Inductive Kickback

[woopy]

Hi Guyla

here is the scope shot following your shematic,  the expanded shot seems really noisy, can we get something out  of it ?

MH
thank's for explanation, just a little doubt, it seems on the second scope shot (expanded  the one that accopmpanied the shot you commented ) that the "braking " negative voltage seems to happen after that the current stops. Is it perhaps an artifact of my scope ?

Laurent

[gyulasun]

Hi Laurent,

Thanks.  Indeed the expanded shot is noisy, this is probably due to the 1 Ohm which may be small for sensing the lower current the flyback spike would provide towards the 210 Ohm coil plus the 0.3 uF capacitor after the reed is OFF.

If you could use the 10 Ohm instead of the 1 Ohm perhaps we could see the better shot on the current when it starts flowing in the coil after the reed is off.

OF course the 10 Ohm is a much higher value than the 0.5 Ohm coil DC resistance so you may have to increase the input voltage to arrive at the 110-120 mA input current range.

Gyula

[MileHigh]

thank's for explanation, just a little doubt, it seems on the second scope shot (expanded  the one that accopmpanied the shot you commented ) that the "braking " negative voltage seems to happen after that the current stops. Is it perhaps an artifact of my scope ?

Thank you for your comments.  I don not think there are any issues with your scope.  I believe Gyula is correct, and the scope shot that I marked up did not show the rapidly decaying current in the coil because the CVR was in the "wrong" place.  That shows you the problems that can happen when you work without a schematic diagram to backup your scope traces.  However, what I marked up on your scope shot still applies.  Assuming that you do the test for Gyula you will see the rapidly decreasing current in the coil as the high voltage spike happens.  The higher the load resistance, the faster the current will decrease and the higher the negative voltage spike you will see on your scope trace - just like applying the brakes to a spinning flywheel.

P.S.:  Vive la France!

[MileHigh]

I'm curious......Are you aware of a method which can be applied to electro mechanical devices which grants one the ability to produce "voltage spikes", a method which does not put the brakes on the prime mover? 

Regards

I am honestly drawing a blank here.  However, reading between the lines I am guessing that you are concerned about the fact that the voltage spikes represent the loss of useful energy as heat via a resistance.  And yes, what that does mean in many cases is that when a beginning experimenter gets excited about the high voltage spikes, he or she probably doesn't realize that that represents the loss of energy.

Switching to my preferred terminology, the current pulses from a discharging inductor don't have to always be dissipated in a resistive load.  They are used all the time in switching power supplies to create just about any voltage you want.

Note that if you are going to extract energy from a prime mover like a coil via current pulses, then that will put "the brakes" on the prime mover to a certain extent.

[synchro1]

@Tinman,

What do you make of this:

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

When a battery is connected to a coil, the battery producing D.C., there is 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".