Ignition coil

Discussion in 'General Electronics Chat' started by samjesse, Dec 25, 2013.

  1. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Hi
    When connecting a car ignition coil to a 12v battery and disconnect the negative, shouldn't there be a spark? but I am not getting a spark.

    Any one can help explaining why even though it is a new coil being bench tested.

    Many thx
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Why do you think you should get a spark ???
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    He thinks that because that's what ignition coils make: sparks.

    Sam: I'm not a car guy, so I have to ask does the coil just have two terminals or more then two?

    If more then 2 it sounds like it may be burned out and open.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If it is a coil from an older car that uses a coil, capacitor, points, distributor (Kettering system), then you are forgetting the capacitor (condensor). The capacitor resonates the primary. Energy stored in the inductance of the coil primary transfers into the capacitor, forming a RLC tuned circuit. The spark happens on the first oscillation as the point open. AFAICR, the value of the capacitor is about 0.15uF at 500V.
     
    Last edited: Dec 25, 2013
  5. eeabe

    Member

    Nov 30, 2013
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    I agree with MikeML that for a typical ignition coil, you need a condenser (capacitor) on the primary side. It will allow the voltage to charge up on the primary, and therefore also on the secondary, because in this type of ignition, the spark occurs as part of a transformer flyback action. You also need an appropriate spark gap on the secondary side between the output high voltage contact and the ground reference (which may be the casing).

    I've recently done an ignition design like this, and the capacitor was about 0.3uF, and it charged to about 400V before a typical spark occurred. In testing an old coil recently, we were easily able to get sparks over an inch long.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The capacitor is not necessarily required to get the spark, but it does increase the sparking intensity when present.

    What IS required is a LOAD. Just having voltage present on the coil will not produce current flow. Current flow will produce the magnetic field, which stores the energy of the electric current. breaking the flow of current will cause the field to collapse and produces another current that causes the spark. briefly shorting the coil across the battery will result in a large spark when the short is removed. You MUST have some resistance in the circuit. or the short will overload and overheat the coil and destroy it. A 25 ohm resistor of 1 watt or greater should let you bench test the coil. Wire the resistor in series with one end of the coil and bobs your uncle.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Agreed.

    Its other function is to prevent sustained arc over when the points open.
    Max.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    I remember the condenser (capacitor) being 0.22uF in value, but we're all in that ballpark anyway - it shouldn't be too critical as long as it is present. Without that capacitance, the secondary spark will be weak to nonexistent. It may also damage the coil.

    '70 Fords and earlier also had a condenser from the + side of the coil to chassis ground (battery -).

    Fords and GM had a resistive wire for the coil's + supply; I don't know how many Ohms it was. The Ford's resistive wire was bypassed during starting via the starter solenoid; that gave it a hotter spark while cranking. Chrysler/MOPAR products had a big wirewound resistor mounted on the firewall in a rectangular metal bracket; you can still find these in the HELP! section at your auto parts store. I don't recall what the wattage rating is; must be pretty high though. I think they're around 8 Ohms. Without the current limiting, the coil would get fried; it's also hard on the points to have to interrupt so much current.
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Connect the "+" side to a battery, and just a wire to the "-" side. Then, quickly strike the other end of the wire that is connected to the "-" side to ground. You should see a weak spark. Look closely, it will be very weak.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Could you show us the circuit that you are using?

    Where is it that you are looking for a spark?

    My guess is that you are getting a spark across the gap as you are disconnecting power and that most of your energy is being dissipated there.
     
  11. Brownout

    Well-Known Member

    Jan 10, 2012
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    That reminds me... if you remove the spark plug to 'see' the spark, make sure you ground it. When you remove the spark plug, you also remove the ground, which is necessary to see the spark.
     
  12. debe

    Well-Known Member

    Sep 21, 2010
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    If you don't use a capacitor the spark will be almost non existent. In picture using a .22uf capacitor & intermittently shorting across it you get a nice fat spark.
     
  13. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Many thanks for all the help/comments.

    I have a nice amp ramp up to 8amp showing on the scope and a nice cut out "fast" and the coil is not producing the spark which tells me I have a bad coil. I changed it with a new one but still have no spark "I got a defective new coil showing open circuit between + and - on the ohm meter", will get another when the store opens after the holiday.

    In automotive, the capacitor is in parallel with the coil -, though in this case it was in parallel with the + and I am not sure if this is the correct design or the customer played around with the wires before car towed to my shop.

    kermit2:
    The coil primary is that resistance and cars do not have any other in series, old cars like SgtWookie indicated have "Ballast resistor" which is removed from the circuit during starting, it's to maintain around 9v to the coil, not the case with new coils/cars in my case.
     
  14. debe

    Well-Known Member

    Sep 21, 2010
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    If we are now talking about Electronic Ign coil then there should not be a condencer across the coils + & -- . There may be a condencer from the coil + to engine block for noise suppression. There is no condencer used in Electronicly switched ignition systems. Yes if there is no continuity on primary of coil then its definitely faulty. I would be looking at why the coil is blown on the primary as its not a common fault. There is possibly a fault in the ign driver or its wired wrong. Not giving the full story at the start of a thread is where missunderstandings happen. It would help if the vehicle was known that this coil is actualy on. But then this thread would be shut down!!! as it would then be vehicle related. Not a good scenario as this is electronics related &what the OP is trying to work out.
     
    Last edited: Dec 25, 2013
  15. inwo

    Well-Known Member

    Nov 7, 2013
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    How are you checking for spark?
    Gap or plug?
     
  16. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
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    Gap. > 20mm
     
  17. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    The original question is intended to be taken at face value. no misunderstandings here as I learned new things from the comments and the help before revealing the automotive involvement which it may not have been the case had it been known in advance that we are dealing with a car, plus I need to understand the electronics side regardless of the application.:)
     
  18. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    If that is the gap your using that's the problem. It should be ~1.3mm. A single spark would never get to 20mm, maybe while the engine is running, due to resonance, but not just with a single make/break.
     
  19. WBahn

    Moderator

    Mar 31, 2012
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    A gap of nearly an inch?!?!
     
  20. debe

    Well-Known Member

    Sep 21, 2010
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    In the pic above I use at least 3mm gap on an old plug, if it cant jump that with a good Blue spark, it wont cut it under compression in an engine.
     
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