Back-EMF Manifesto - A Key, hidden in plain Sight.

[dieter]

As far as I see, this has been confirmed to some degree eg. by Gotoluc and Woopyjump, even tho they did not draw this fundamental conclusion.

[Magluvin]

When the Pulse ends, the current reverses direction. The Voltage is higher in the coil than in the power supply, therefor that is a potential diffrence, the coil charges the supply for a brief moment.
You can use Diodes to make this reverse current flow over a diffrent path. Sometimes called Flyback Diode.

When the pulse ends, the current from the coil afterwards is in the same direction as it was during the pulse.

When the pulse current is applied, the field of the coil is building outwards, and that field cuts the windings of the coil causing a reverse influence of the input current, CEMF, which causes the delay in the coil to reach max current from the input. The higher the inductance, the longer it takes to get to max current/max field. Now when the pulse current is taken away, that field that was built collapses inward of the coil, opposite to when it was building. So now the collapsing field is cutting the windings inward causing the current in the coil to be in the same direction as was the input pulse.

I had thought the same as you back some years ago. But no, the current developed in the coil after the pulse is taken away IS in the same direction as was the input currents. The 'back emf' statements can confuse us into thinking as you are, as did it me, back then.

Mags

[tinman]

 author=dieter link=topic=17611.msg516959#msg516959 date=1519264733]

When the Pulse ends, the current reverses direction.

Wrong.
The current continues to flow in the same direction through the coil during the inductive kickback part of the cycle/when the source current is disconnected..

Only the voltage inverts across the coil when the source is disconnected.

the coil charges the supply for a brief moment.

No it dose not,as the current flow will be in the wrong direction through the coil,when the coil is disconnected from the current source.

You can use Diodes to make this reverse current flow over a different path.

There is no reverse current.


Brad.

[dieter]

Hi Mags,


As much as I appreciate an opinion that is against accepted beliefs, here you arr definitely wrong.


Just add two diodes at the positive supply side of your coil, one into the coil, and one out of the coil. In the path from out of the coil to the positive Supply (don't say I have to explain Diodes) you add an LED, with positive/long pin tobthr diode side.


If your statement were true, the LED may not be lit, never. But it is.


That's all I can say about that and maybe, this, verified by myself and many others (that back emf current flows backwards) us if such a basic knowledge level, that I normally don't feel obliged to explain it. Especially since I just mentioned the potential diffrence. An understanding of the concept of potential diffrence is assumed, as it fits in one sentense: Current flows from high to low potential.


Do the experiment again. Note: there is also a feature of current inertia, meaning flowing current wants to keep on flowing ( and can therefor even arc between contacts if disconnected). This may not be confused with Back-EMF from a collapsing magnetic field. If your test coil has low back EMF features, you may have mixed up the two effects.

[tinman]

 author=Magluvin link=topic=17611.msg516966#msg516966 date=1519270261]

When the pulse current is applied, the field of the coil is building outwards, and that field cuts the windings of the coil causing a reverse influence of the input current, CEMF, which causes the delay in the coil to reach max current from the input.

What is the reverse influence Mags?

The higher the inductance, the longer it takes to get to max current/max field. Now when the pulse current is taken away, that field that was built collapses inward of the coil, opposite to when it was building. So now the collapsing field is cutting the windings inward causing the current in the coil to be in the same direction as was the input pulse.

Are you sure it's a magnetic field?

If we take a toroid core inductor for example-->is not the magnetic field contained within the core?
If so,then what field is collapsing inward around the wire ?

The 'back emf' statements can confuse us into thinking as you are, as did it me, back then.

EMF is measured in volts--has nothing to do with current.

Brad