Ignition Coil question

Discussion in 'The Projects Forum' started by sekhmet, Nov 22, 2010.

  1. sekhmet

    Thread Starter New Member

    Nov 22, 2010
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    Hi I'm using a 12VDC battery to power an ignition coil as well as a voltage regulator that powers some IC chips. I've found that if the spark plug is corroded or the secondary is open, the voltage regulator heats up and starts to fail. I'm guessing that there is a voltage spike on the primary of the ignition coil if the secondary fails to spark and dissipate. Can someone explain to me why this is? Also, I tried tying a 12V zener diode across the primary of the ignition coil to protect the voltage regulator but it doesn't seem to help. Can I not run the ignition coil and IC circuits off of the same power source? Do I need a RC snubber or something? Are there any best practices for protecting the ignition coil and IC circuits when running them off of the same source?

    Thanks for your help!
     
  2. sparksanarks

    New Member

    Nov 22, 2010
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    I believe that if you want to use the ignition coil you will have to have separate supplies. In the old days they used to run ballast resistors with the coils. They are meant to cycle as the power will be running through the primary to ground, ( A LOW) Resistance situation constantly since there is no cdi or points unit turning the current off and on. You got me on that?? Coils are just pulsed in other words , or theres a steady current to ground and probably pretty high too. Tim
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The ballast resistor was used to lower the voltage and prevent the coil from burning out during long periods of driving or the engine running. The ballast resistor was bypassed during starting to make the spark ''hotter''. Even though the coils were marked as 12 V they were only 6 or 8 V in reality. Actual 12 V coils didn't come about till the advent of electronic ignition.
     
  4. sekhmet

    Thread Starter New Member

    Nov 22, 2010
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    Okay, so I think I understand what's happening now: When the contact of the relay coil closes, current flows through the primary side of the ignition coil charging the core with EMF. When the contact opens, the sudden change in voltage and field colapse causes out rush of the stored EMF as current. Normally, there is a small gap in the spark plug and very high voltage on the ignition coil secondary so the current dissipates through a spark in the plug. However, if there is no path on the secondary (ie an open or a resistive spark plug), the outrush is probably dissipating on the primary side through the capacitive gap of the relay contacts and back onto the 12V bus that is powering the voltage regulator.

    So, I guess my question now is, can you use an RC snubber on the primary side to provide another path for the current and prevent the contacts from arcing to the 12V bus? If so, how would one size this?
     
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