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

[CuriousChris]

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

[pese]

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]

@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]

(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.

[CuriousChris]

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