Inductive Kickback

[MileHigh]

So effectively the "negative voltage" seems to  "brake"  strongly the current  (flywheel) which goes extremely fast to zero in my particular arrangement.

Laurent,

Think about a very simple thought experiment where you have a coil with some current flowing through it and a variable resistor across the two terminals of the coil.   If the variable resistor is a low value then it will take a longer time for the coil to discharge its stored energy.  If the variable resistor is a high high value - more RESISTANCE - the coil will discharge its stored energy more quickly.  I assume that you would agree that that makes sense.

Now you know that more resistance equals more voltage across the resistor.  The initial current is always the same - there must be more voltage across the higher resistance because voltage equals current times resistance.

It's all the same thing as how hard you apply the brakes to a spinning flywheel.  The harder you apply the brakes the higher the back-torque acts to stop the flywheel.  Most people have an intuitive sense that it is essentially impossible to instantly stop a spinning flywheel because that would take an infinite amount of force.  Almost everybody knows that force equals mass times acceleration.

If all this makes sense to you then you should realize that when you see a big spike from a coil when you are working on your bench that that represents the burning off of the stored energy in your coil because the coil discharged through a very high resistance.  That's all there is to it.  It's no magic, it's no mystery, it's just a discharge of energy and normally that represents the loss of electrical energy being turned into waste heat in your circuit.  In that sense it's bad news and nothing to get excited about.

Finally, in theory, any coil no matter what size can manifest a high voltage spike.  And if you have been following all of my postings, it's more the load that generates the high voltage spike, and not the coil itself.  It is certainly not "radiant energy" in any way, shape, or form.

A simple rule of thumb:  The higher the load resistance across a discharging coil, the higher the initial voltage spike, and the faster it discharges.  When in doubt, think of putting the brakes on a spinning flywheel because the behaviour is identical.

MileHigh

[MileHigh]

Duplicate posting, I will have to find an interesting picture or two.

[forest]

Again, you need a circuit with coil energized from battery , then disconnected and connected to the capacitor and diode to investigate the problem.

[citfta]

Hi forest,

Brad's post 101 has a scope shot that clearly shows what happens when a coil is charged and discharged.  So I am not sure what problem you are referring to.  Could you be more specific please?  I saw your other post about using a DPDT relay.  The problem with that is the noise from the switching of the contacts will make it much harder to see the changeover on a scope.

Carroll

[Magluvin]

Hi forest,

Brad's post 101 has a scope shot that clearly shows what happens when a coil is charged and discharged.  So I am not sure what problem you are referring to.  Could you be more specific please?  I saw your other post about using a DPDT relay.  The problem with that is the noise from the switching of the contacts will make it much harder to see the changeover on a scope.

Carroll

If you look back at the Ossie motor, he used 2 reed switches to disconnect both ends of the source. The purpose of that is that now you can have 2 diodes send the bemf back to the source directly, where with only 1 switch or reed that cannot happen directly.

Mags