Storing Cookies (See : http://ec.europa.eu/ipg/basics/legal/cookies/index_en.htm ) help us to bring you our services at overunity.com . If you use this website and our services you declare yourself okay with using cookies .More Infos here: https://overunity.com/5553/privacy-policy/If you do not agree with storing cookies, please LEAVE this website now. From the 25th of May 2018, every existing user has to accept the GDPR agreement at first login. If a user is unwilling to accept the GDPR, he should email us and request to erase his account. Many thanks for your understanding

Custom Search

### Author Topic: Understanding the sparks created when using a relay to switch a coil.  (Read 26082 times)

#### CuriousChris

• Sr. Member
• Posts: 280
##### Understanding the sparks created when using a relay to switch a coil.
« on: October 19, 2010, 06:33:21 AM »
It seems to me that many people don't understand the cause of the spark exhibited when a relay contact opens after supplying power to a coil. So I thought I'd explain it.

If you understand inductor theory please move on

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.

A coil is a winding of conductive metal, usually insulated copper around a former, the former can be any material often its purpose is to give the coil shape, most of the time it is used to increase or more accurately concentrate the magnetic lines of flux produced by the coil.

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.

This part is important...

The speed at which the field collapses controls the voltage developed across the coil. The faster the collapse the higher the voltage.

When you interrupt the current flow into the coil using an unprotected relay/reed switch a visible spark jumps across the contacts. This spark is caused by exactly the same mechanism used to create the spark in a cars engine. Its the back EMF of the coil creating that spark. It is not some sort of battery effect from the contacts as I read once. In a car you have the ignition coil. its just a fancy coil designed to harness back EMF.

To go a little deeper

As soon as a gap appears between the contacts of the relay, current stops flowing into the coil. The magnetic field immediately starts to collapse. Because there is no load on the coil (current can't flow as the relay is open - just) the magnetic field collapses very fast. This as stated before creates a higher voltage, as long as no current flows this voltage increases, until a point where the voltage across the relay contacts reaches high enough to ionise the air in the tiny gap between the contacts, creating a plasma. Current starts to flow through the plasma and we see that as a spark.

As the contacts continue to open the plasma grows between the contacts and current continues to flow until finally the magnetic field in the coil collapses to the point no more current can flow and the spark disappears.

While this current is arcing across your contacts in a little light display it is also destroying them. That's why if you look at the contacts after a few uses they are pitted and burnt. It takes energy to do that and the energy is supplied by the collapsing magnetic field.

When designing a circuit which uses an inductor (coil) one must take into account this problem and design around it. the easiest way to solve it is to place a small capacitor across the contacts. What that does is gives the current an alternate path to flow while the contacts are opening. Eventually the contacts are open wide enough to prevent the plasma developing and therefore prevent the spark occurring. The capacitor must be big enough to absorb the energy from the coil without failing which means it must deal with very high voltages or lots of current. Also the coil itself must be capable of dealing with those high voltages or the insulation will break down and your coil will short out.

Usually a capacitor across the relay contacts is insufficient protection. In this case (most of them) another way is needed to dissipate the energy from the collapsing field. A Bendini motor attempts to capture that energy and reuse it. But for most applications you simply need to place a diode across the coil. Remembering that back EMF is the opposite of original EMF, if you place a diode in such a way that the back EMF is shorted across the coil. What happens is the current from the back EMF flows through the diode and back into the coil creating a loop. This puts a load on the coil which slows down the collapse of the magnetic field which in turn reduces the voltage developed (remember the speed of the collapse controls the voltage) thus the excess energy is safely dissipated in the coil itself (as heat). The capacitor across the contacts now only needs to deal with the original voltage which is less than the hundreds or thousands of volts back EMF can generate.

Before you start thinking about uses for these hundreds if not thousands of volts, keep in mind volts is not energy. If it was we could power the house by brushing our hair.

CC
« Last Edit: October 19, 2010, 07:13:08 AM by CuriousChris »

#### pese

• TPU-Elite
• Hero Member
• Posts: 1597
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #1 on: October 19, 2010, 12:12:48 PM »
This is the best explantion that i seen over years.

i ask to collect them (as link)
in my URL /Homepage

www.alt-nrg.de/pese

G Pese

#### wattsup

• Hero Member
• Posts: 2606
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #2 on: October 19, 2010, 03:06:18 PM »
@CuriousChris

Thanks for a good write up as @pese suggested.

The only problem I have is with your use of the term back EMF (BEMF). In my book that term should not be used or confused with the term flyback. There have been many threads on this topic and most will end with each persons personal vision of what each are.

But in my book, you are describing flyback.

When the contact closes, the feed supply is the EMF and it is automatically met with the coils inherent level of BEMF or (CEMF for some countries). You only get BEMF while you apply EMF.

As an analogy, if you put 10 pounds on one side of centrally supported length of wood, that side would be on the ground and the other side would be raised. As you then apply downward force on the high end (EMF), you will automatically feel the weight of the 10 pounds working against you (BEMF). This working against can only happen while you push down. Ounce you let go of your end, the back force can no longer exist and becomes a fast drop (flyback). The first two forces happen at the same time (EMF/BEMF), and the last force happens alone (flyback).

When the contact opens, there is no more EMF hence there cannot be any more BEMF. That return collapse of the field as you put it is what I would call simple pure flyback.

But this has always been a point of debate. I do however like the way you explained this in step by step method.

The other point is the spark itself. Yes it is a common idea that it is the ionized air that is providing the bridge onto which the voltage can jump from one contact to the other. There again, I do not believe this is what is happening. There is more there then we would think possible. Saying that the air is being ionized is like saying a lighting strike also ionizes the air in which it travels downwards and meets the rising lightning bolt that meet 1/3rd by 2/3rd of the way. The ionizing process is just a subset of the total reaction because the air happens to be in the way. The magnetic stress levels at the contact points are so high that it is this high magnetic field attracts ambient ether to super concentrate between the stress points to generate a plasmic discharge. They say your hair will rise before a lightning strike. Imagine the magnetic forces involved. The contacts getting pitted is also a subset of this.

Case in point is that it is now understood that many of the craters on the moon were created by lightning discharges. You can also have lightning discharges in space. There is no need for air. Also, the simple fact that one can use magnetically quenched spark gaps indicates that there is some other force there that can be manipulated.

So you see, there are many ways to look at the same condition and derive different understandings. I do not say mine is right, but only that one has to stay open to all the possibilities.

#### PhiScience

• Jr. Member
• Posts: 86
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #3 on: October 19, 2010, 04:47:56 PM »
(fly-BEMF) Short and Sweet

When a current is started, or increased, in the primary circuit, the induced current in the secondary will flow in the OPPOSITE direction, which seems to indicate that the free electrons in the secondary are given an impulse in a direction opposite to the direction of movement of the electrons in the primary.

Such an impulse can be imparted only by a magnetic field around the primary.

The induced current in the secondary will continue to flow only as long as the acceleration of the electrons continues.
After the current in the primary on longer increases in strength, there will no longer be any induced electromotive force in the secondary, but the free electrons in the secondary will still be held in their oriented positions by the magnetic field.

If the current in the primary then diminishes or stops flowing, the magnetic field will be removed from the secondary, and the free electrons which were held in oriented positions by such a magnetic field, will be released, whereupon they will spring back into their natural positions which will constitute a flow of current in the same direction as the current in the primary.
« Last Edit: October 19, 2010, 06:12:47 PM by PhiScience »

#### CuriousChris

• Sr. Member
• Posts: 280
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #4 on: October 20, 2010, 03:04:55 PM »
wattsup,
A flyback transformer is a special transformer that uses back emf to achieve the necessary voltage (EMF) to cause the electron beam on a TV to "fly back" to the start of the scan line.

Back EMF is actually caused by the collapsing magnetic field. The name comes from the electromotive force generated by the collapsing magnetic field, this emf resists the collapse of the magnetic field, thus the current is in reverse or backwards to the fields original generating current. If their was no losses in the coil, in other words if the copper was a superconductor (and the terminals shorted), then the back emf  would prevent the collapse of the magnetic field. When discussing the spark developing across the contacts of a relay this is the only EMF we need to discuss, to muddy the water with other induced losses does not help the discussion at hand.

Lightening strikes are the result of ionisation of the air. That's what the smell of ozone is. When a lightening strike occurs its because of the potential difference between the ground and the charged molecules in the air. The ionisation must occur before any current can flow and any visible spark occurs. But there are many types of lightening strikes and much of the fundamentals are still unknown. ground to cloud lightening is where the ionised air rises to a point where the lightening strike can occur. This upward rush of negative ions is called the leader stroke, and it comes before the downward rush of current.

Lightning strikes because of the ionised air between the ground and the sky it has nil to do with magnetism, The magnetic field is induced only after current flows. Ionising the air is a result of the very high level of static electricity.
Hair responds to static electricity. that's why your hair stands on end before a lightning strike. Not because of magnetism. try running a strong magnet over your scalp see what happens.
Magnetism, solar wind, normal wind friction all have a lot to do with creating the potential difference that results in lightening. but before the current causing the effect known as lightening can flow, ionisation must occur.

Ionised air is a much MUCH greater conductor than non ionised air and a vacuum is not a conductor at all. So the ionising of the air caused by the increase of voltage across the contacts is the PRIMARY reason for the spark. If you operated the contacts in an absolute vacuum the spark would not occur. For the spark to occur in a vacuum you need a voltage so high the electromagnetic field (static of course) rips electrons across the gap OR as any tv tech will tell you you heat up the cathode until the electrons start to boil off and then the static field (provided by the beck EMF of flyback transformer) directs the electrons to the anode. This is exactly how a tv monitor works. If the voltage was high enough for a spark to occur across the contacts in a vacuum without heating of the cathode then it would be so high the insulation on the copper winding would fail. it would fail LONG before the spark could develop because even the very best insulation is a better conductor than a total vacuum.

A "magnetic spark quench", quenches, which means suppresses the spark. It does not prevent it. How it works is when a spark occurs, its because there is current flow. current flow induces a magnetic field. this magnetic field responds to the presence of another magnetic field. which can draw out and stretch the original arc, by doing so it can cool down and break the ionised path that was allowing the current (and spark). The current MUST flow first therefore their must be a conductive path first, that's where ionisation comes in.

The pitting is (mostly) caused by the violent spark jumping from one contact to another. to say its something to do with magnetism is rather strange. Where does the magnetism come from? If the relay is 500 Metres from the coil it can still be burnt and pitted by arcing, yet there is very little source of magnetism around it.

phiscience we are not discussing transformers but coils. They use the same technology but in many respects are very different, for starters there is no secondary. But having said that you are correct except in your last paragraph. I think you are confusing particle spin with current, spin has a lot to do with magnetism, as best I understand it, it gives rise to magnetism and in turn is effected by magnetism. The electrons don't return (spring back) to their natural position causing current. Where would they go if their natural position is back inside a battery now disconnected by the relay?
The current is a result of the acceleration of the electron caused by potential difference or EMF in turn caused by the collapsing magnetic field. The spin may or may not return to normal depending on the material. For harder materials they don't return to their original positions or more correctly orientations and therefore some magnetism is left behind and the material shows a remnant magnetic field. In effect you are talking about the hysteresis of the magnetic material, this needs to be factored in when one is discussing the core of a transformer.

There is much more to learn about electromagnetism. And I believe the fundamentals of magnetism itself is not understood very well at all. but we must accept that in most cases the text books are correct. As an electronics technician I know this to be true. Its how I earnt my income back then. If they were wrong, then I wouldn't have been able to do my job.

CC

#### gyulasun

• Hero Member
• Posts: 4117
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #5 on: October 22, 2010, 12:41:35 AM »

....
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

• Sr. Member
• Posts: 280
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #6 on: October 22, 2010, 08:09:59 AM »
gyulasun

Quote
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.

Quote
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

• Hero Member
• Posts: 4117
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #7 on: October 22, 2010, 02:47:11 PM »
...
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

• Sr. Member
• Posts: 280
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #8 on: October 23, 2010, 03:29:33 PM »
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

• Hero Member
• Posts: 5884
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #9 on: October 23, 2010, 07:10:25 PM »
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

#### gyulasun

• Hero Member
• Posts: 4117
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #10 on: October 23, 2010, 07:21:15 PM »
Hi CC,

I am afraid it is you who is uncertain here on what is the connection between the collapsing field and the current flowing after the switch-off. You wrote:

"Counter emf is not countering the original current its countering the collapsing flux."

Emf cannot counter any flux.  However, the emf can start a current flowing in the coil.  However, current can only flow in a closed circuit and because the switch has been opened, seemingly there is no closed circuit for the coil: it has the induced emf across its endings but no closed circuit...

You wrote:
"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."   This latter part is ok but you did not clarify why current flows?

This is where the coil self-capacitance comes into play:  whatever (small) capacitance the coil possesses it will constitute a parallel LC circuit with the coil's self inductance (a high L/C ratio is involved), thus the induced emf  "finds" immediately a closed LC circuit from the very moment the relay switch is opened and the energy from the collapsing field will start swinging in this LC tank circuit. Now the current starting to flow in the coil part of the LC tank will of course have an opposite polarity and a lower amplitude with respect to the previous coil current so the original current from its instant original value will be gradually reduced to zero in a decaying "ringing" fashion (depending on the L/R time constant and any damping).

Here is another quote from you:
"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."

Now it is not clear which counter emf you speak above?  The one you use for naming the induced emf due to the collapsing field after the current switch-off  OR you meant the counter emf inherently created in any coil while its input emf is connected to it?  Because in the latter case you are right: the losses in the coil makes counter emf be slightly lower indeed than the input emf and this is valid for a continuous operation like an electric motor or a transformer etc works.  However in case of the former case the induced emf can be many times higher in amplitude than the input emf and this is valid for the current switch-off case, no more input emf.    This is the example why you have to differentiate between the two cases by using different terms... even you seem to be confusing the two.  While you know correctly the induced emf after the current switch-off can be many times higher than its input emf as you wrote it in your very first post.

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

Yes this is ok in itself, emf is created by a changing magnetic flux but I did not object emf in itself but objected using the term back emf  (or counter emf) when it is meant referring to the induced voltage or induced emf that is created after coil's current switch-off.

Re on the term flyback pulse:  I agree it is not used at university or college levels for referring to the induced emf that is created after coil's current switch-off
and I admit I also used it in the past for referring to the same but just due to the lack of the correct term: to make it clear I meant the induced emf produced by a collapsing flux after coil's current switch-off.  I used it because it described the situation very well: the sawtooth waveform switches current off at the end of its up-ramping in the primary coil of the line output transformer and then the CRT is blanked while the electron beam is brought back to start the next horizontal line:  the  time period under which the beam returns is called the flyback time and exactly under this time the collapsing field in the primary coil creates the voltage spike (usually 1000-1200V peak), this is where the term flyback pulse originates from (just for those not familiar with). So to use the term flyback pulse for naming the induced emf that is created at flux collapse has much more sense than using the back emf for referring to the same situation (but still not fully correct, I agree).

Re on bifilar coils: I agree and I think you may wish to continue on bifilar relay coils...?

rgds,  Gyula

EDIT I was writing this answer in Notepad and did not notice Magluvin already posted his text above.

#### gyulasun

• Hero Member
• Posts: 4117
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #11 on: October 23, 2010, 08:08:02 PM »
....
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.
.....

Hi Mags,

Very interesting and I assume you tested the series LC + diode setup and found higher voltage across the cap than the source gave. I wonder if you used a solid state switch or a mechanical one, in case of the latter perhaps simply touched two wires together and then pulled them apart by hand?
It would be good to operate the switch in a controlled fashion so that the ON time could be variable in the microsecond range, depending on the L/R time constant, maybe there is an optimum length of ON time whereby the voltage in the cap gets to a maximum value.

I hope this is not off topic for CC's thread.

rgds,  Gyula

#### Magluvin

• Hero Member
• Posts: 5884
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #12 on: October 23, 2010, 09:56:40 PM »
Hey Gyula

I agree with the short timely pulses.  Think about a pendulum. At what time in the swing cycle, also where in the swing cycle would you apply the KICK to keep it going.

Or if we want to keep the flywheel going, more likely we have to discharge a cap into it on a timely basis. The larger the inductor, the longer the flywheel will turn. A large inductor will take a bit of kicking to get it up to speed, but as it gets going, it can be kept turning on a practical continuous basis with kicks.  Is this SM's secret?
Its output would be DC.

Mags

#### Magluvin

• Hero Member
• Posts: 5884
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #13 on: October 23, 2010, 10:34:07 PM »
Oh Gyula

It was just simple testing, reed switches, relays.  Im in the middle of moving, but I will be back on it shortly.

The test I proposed above to charge the cap to nearly double should be very clear on my theory, and easy to do for anyone.  Its not extra energy in the cap, as in more in the cap than what came out of the source, it is equall.
But, if we can cut off the source before the cap is charged higher than the source, can we loop the cap, coil and diode, to let the coil continue to wind down and pump anything extra from the other side of the cap.  If we cut off the source, for example, exactly when the cap is equal to the source, if the coil causes the cap to charge any higher than the source, its free.   Would you agree?  ;]

Mags

#### gyulasun

• Hero Member
• Posts: 4117
##### Re: Understanding the sparks created when using a relay to switch a coil.
« Reply #14 on: October 23, 2010, 10:50:05 PM »
Oh Gyula

It was just simple testing, reed switches, relays.  Im in the middle of moving, but I will be back on it shortly.

The test I proposed above to charge the cap to nearly double should be very clear on my theory, and easy to do for anyone.  Its not extra energy in the cap, as in more in the cap than what came out of the source, it is equall.
But, if we can cut off the source before the cap is charged higher than the source, can we loop the cap, coil and diode, to let the coil continue to wind down and pump anything extra from the other side of the cap.  If we cut off the source, for example, exactly when the cap is equal to the source, if the coil causes the cap to charge any higher than the source, its free.   Would you agree?  ;]

Mags

Yes, I would...

and I would like to understand how the extra voltage can get into the cap once there is an open circuit? Maybe in the reed switch's inner air gap there is still some leakage-through possibility from the coil's collapsing field induced spike?

Gyula