# Induction coil question

Discussion in 'General Electronics Chat' started by R_W_B, Nov 11, 2011.

1. ### R_W_B Thread Starter New Member

Oct 19, 2011
15
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Hello I am trying to learn the deeper theory behind automotive and motorcycle electronics. I've read that current is induced when a conductor is moved through a coil OR the coil is moved around the conductor. I get the jest that movement is key.

More specifically I'm trying to see exactly what's happening when the spark jumps the spark plug. I'ver read that for this to occur the positive side connection to the primary coil is cut, thereby creating a voltage spike in the secondary coil resulting in the spark jumping the gap.

My confusion is I don't readily see what this disconnection does to cause the voltage spike. I understand that the secondary windings are 10 times the amount of primary windings and would have 10 times more volts (or around 120 volts with a 12 volt battery).

But why does collapsing the field in the primary cause the secondary to soar even higher creating enough voltage to jump the plug gap. Is this disconnection another facet of the "movement" semantics ?
Appreciate any info on this.

2. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
300
The generation of a large voltage upon disconnection of the supply depends on a fundamental part of the behaviour of inductors. There is no room for doubt that it happens.

Basically, when a coil is connected to a DC supply a current builds up in it, generating a magnetic field. If the current is then forced to reduce suddenly by disconnecting the supply, the magnetic field must also reduce suddenly, so that the coil finds itself in a rapidly changing magnetic field.

This in turn induces a large voltage in the coil, according to the equation E = -Ldi/dt, where E is the induced EMF in volts, L is the coil inductance in Henries, and di/dt is the rate of change of current in amps per second.

You may expect that this could produce an infinite voltage because the current is forced to zero upon disconnection, but various limiting factors apply such as arcing at the contact breaker and at the spark plug, and various capacitances. The deliberate capacitance wired across the contact points among other things restrains the rate of rise of voltage somewhat, helping to avoid much of the energy being wasted in useless destructive arcing.

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3. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
I think your statement "I've read that current is induced when a conductor is moved through a coil OR the coil is moved around the conductor. I get the jest that movement is key." is not quite accurate.

I would correct it to say "Voltage is induced when a conductor is moved through a magnetic field or a moving magnetic field cuts a conductor."

And you are right that movement is the key whether it is the conductor moving or the magnetic field moving.

In the coil you mention, when the primary current is interrupted the magnetic field very quickly collapses past the many turns of wire in the secondary inducing a very high voltage that can cause a spark to jump the spark plug gap.

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4. ### R_W_B Thread Starter New Member

Oct 19, 2011
15
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Thanks for the replies. Ok so when the magetic field collapses it effects the same thing as movement. That makes sense as far as being somewhat equivalent in semantics.

I guess what's puzzling me is how the movement causes such a dramatic increase of voltage on the secondary side, i.e. much more than the 10 times number of windings ratio. I obviously have not fully gotten under what happens in the field as the collapse occurs.

I.e. would a conductor moved thru the same coils create this same voltage amount ?

5. ### gerty AAC Fanatic!

Aug 30, 2007
1,154
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Think of the ignition coil as a "step up transformer", a small voltage induced in the primary is "stepped up" to the secondary. The magnectic has to be colapsing for this to happen, merely applying dc to a transformer will not have this effect as transformers are for ac.

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6. ### crutschow Expert

Mar 14, 2008
13,492
3,372
The primary to secondary turns ratio of an ignition is more typically 100:1 not 10:1. Thus, for example, a 100V spike on the primary due to the primary inductance will be transformed into 10,000 volts on the secondary.

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7. ### R_W_B Thread Starter New Member

Oct 19, 2011
15
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Oh ok I see what you mean, collapsing is just the DC's way of pulsing the flux field as AC does.

So I think my confusion then is from being overly preoccupied with the logistics of an example I saw whereas the step ratio was 10 to 100. I surmise for the plug gap voltage requirement in reality there is a much larger step up ratio ?

And further from the previous reply in that (without absorbtion from an inserted capacitor) the voltage would go to infinity. This totally went over my head. What causes it to go to infinity ?

8. ### R_W_B Thread Starter New Member

Oct 19, 2011
15
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Oops sorry I was typing while you posted an exited earlier before seeing your post. But I think all of these replies are starting to help me see this. I gather that it's not just the collaspe but the speed of the collaspe (which would be great) that effects such a larger voltage spike.

Can I then assume that an alternator turned faster will also produce a higher voltage per revolution or is this another aspect to it ?

9. ### crutschow Expert

Mar 14, 2008
13,492
3,372
The voltage would theoretically go to infinity if there were no capacitance on the primary or no capacitance or other load on the secondary of the coil. (A finite amount of energy dumped into an infinite impedance load gives a infinite voltage). But of course there always is some stray capacitance so the voltage will actually go to some finite (but high) level.

The alternator turning faster has little to do with the coil voltage as long as the alternator voltage is regulated. The coil voltage is determined by the amount of current going though coil that's being interrupted, and that current is determined by the resistance in series with the coil and the battery voltage.

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10. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
You have to understand the basic law of Inductance:

V = L dI/dt

"dI/dt" means how fast the current flowing in the inductor changes. That induces a large voltage if the current changes abruptly.

BTW: the standard automotive ignition (sometimes called the Kettering ignition) is actually a basic flyback converter. The thing incorrectly called the "coil" is a transformer with a turns ratio of roughly 100 so it boosts the 200-300V kick on the primary side up to about 20 - 30 kV on the secondary (spark) side.

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11. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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What limits the voltage is that it's impossible to get an instant break. Even with mechanical points, electrons will jump across the contacts as they open and the voltage builds up allowing flow so it doesn't instantly stop. That's why points burn away over time.

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12. ### R_W_B Thread Starter New Member

Oct 19, 2011
15
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Thanks to all replies they have helped me visualize this more. I understand the alternator has nothing to do with coil voltage since the voltage regulator clamps that. What I meant was that since the 'speed' of the flux pattern movement affects the voltage raised, would an alterator spinning faster produce a higher voltage than the same one spinning slower?

13. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
It would if it was running open load. In a car, the output of the alternator goes through diodes that clamp it to the 14V system voltage so the voltage is relatively constant. The current is varied by the regulator feedback to supply system demand.

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14. ### tinkerman Member

Jul 22, 2012
136
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Does any one have some information or schematics on the new CD ignition sytems used everywhere on small engines? Kettering systems are pretty much old school now. I've not found anything describing how they work or what's inside that molded coil assembly.

15. ### debe Well-Known Member

Sep 21, 2010
955
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This is the circuit of a brush cutter CDI i dismantled. The HV & trigger windings are wound to gether on one leg. The ign coil is seperate but is moulded in to one unit.

• ###### Brush cutter CDI.JPG
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16. ### R_W_B Thread Starter New Member

Oct 19, 2011
15
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Wow thanks. That gives a clear view of things. I've never seen that before. On my Motorcycle the ECU (computer box) has a B- pin that pulses the Primary side of the coil, but I had never seen how a magneto affair would do it.
Course many yrs ago when CDI first came out as an add on to battery ignition systems, I remember you just put the CDI box in to replace the points and condensor. I surmise then the CDI got it's power off the battery in that case.