Inductive Kickback

[Magluvin]

Id call it Inductive Kick Forward.   

Mags

[Belfior]

But in addition as the cap charges and the internal impedance of the cap goes up the discharging coil keeps raising the voltage to overcome the resistance of the return path. 

Thx Carroll!

What If I put a 10MOhm resistor on the return path? Can I get the voltage to go into thousands and then have it jump a gap to create another return path?

[synchro1]

Hi Belfior,

It doesn't seem like anyone has actually answered your question yet so I will try to answer it.  First I need to warn you that you should totally ignore any of the foolishness posted by Synchro1.  I don't know if he is deliberately being misleading or he just is really that confused about how inductors work.  But if you have read the whole thread you have seen several try to convince him he is wrong about inductors.

The first thing you need to understand is actually how inductors work.  A mechanical equivalent is the flywheel.  An inductor resists a CHANGE in current flow just like a flywheel resists CHANGE in speed.  A former member of this forum (Milehigh) taught me that equivalent many years ago.  So what is happening in your circuit?

When you first turn on your circuit the voltage from the battery is flowing through the inductor to the cap and charging the cap up to close to the battery voltage.  Then the charging of the cap stops and all current flow stops.  Now you turn on the fet.  Current again begins to flow from the battery through the coil and fet and back to the battery.  What you have to remember is the current flow is not instantaneous.  Just like when you first turned on the circuit you saw at the bottom the current go up at an angle.  This is how an inductor works, it resists current CHANGE.  So depending on the voltage applied and the inductance of the coil it may take a few milliseconds to several milliseconds for the current to reach its peak value.  As the current reaches the peak value the voltage across the coils drops.  You can also see that in the bottom trace.

So you now have full current flowing through the coil and back to the battery through the fet.  So what happens when you turn the fet off?  Remember the coil or inductor resists current CHANGE.  The goes the same for whether we try to increase the current or decrease the current.  When you turn off the fet that current flowing through the coil wants to keep going just like a flywheel wants to keep spinning after you remove power.  But it now can't keep going through the fet because it is now turned off.  One of the interesting things about inductors is they will increase the voltage during inductive kickback until that current can go somewhere.  Rather strange but not really if you continue to compare them to a flywheel.  What happens if you try to stop a flywheel from spinning?  The energy of the flywheel will be transferred to whatever you are using to stop the flywheel.

So where can the stored energy in your coil go?  It wants to go back to itself.  The coil which was the load has now become a source of energy instead of a load.  That means that if the current is still wanting to go the same direction as it was going, then the bottom of the coil has now become positive in relation to the top of the coil.  Or another way of looking at is to replace the coil with at a battery at the time of turning off the power.  And you connect the battery so that it wants to keep the current flowing the same way.  So the positive of the battery would  be at the bottom to keep the current going the same way.

Now since the current can't continue through the fet it will have to go through the blocking diode and into the cap and back through the battery and then back to itself.  Of course this depletes the magnetic field and the coil discharges.  You will notice that the voltage from the discharging coil gets added to the voltage from the battery.  But in addition as the cap charges and the internal impedance of the cap goes up the discharging coil keeps raising the voltage to overcome the resistance of the return path.  This is why coils are used in boost converters.  You need to study them to get a better understanding of what is going on when charging and discharging coils.

I hope this has been some help to you.  The main thing and most confusing to those new to electronics is to remember that the coil switches from being a load to being a source of current when the power is removed from the coil.  And that explains why the current can continue in the same direction while the polarity of the coil changes.

Respectfully,
Carroll

This is just a bunch of baffle gouge Carroll in his limited knowledge simply parroted from Milehigh to pass for a savant. He doesn't answer the question put to him, he just skillfully mis-directs the answer to a different subject.

I asked Carroll which direction the inductive kickback would take in Igor's "Reed switch spinner 2" circuit if the resistance to the battery from the top of the pulse coil was lower then the resistance to the ground, and he vanished off the thread.

[citfta]

Thx Carroll!

What If I put a 10MOhm resistor on the return path? Can I get the voltage to go into thousands and then have it jump a gap to create another return path?

Hi Belfior,

You have to remember what else is in the circuit.  If you raise the voltage too high it will find another path.  In the circuit you have shown it will most likely break down the fet and go through it.  Repeatedly doing that will destroy the fet.  How high the voltage can go is limited by how high the applied voltage is and how large the inductor is.  And of course by what other path it can take.  But I have easily gotten caps to charge as high as 300 volts from a 12 volt supply.  I have used a slightly different circuit than what you have shown.  In my circuit I just put a diode in series with a cap and the return of the cap back to the top side of the coil instead of to the battery.  Put that circuit on your simulator and try it for a while.

Glad I could help some.  Keep studying and learning.  Electronics can be a lot of fun.

Take care,
Carroll

[synchro1]

Hi Belfior,

You have to remember what else is in the circuit.  If you raise the voltage too high it will find another path.  In the circuit you have shown it will most likely break down the fet and go through it.  Repeatedly doing that will destroy the fet.  How high the voltage can go is limited by how high the applied voltage is and how large the inductor is.  And of course by what other path it can take.  But I have easily gotten caps to charge as high as 300 volts from a 12 volt supply.  I have used a slightly different circuit than what you have shown.  In my circuit I just put a diode in series with a cap and the return of the cap back to the top side of the coil instead of to the battery.  Put that circuit on your simulator and try it for a while.

Glad I could help some.  Keep studying and learning.  Electronics can be a lot of fun.

Take care,
Carroll

@Citfta,


Maybe Igor, in his infernal genius, simply tapered his ground lead.