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How does a series resistor inside a spark plug rduce EMF noise?

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langwadt:

--- Quote from: gnuarm on February 12, 2021, 07:58:36 pm ---
--- Quote from: GlennSprigg on February 12, 2021, 12:10:30 pm ---This is a 'little bit' off topic, but there is 'another' Resistor integration that is worthy of note!!
A lot of older cars, (mainly Japanese etc), had a Resistor in series with the Coil, (low voltage).
The coils are actually rated at about 6v or something, and when the engine is running, the 12v supply is
dropped to what is really needed. The reason for this, is that while the ignition key is cranking the engine
to initially start it, the system shorts out the resistor, putting over-voltage on the coil, to assist starting!
Often, when that's not working, they can be very sluggish to start. A problem I've found a few times!  :-+

--- End quote ---

You must be an old guy... like me...  The ballast resistor went away with the distributor when fully electronic ignition was adopted.  They charge up capacitors to fire the spark and it is plenty hot these days.  In the old days cars actually barely ran.  Get some cracks in the high voltage wires and they stop working on damp days.  Cold out so your battery voltage is low and the engine turns over but won't fire up.  Now with all the improvements cars have a lot more margin and run under nearly all conditions and don't need as much attention as they used to.

--- End quote ---

I  think most cars use inductive discharge not capacitive discharge ignition. But the the ignition coil primary is only  a few hundred milliOhm so the computer can just increase the dwell time at low voltage to get the needed energy


DavidAlfa:

--- Quote from: Beamin on February 08, 2021, 09:33:30 pm ---Wouldnt a capacitor work better? Plus you want the spark as hot as possible and a resistor would lower that. I have no idea what the ohms are if its like 10 ohm or 100k, I dont have one to test. I know putting a ceramic cap across your alternator will reduce noise but a resistor in the plug? Plus its going to get super hot and change value.

--- End quote ---
From my point of view:

Without the resistor, once the spark jumps, the wire capacitance would cause a very short but high current pulse, generating a lot of EMI due the oscillations at the cable resonance frequency (depending on its LC characteristics), this could cause the voltage to reach even higher levels than it originally had and cause damage the driver. Not to mention the noise taken by all nearby electronics.

That's where the resistor comes to help. It limits the peak current, greatly avoiding the oscillations, but lets the voltage rise as fast as it's generated, so the spark will work the same.

A capacitor would filter the spike, reducing the peak voltage, requiring longer pulses for the spark to jump.
Once it jumped, the current would be higher, worsening the EMI worse and shortening the life of the electrodes.

You can see it yourself very easily. Take a small capacitor (ex. 10uF 16v) and attach a 1m cable to it.
Connect the oscilloscope in 10x mode at the capacitor pins.  Charge the cap.
And check the difference when directly shorting the wires or when doing so through 0.5ohm resistor.
See the difference when shorting out di

SilverSolder:

So the resistance is effectively lowering the Q of the spark plug circuit, reducing its ability to resonate?

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