygen-gun-simulator-schematic.118860/
    One of the circuits is to fire a spark plug to ignite a gas oxygen mixture.
    I have a circuit that I think works, but if someone has a better one or any input on this one, I would appreciate the input.
    It's pretty simple: A pulse triggers a 555 which turns on a high voltage FET for about 4ms.. From what I have read about the coils the current should rise to about 7 amps in that time. When the field collapses the spark plug fires.
    I used a TVS to protect the FET just in case.
    So some questions:
    Is 5 mh about right for the coil primary?
    250H okay for the secondary?
    4 ms and 7 amps okay?
    Anything else you can think of?
     
  2. Lestraveled

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    Just this last weekend I helped a friend with an ignition problem on his VW dune buggy. The root problem was a shorted primary on the spark coil. Regular coils read at 5 to 6 ohms. The shorted coil read 1.5 ohms. The shorted coil blew out the electronic ignition module. So, what am I trying to say? I think you need to build into your model a DC current limit of 2 - 3 amps with an equivalent resistance in the primary.

    Also I think where you have the TVS protects the mosfet but couples any inductive kick on to the Vcc. Maybe something directly across the primary.
     
  3. shortbus

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    There are different ohms/resistances on different 'types' of ignition coils. Coils for the old type points ignition are different than the new electronic ignitions. And coil on plug types are different from both of the others. So you might want to find out what coils will be used.
     
  4. Externet

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    ronv :
    Try feeding the spark coil with 12VAC or more from a plain AC wall transformer, -no 555 circuitry-, and please come back with results.

    If you have a spare CRT ´flyback´ transformer, can try to feed its primary also with a plain 12VAC adapter.
     
  5. Bernard

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    Battery power only, no AC available.
     
  6. Lestraveled

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    How about some cheap and dirty. A relay oscillator.

    [​IMG]
    The R and C adjusts on/off time. Maybe a cap across the relay contacts. I made one of these as part of a science project when I was in high school.
     
  7. MikeML

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    If it is an old Kettering ignition coil, then you need a capacitor across the coil primary to get a nice hot spark.
     
  8. Lestraveled

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    So that is what that capacitor was all about....
     
  9. ronv

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    Yes, I think it is a standard coil. 4-We use a standard automotive coil like the AC Delco U505 or the Emgo 24-7512
    So I guess in addition to saving the points from the arc the series resonant circuit keeps it firing until it arcs. It's also why you see the diode in series with the FET that looks like it doesn't do anything?
     
  10. Bernard

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    Ron, I was able to scrounge an old Delco-Remy 293 B-R coil from a 1968 Camaro, pri. 1.6 ohms, 10 k on sec.
    Called for cap. might be # 10495103 D203 K7 no inf. on capacity or V.
    With 3 ohm ballast R, ( power supply rated 4 A ) tried several C values across points, .068 seemed the best, none & 1.5 uF seemed the worst. C across points. Still sparking from points on break. V unknown as scope died some time ago.
    You are old ? I'll be 89 this month.
     
    Last edited: Jan 8, 2016
  11. ronv

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    Thanks Bernard! 0.068 it is then. I really can't see much in the simulation. Probably because I'm doing something wrong with the plug in the simulation. But I can see it ring with the cap which seems like a good thing.
    89. Wow! I'm just about to be 72, so you have me by a few.
    Thanks for your help. I always like your experiments better than a swag.:D
     
    Bernard likes this.
  12. Lestraveled

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    Today I picked up a spare spark coil for my VW sand rail. PN 043-905-115C, Primary: 3.5 ohms, 7.5 mH, Secondary: 7.5 Kohm.
     
  13. ian field

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    If you only want a spark when you trigger it manually, you could use a capacitor discharge system with a SCR to dump into the LT winding. Another way is the Siemens gas discharge surge protectors, you'd need to rig the inverter to charge the capacitor to just under the tube breakover voltage and have a boost button to tip it over.

    A regular automotive coil is always the easiest option, but you can get a decent spark from a backwards SMPSU transformer if you do it right.
     
  14. ian field

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    The capacitor mainly slows down the rise time, so relatively speaking the contacts part quickly enough to quench the arc before it dissipates most of the energy you want to come out the HT lead.

    Less important, but not irrelevant - it makes an LC tank circuit that resonates (rings).
     
  15. Bernard

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    As I remember from 1950's, it took a capacitor charged about 300 V DC & dumped into primary via SCR ? to produce a nice fat spark. Made one from Popular Mechanics article & used on my Co. truck until power supply died.
     
  16. Brownout

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    Jan 10, 2012
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    ronv,

    I built and tested a circuit nearly identical to your, only I used a BJT instead of a FET. It worked fine without a capacitor, getting a spark over an inch or two. The capacitor seems to be needed for mechanical contact switching. But it destroyed the 555's. I always meant to go back and see how I can keep the four hundred volt pulses generated in the primary out of the twelve volt supply to the 555, but never got around to it. My plan was to use a opto isolator and separate power supply for the switch/primary until I was able to remove the pulses from Vcc.

    To summarize, I think you're going to need a more robust transient suppression network on vdd between the FET and 555.
     
    Last edited: Jan 9, 2016
  17. ian field

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    A good way to do it is; use a SMPSU MOSFET rated about 9A 900V in grounded gate mode - the switching is *EXTREMELY* fast. This works very well with the points in the source circuit as it provides the wetting current to break through any oxide/glaze on the points faces.

    The source circuit can be driven by a high current low voltage MOSFET in common source configuration. The bottom MOSFET drain will never see more than the upper MOSFET gate voltage - which would typically be 12V. The bottom MOSFET can be driven by most fairly basic pulse generators - gate capacitance means high current is only needed for high repetition rate.
     
  18. ronv

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    Thanks.
    I'm not sure what to think about the cap now. I can see for sure where you need it with points and the regular (meaning points) circuits seem to talk about resonance.
    In the point system it's across the points, but @MikeML said to put it across the coil. So I'm a little confused. :oops:
    If I add a little inductance to the +12 line I can see the ringing show up on the +12 so I'm going to add some filtering there. Thanks for the tip.
    @ian field it's an all electronic circuit, so no points.
     
  19. Lestraveled

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    @ronv
    On my VW dune buggies I have replaced the points and capacitors with an ignition module that fits in the same location as the points. The volume of the module is about the same as the capacitor. So, I don't think there is a "points" capacitor in this module.

    [​IMG]
     
  20. ronv

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    Maybe I'll put in the noise suppression and leave a provision for the cap in either place unless I here from Mike. I'm sure you get a spark in either case.
     
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