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Farmall Tractors Discussion Board |
Re: Re: Re: Back EMF from coil..
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Posted by Rick K Harvey is going in right direction (don't read this if you hate technobabble) on September 03, 1999 at 10:09:35 from (205.181.231.254):
In Reply to: Re: Re: Back EMF from coil.. posted by HR-use AC volt view on September 03, 1999 at 07:25:52:
The coil is more than just a transformer. While there is a transformer action in it, there is also energy storage in it. When the points close, current starts to flow in the primary. The "inductance" of the coil does not allow the current to change quickly, but rather, it builds up slowly. When the points open, the opposite thing happens. The inductance does not want the current to instantly return back to zero. The problem is that the points are open, so the current cannot flow through them. The energy is still in the coil, and wants out. The energy is a function of the product of the current and the voltage. If the current falls towards zero, the voltage has to go up, as the same total amount of energy is still there. The current doesn't actually instantly go to zero, as the condensor allows current to flow into it. But it does drop fairly quickly anyway. What was once 6 volts across the coil becomes a lot more than 6 volts as the current drops to a very low value. This is sometimes know as "flyback" action. Now that the primary voltage is a large value (300 volts is not uncommon), the transformer action comes into play. The 300 volts is stepped up to whatever voltage it takes to fire the spark plu. When the plug fires, current in the secondary flows. Now that current is flowing, the voltage will not rise any more. The current in the spark plug drains all the stored energy out of the coil. As far as running off of 12 volts directly... The ignition coil is storing all the energy in the "inductance" of the coil. In a practical inductor, there is a limit to how much energy that can be stored. If you exceed the maximum amount of energy, the inductance drops to zero. Remember that it was the inductance that limited the rate of change in current. If the inductance falls to zero, the current can now rise to a very high value very quickly (this is called "saturation", by the way). The thing that limits the current is now the resistance of the coil. The extra current is turned to heat, which can damage your coil. It doesn't produce a higher voltage spark, it just goes up in smoke. This is one of the reasons that you have to set the dwell angle on the points. It is set so that you have time to get current flowing (even at high speeds), but not so long that saturation occurs (even at low speeds). Putting a resistor in series with the coil limits the current to a safe value when trying to run off of 12 volts. I wonder how big the hard disk on the server that this Board runs on is, cuz we are gonna fill er up before this is over.
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