Understanding the sparks created when using a relay to switch a coil.

[gyulasun]

....
Please excuse the fact that I am a little rusty on my old electronic theory. If I slip up dont be afraid to speak up.

....
When you apply a voltage across a coils windings, such that current flows through the coil, a magnetic field develops around the coil. It is maintained for as long as current flows.

When you break the current flow, the magnetic field starts to collapse. As it collapses a voltage develops across the coil. That voltage is in the opposite direction of the original voltage. This is the famed back EMF or back ElectroMotive Force.

Hi CuriousChris,

Sorry to chime in, you nicely explain these things, the problem is with the term: it is NOT called back EMF, it is called induced voltage, or voltage spike.

Here is what back EMF or as often called also as counter EMF is:

http://www.williamson-labs.com/480_rlc-l.htm

I consider the animation with the appearing and diminishing battery symbol that is shown with changing its polarity a very spectacular and true explanation for the back or counter EMF created in the coil's circuit.

There is another accepted usage for the term back EMF: in DC motors when the rotor starts rotating in the stator's magnetic flux, a voltage is induced in the armature coil and this voltage has an opposite polarity with respect to the input DC voltage. The input voltage and the back EMF (i.e. the normally induced voltage due to the flux cutting rotation of the rotor coil, i.e. Faraday induction) are always in an opposing balance with each other, being the back EMF slightly smaller than the input emf (due to coil and core losses).
This same situation is true for any transformer with a primary and secondary coil, as long as you do not load the secondary coil, the input AC voltage is opposed by the back EMF AC voltage created by the AC induction in the primary coil, they are in balance, this is why the current is minimal into the primary coil, it is just needed for making up the copper and core losses too.

Now back to you title topic: if you switch off the current in a coil, the flux collapses, this is a sudden flux change and the normal induction law happens as you nicely described. BUT the induced voltage is NOT called back EMF, please do not use it for naming the induced voltage thus created.
The term back EMF has already been used for many decades to speak about the two voltages I mentioned above that are appearing with opposite polarity with respect to the input voltage across the coils and that any inductance produces when current changes in them. The input voltage or EMF and the back EMF has almost the same amplitude, the back EMF is ALWAYS smaller than the input EMF.

BUT the induced voltage or the voltage spike that is created by the collapsing flux field in the coil after the current switchoff has a much higher amplitude then the input EMF has.

The term flyback pulse that is used by wattsup too is a much better name for referring to the induced voltage or voltage spike created at current switchoff. You nicely described it and then used the term back EMF for it... 

Please do not use it, accept it as an induced voltage or as a voltage spike or even as a flyback pulse.

Thanks,  Gyula

[CuriousChris]

gyulasun

Please do not use it, accept it as an induced voltage or as a voltage spike or even as a flyback pulse.

Huh! are you trying to instruct me in electronics?

Electromotive force is the force that creates what we measure as voltage.

Thats why they called it electromotive force. Simplistically, the force that gives electrons motion.

Voltage is the result of emf. In a circuit without resistance emf is equal to voltage.

A battery has emf. We put it on the label and call it the voltage of the battery.

Now, real batteries are constructed from materials which possess non-zero resistivities. It follows that real batteries are not just pure voltage sources. They also possess internal resistances. Incidentally, a pure voltage source is usually referred to as an emf (which stands for electromotive force). Of course, emf is measured in units of volts.

http://farside.ph.utexas.edu/teaching/316/lectures/node57.html

When the magnetic flux collapses it produces an emf which opposes the collapse of the field. Therefore it is called a back emf or counter emf which is really the more accurate term.

When simplifying a description the term voltage is often used instead of emf. Its done to reduce confusion amongst those who don't have the prerequisite knowledge. I deliberately used emf because emf is often referred to when talking overunity.

If you were to take a voltmeter and measure across the terminals you would get the result of addition of the emf induced into each coil (individual circle of insulated copper) less the voltage drop across the resistance in those coils combined. This would give us the induced voltage across the coil.
If our voltmeter was perfect. In that it drew no current from the coil and there was no other component drawing current from the coil then the measured voltage would also be the emf. In practice that cannot happen so the voltage is less than the emf.

As described earlier flyback is the name given to what the back/counter emf or induced voltage was used for. Not what it is! It was used for the purpose of making the electron beam in a TV (vacuum tube) fly back to the start of the scanline. To use that term when describing normal inductive coils or solenoid coils is erroneous. That doesn't mean it has not entered the vernacular, it has and so a lot of people call it that. They even mistakenly call the diode placed across a coil to suppress the induced voltage as a flyback diode. That is also wrong and makes no sense, a flyback transformer never has a flyback diode to suppress the induced voltage.

If I wanted to discuss motors and or transformers I would have.

I could also have gone on about protecting semiconductors in circuits that drive coils/solenoids. but I didn't want to, I just wanted to say there was no "magic" in the spark. Its not created by some weird unknown effect that science has ignored. it is in fact wasted energy that results in damaged contacts that eventually stop your device from working.

I am curious, there are probably a large number of differences in our use of the language. How many years was it you studied electronics/electrical engineering? what course was it? and where? are you currently an electronics engineer or instructor?

I did about 6 years both as part of my trade as a communications technician and another couple of years as self learning in a community college. I know thats not a university but I could not afford uni. All my study was done in Melbourne, Australia, That explains some difference in terminology.

Then I worked for the next 15 years doing electronics repairs. So understanding electronics was my bread and butter. now I am a computer programmer.

CC

p.s. in reality I should not have capitalised EMF its really only emf. so "back emf" or "counter emf" is the correct phrase to use.

[gyulasun]

...
Huh! are you trying to instruct me in electronics?
...

Hi CC,

No, I am not and I only meant to say if you care to read my post correctly that the induced voltage in question is the result of a response to the current switch-off in a coil. It is not present when you apply input voltage (emf) to a coil.  However the back emf is immediately created in the coil the very moment you apply input emf to it. This is a significant difference and I think it is important to name them differently.

And I also wrote I agreed with your text, I know your intention was and is to help understanding the sparks created when a relay switches a coil and it is welcome of course. I noticed I disagreed only with the term back emf usage for that case.

And in most part of the world at universities and colleges the term back emf is taught as an electric potential difference that opposes the current that induces it, hence the induced voltage (that you get across the coil when you switch its input emf off) is not correct to name as the back emf.
Of course you can call it like that, now I do not care, lol. I tried to help separating the terms usage.

I understand that this back emf term is widely misused, just by pure negligence, on many forums or on yahoo technical groups etc. and this is unfortunate. In such forums the members very often use the term back emf and later it turns out they did not mean the natural coil behavior that opposes current change but meant the induced voltage after the current switch-off in the coil.  You may say that the current switch-off in a coil is just a special case of the coil natural behavior against current change but if you do not refer to it exactly, it is misunderstandable. The induced emf at the flux collapse should be differentiated from the real back emf because the latter manifests also in working electric motors, transformers etc and when you switch these latter devices off, the created emf is not the same that rules 'inside' them when they are on.

If forum members wish to get more output power than they want to furnish the first thing is to understand and use correct scientific terms to communicate on their setups, otherwise the success is even more remote.

All in all, I did not wish to teach you but asked you. I am not an instructor, I have been an EE since 1974, graduated at a technical university, I live in Central Europe.

rgds,  Gyula

[CuriousChris]

I understand you disagreed with my use of the term. and I also felt you were instructing me on the correct term to use. "do not" is a very instructive phrase.

What I am saying is I used the 'technically' correct term. Voltage, is what you measure, emf is what it is and back or counter emf is the correct term to use.

It seems to me you are getting confused with what the counter emf is actually countering. Counter emf is not countering the original current its countering the collapsing flux. Counter emf is the reaction to the action that is the collapsing flux.

In motors and transformers there is also a counter emf. Its causation is the same, magnetic flux varying over time. but I am not talking about motors or transformers I am only talking single coils (electromagnets). I am also only considering what happens from the moment the contact opens and current initially stops flowing. Prior to that is a whole other series of events also involving counter emf.

My explanation of what causes the spark and discussion on how it is prevented starts at the precise moment the contacts open.

At this point in time the current stops flowing, therefore magnetic flux starts to collapse, it now has no 'supporting' current. Because collapsing flux is an action it must have a reaction. The reaction opposes the action. That reaction is the creation of an opposing (back or counter) emf, this opposing emf attempts to stop the flux collapsing.

Because an emf is developing, current wants to flow in the circuit, in effect the coil has become a battery with its own emf (and its own resistance). because current can't flow, remember the contacts have opened. There is no opposition to the collapsing flux. so it collapses very fast. for there to be opposition, current must flow, thus creating its own flux which in turn opposes the collapsing flux. (side note below about bifilar coils).

In an inductor (of which a coil is), two things control the emf produced and therefore the voltage measured. One, the speed the flux cuts the windings and two, the number of turns(windings) in the coil.

Therefore we have a coil with a lot of turns and a rapidly collapsing flux cutting those turns of insulated copper very fast. The voltage across the coil rises rapidly until the air between the contacts ionises. The story then continues as described in my first post.

Now I spoke of counter emf and then I spoke of voltage, they are essentially one in the same. the counter emf is measured in volts and is therefore the voltage. but the voltage measured includes the voltage drop across the internal resistance of the coil so it is lower than the emf. not by much but it is. if the coil had no resistance at all then the voltage and emf would be exactly the same.

Do you now see why I used the term emf? emf is the correct term to describe what the varying flux creates.

The uses of the term flyback is not correct it really only applies in a very specific situation.
http://en.wikipedia.org/wiki/Flyback_transformer

Just because other people adopted the term and used it to describe other things which utilised the same process does not make the use of it correct.

Why not call it the tesla voltage? The tesla coil is the predecessor of the flyback coil, so why not call it that? same for the car. why not call an ignition coil a flyback coil, it uses exactly the same theory. Well because the ignition coil was invented long before the flyback coil and it was used to ignite stuff. The fly back coil was specifically designed to make the electron beam in a tv flyback (actually if I recall it does more than that). Therefore calling the resultant voltage of the collapsing flux in a coil, the "flyback voltage", only displays an ignorance of the correct terminology to use.

CC

bifilar coils.
A bifilar coil is a specially wound coil, two wires are wound side by side, when flux cuts through the pair of windings a counter emf is created, each winding develops its own emf. If you wire those two windings together in such a way that the emf's oppose each other then the result is no counter emf as the two emfs cancel out.

[Magluvin]

This might be of interest.

First   The term flyback came from television design. Normal glass tube tv sets used 1 dot at a time to create the whole picture.  The dot is scanned across the screen at varyous intensities to create levels of brightness according to signal. When the dot or trace reaches the far right side of the screen, started from the left, the trace is drawn back to the left side very fast, Flyback, to create the next line of visual information, 600 lines, 60 times per second.

I have recently been seeing different things in inductors and bemf/cemf, than is described above or back in tech school. 
We are taught that when the current is disconnected that the mag field collapse creates a reverse current, bemf.
But thats not totally true. ;] The bemf is a product of the field collapse and the capacitance in the coil itself. A self ocsillator, very low capacitance very high freq.

Lets say we have a coil, relay will do, and we put a diode across the coil so that we can dump the so called bemf back into the coil to avoid the high voltage from getting back into the circuitry or damaging the switch that disconnectsthe coil.  Just like they taught us to do.
Notice the polarity of the diode and the direction of the current in the coil.  If we believe in electron flow, from negative of source to positive, the diode is across the source when the source is producing current in the coil, but the diode does not conduct from the source, just the coil, like the diode was not there at all.  When we disconnect the source, the magnetic field collapses of course, but notice that if the coil were actually producing a reverse current, it cannot flow through the diode in that direction, or else the source would have chosen the diode as a path when it was connected.  Its true. 
What we are really doing with the diode is dissipating forward current from the coils collapse into the diode. 
If there were no diode, when the source is disconnected, as the field collapses, it will run into its internal capacitance and only then BEMF of a high voltage, and that will be where you get the arcing across the switches terminals.
The inductor is like a flywheel. Add a capacitor to the coil and it offsets the center of the flywheel and becomes a pendulum/oscillator.  And the smaller the cap, the faster the oscillation.  The internal capacitance of a coil is usually small.


Try this.  Get a source, 12v is good, a coil , a switch, a diode and a cap.   With the switch in an open condition, connect all components in series, no particular order. hit the switch then release. Now measure the cap. Depending on the coil and cap, there should be nearly 2 times the voltage in the cap than the source.
When current is first applied, the coil starts to conduct, through the diode and Empty cap, we are getting the flywheel going. But by the time the cap reaches source voltage, the flywhel is not done yet and actually Pulls more current from the source as it winds down, and it pumps the cap to greater than the source. The diode prevents any oscillation once the flywheel stops. If the diode were not there, the higher voltage of the cap would want to level out with the source till the oscilation dwindles.

So from that, you can realize that the collapse that we all know of, continues to pump FOREWARD, FEMF after the source is disconnected, all that presure is stored in the cap, and that presure is voltage. The smaller the cap, the higher the voltage that will be developed.  And in an LC oscillator, KICK it with 12 v, and the oscilation voltages can be very very high depending on the cap and coil value.

Hope that makes sence.   If you really look close at the coil/relay with the protection diode in place and the circuit I described above, you will see things in a different light.   =]

Mags