Ignition coil bench test?

Discussion in 'Electrical and Ignition' started by RichB, May 12, 2018.

  1. RichB

    RichB CherryCuda65

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    How can I set up a coil on a bench test and get 10-30k volts out of it? So far, I've tried two different coils with a 12v battery and can only get 12.5 volts out of the coil wire.
     
  2. 67Dart273

    67Dart273 Well-Known Member

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    First you cannot measure it. The pulse width will probably not display much AND MAY RUIN YOUR METER

    Second you need a capacitor, if its the older "round" coil we all "used to use."

    Don't leave it hooked to 12V for long

    Wire it up 12V--------coil+--------coil---------coil neg------capacitor-----to ground

    Now take a jumper wire and repeatedly touch from coil neg to ground. Every time you pull it off, a nice fat spark should happen at the tower

    "Rig" a spark tester, or old plug at the tower to ground

    THE COIL CAN IS NOT GROUND. "Ground" in this case is your battery NEG
     
  3. RichB

    RichB CherryCuda65

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    O.K. thanks. I thought a capacitor may have to be involved! Do you have any idea how much current may be generated?
    I'm trying an experiment and I wonder If I will need a high value resistor to keep the wires from frying.
     
  4. 67Dart273

    67Dart273 Well-Known Member

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    No not for that short time. I would not leave it connected for "a minute" but you should be "flicking" it with your jumper anyhow. LOL current? I've never actually measured a coil current draw....2-5 A I'd guess
     
  5. AJ/FormS

    AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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    Another method I have used is to drag the Coil negative wire over a long flat-bastard file,that itself is grounded back to the battery;letting it bounce along over the high-points . This method does not require a capacitor. A stream of sparks should issue from the coil tower to anything grounded back to the battery.
    Watch out; it is very addictive to watch. It's like playing God; Look what I can do! Zzzzzzzzzz zzzz zzzz zzzzzz z zzzzzz z z z z zzz, oh look it's a mini-welder!
     
  6. KitCarlson

    KitCarlson Well-Known Member

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  7. BigBlockMopar

    BigBlockMopar BigBlockMember

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    Another hard way to do it involves a lathe, trigger wheel, MegaSquirt ECU, HEI module, coil, some wiring and ofcourse a sparkplug...

     
  8. RichB

    RichB CherryCuda65

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    I am not getting a "fat" spark anywhere. Where's my error in wiring? Here's a picture of my bench setup and a diagram. Shouldn't the center "coil wire" fire to ground (negative) ?

    coil test 004.jpg

    coil test 001.jpg
     
  9. halifaxhops

    halifaxhops It's going to get stupid around here! FABO Gold Member

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    we used to use a flasher can when we used a coil in the exhaust pipes worked well.
     
  10. KitCarlson

    KitCarlson Well-Known Member

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    The cap goes in parallel with switch, not in series. You should also have a ballast resistor in series with switch. Put the ballast where you had the cap. It only takes bout 3ms to charge the coil when switch is on, the ballast limits the current to a safe value.
     
  11. KitCarlson

    KitCarlson Well-Known Member

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    That is a good idea! The bi-metal heater serves as ballast.
     
  12. RichB

    RichB CherryCuda65

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    It works just as you said. I used a flasher instead of a ballast resistor. I can now label my coil as "good". Thanks to all for the help.
     
  13. halifaxhops

    halifaxhops It's going to get stupid around here! FABO Gold Member

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    used to have zoomies on a wagon we used to light with that method, was great at the bus stops!
     
  14. BillGrissom

    BillGrissom Well-Known Member

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    You don't need the ballast resistor if only charging up the coil for an occasional spark test. The coil charges whenever coil- is held to BAT-. It only takes a few millisec to fully charge, i.e. reach a steady DC current in the primary coil, so don't hold it ON long. It is a misunderstanding that a coil outputs a certain kV. That isn't helped by stupid ads. The coil+ output voltage rises to whatever is needed to make a spark jump somewhere. It could spark thru the coil wire to gnd or even internally inside the coil. The later is likely what the kV rating means. At 1 atm (open air), the coil can easily throw a spark several inches (many feet in a vacuum chamber). Much harder at high cylinder pressures, which occurs most w/ turbo boost. That is why a Chrysler TSB for the 2.4L turbo engines said to reduce the spark gap to cure mis-fire (from ~60 mil to ~50 mil). That worked for me in my non-turbo 2.4L. It used to mis-fire bad when flooring it on a freeway ramp.
     
  15. KitCarlson

    KitCarlson Well-Known Member

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    There are a few good reasons to use a ballast. The ballast will limit the coil current, so the coil test is realistic for normal condition. Without the ballast the coil current can be much higher, and coil energy increases by square of current increase. The ballast originated when 12V replaced 6V, the ballast typically has a value close to coil resistance. Without the ballast, coIL current would be double, with energy 4x. With energy 4x, yeah there may be spark for a sick coil, and perhaps not with ballast.
    The next reason, to test in a safe way and not destruct the coil under test. Example is testing a low Ohms (0.4) e-core coil. At 12V, the coil current could reach 30A, being inductive load, not many spare parts bins have a switch to clear that! The 360W of dissipation in the plastic potted coil, will likely char the enamel winding insulation of the primary coil. Also the human shock hazzard of death, in the intervetion as wires are smoking on the bench.