Its the same with a car ignition coil which works using the back emf from the suddenly interrupted primary current...

The turns ratio is only about 100:1 so 12 volts will only give a maximum of about 1200 volts output, but if the primary current is suddenly forced to zero the back emf produces a primary voltage of many hundreds of volts and so the secondary gives a very capable output of many tens of thousands of volts to strike an arc across the spark plug.

As for why you didn't get a 'kick back' from the transformer you mention, you were probably just lucky or more likely the current flowing from the multimeter was too small to give a noticeable current flow.

John.

To try explain it in qualitative terms.

An inductor doesn't like changes of current....
Thus when the 12v supply is removed the inductor tries to maintain the current flow, A voltage is induced which tries to maintain the current flow, as there is no low resistance path a very high voltage is generated to force the current to flow across an airgap or through an unsuspecting human.

Any winding has inductance and it is the property of inductance to store energy. When current flowing through an inductive coil is suddenly stopped the stored energy appears in the form of self induced voltage and the contact gap will be bridged by a high voltage.

The value of voltage will be = Inducance of coil X rate of change of current

When u r using multimeter the current flow will be negligible and back emf will be less.

Current through a coil produces a magnetic field. CHANGING magnetic field through a coil produces a voltage. When you disconnect the battery, the field disappears (changes very rapidly) inducing a large voltage. The multimeter does not produce much current so the change in field and inductive kickback is small.

Voltage across a transformer may be modeled as voltage across and inductor, therefore:

V=L*di(t)/dt

Where i(t)=instantaneous current through the transformer.

When you disconnect the transformer you are shutting it off, effectively trying to cut the current from i(t) = some value to i(t)=0A. Since the voltage is related to the change in current with respect to time, quickly trying to change the current from a finite non-zero value to zero quickly make the derivative of current di(t)/dt tend to infinity (or at least a very large number) (the slope of the graph of current versus time is very large); therefore V also becomes very large, hence the inductive "kickback" you refer to.

Folks,

With all due respect, I know some of you are far better educated than I, but I think you fall down on explaining "reverse EMF" or "reverse electromotive force" to someone who had to ask that question. If I may..

When you pass an alternating current, i.e., a current that alternates in polarity from "0" to a given positive peak of voltage, back down through "0" to the opposite negative voltage peak and back to "0," which is one cycle, you have described the voltage you find in your house.

Because that electric current is always in motion, you may graph such voltages, for reasons we won't get into here, as a sine wave, it creates a moving magnetic field around the wire that is conducting it. If you place another wire near the conducting wire and take a reading with a volt meter from the ends of the wire, you will find that the moving magnetic field has "induced" a corresponding and opposite flow of electricity in that wire.

The rule is that a moving magnetic field will induce (There is that word again) a current in any conductor within that field. You can do the same thing by moving a wire back and forth in the magnetic field. The process is called induction. We induce a current in the wire by virtue of the motion, either of the magnetic field or the wire in the field.

Wrap the two wires as two coils around an iron core and the effect is much amplified. You are no doubt aware of such devices. They are called transformers. They are called transformers because the ratio of the windings, i.e., more or less turns of wire, determine the ratio of the voltages in the transformer; transformer because obviously such a device can be used, is used, to change voltages to higher or lower values than the source provides.

Connect a direct current, i.e., one that maintains a given voltage level with out alternating, that is without motion, you will get a voltage in the second coil at the moment of connection and then, for our purposes, nothing. That is because a steady direct current in a coil produces a steady magnetic field, one that is not moving. To induce a current in a second conductor either the conductor must be moving through the magnetic field or the magnetic field must be moving around the conductor.

Now let us look at the spark coil in your car which is a form of transformer. The electric current that is connected to the transformer is from the car battery, a direct current source. Except for the one pulse instance at connection it would not induce a current in the coil secondary (High voltage because of windings ratio) winding. That of course does not serve our purpose of changing 12 volts d.c to 10,000 or more volts needed to jump the spark plug gaps as a spark.

To solve that problem circuit between the battery and the spark coils contains, in the distributor, a switch that with the rotation of the engine by the starting motor, opens and closes the circuit to the coil causing the magnetic field to rise and fall and therefore inducing a current in the high voltage winding of the coil, causing a spark to jump across the plugs in the gasoline vapor filled cylinder and starting the engine.

You should now understand that it is the presence of a moving magnetic field that induces a current in a coil winding.

Apply that to your question. When the battery was connected to the coil, the battery producing D.C., there was no inductive effect. But when you suddenly disconnect the battery, you cause the magnetic field to collapse, and in collapsing, that movement, creates in the coil a reverse current and magnetic field which under certain conditions, coil winding, etc., produces enough voltage to shock you.

Because the falling, magnetic field is reversed in polarity and induces a current reversed in polarity, that effect has become known as reversed electromagnetic force or field or reverse EMF.

j.