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Thread: Ignition Coil Pack

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    Ignition Coil Pack

    Anyone know how the current in the ignition coils is limited? I cant tell if it happens internal to the coil pack, in the PTU, or if its controlled by the ECU. Stealth 316 hints that the ECU is the dealer but I cant seem to find anything to confirm this.

    Any leads would be great.

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    Stealth owner since '91 Feedback Score 0 Steve '92ES's Avatar
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    I would imagine, but don't know for a fact (because the ECU schematics are super top secret) that the current is limited by the internal impedance of the coil. What is it that you're trying to figure out?

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    JNS Engineering verified Feedback Score 1 (100%) Jimvr4's Avatar
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    I have, i was just wondering if anyone knew of some more specifics. Ill do some poking around. Thanks.

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    Stealth owner since '91 Feedback Score 0 Steve '92ES's Avatar
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    What specifics?
    The reason I ask is it seems like an odd question.
    Basically you apply 12v to the coil primary, and it draws what it draws limited by the impedance of the coil. To generate spark power is CUT from the primary, which causes it's magnetic field to collapse inducing a voltage in the secondary. The secondary is fed, via the ignition wires, to the spark plug. What limits the secondary current is largely the resistance of the secondary circuit, which includes the resistance of the wires as well as the resistance at the plug gap.
    When you ask "What limits the current in the coil circuit" the answer is "The design of the components", but I'm pretty sure that's not what you're looking for.
    If we knew more about what you're trying to do maybe we could give you a more satisfying answer..

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    I think i found exactly what I was looking for. I must have missed this line as it was very late when I was reading this haha.

    From http://www.stealth316.com/2-ignitionsystem.htm
    "Using an internal resistor, the ECU controls the current to the transistor base to limit the current in the coil to 6 amps"

    That seems to be what I needed.

    Thanks for the response also by the way, ill look more into the coil impedances.

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    Stealth owner since '91 Feedback Score 0 Steve '92ES's Avatar
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    True, but transistors don't have to be absolute on/off switches. They can be partially on, like in an amplifier. It's feasible that they could be used to limit current, but in this case I doubt there's any operational function to it. More than likely it's done that way to protect the PTU if a coil shorts out. In other words if, for any reason, the coil starts drawing more than 6 amps the transistor is biased off, limiting the current. On the other hand it would make no sense at all to use a 10 amp coil and limit it to 6 amps...

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    JNS Engineering verified Feedback Score 1 (100%) Jimvr4's Avatar
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    Quote Originally Posted by Steve '92ES View Post
    True, but transistors don't have to be absolute on/off switches. They can be partially on, like in an amplifier. It's feasible that they could be used to limit current, but in this case I doubt there's any operational function to it. More than likely it's done that way to protect the PTU if a coil shorts out. In other words if, for any reason, the coil starts drawing more than 6 amps the transistor is biased off, limiting the current. On the other hand it would make no sense at all to use a 10 amp coil and limit it to 6 amps...
    You're right in principle on transistors however in this case it's definitely a switch. In fact what is happening is the ECU is turning the PTU on/off periodically. The PTU energizes the primary coil magnetic field and induces a much greater magnetic field in the secondary. For each ECU pulse the magnetic fields are building and collapsing. When the magnetic field is at a maximum in the secondary, the high voltage arcs across the spark plug electrodes.

    Technically its not the ECU that limits the current to 6 amps. Its the resistance of the CE junction of the PTU in series with the primary coil resistance.

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