Lenz’s Law produces a back emf immediately after the circuit is open!
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- Опубликовано: 8 июл 2024
- This video was made to demonstrate the phenomenon known as back EMF in a simple circuit, employing a lightbulb, a DC power supply, and a large coil. The light bulb voltage is monitored using an oscilloscope. What is noticed is very subtle, but there is a small negative voltage spike that arises immediately after the circuit is open (no longer connected to the battery). If you freeze-frame it on one of those downward spikes, you can see that it is behaving as an negative inverse exponential curve, as theory predicts. It goes negative because the EMF that is generated tries to oppose any changes. This is Lenz’s Law (that minus sign in Faraday’s law).
This may not be that significant with only a 3 V power supply as seen here, but when dealing with high-voltage power supplies, sometimes the back EMF is so strong that the electric field across a simple open switch can pull the switch closed again-which can be extremely dangerous.
Also keep in mind that back EMF can be several hundred (or several thousand) volts. I once made a 12 inch coil of only 26 turns, and energized it with 12V. It generated 400V back EMF. That's why diodes are placed across relay coils.
Why doesn’t the coil in this video produce a large back EMF?
@@clownhands Current from the inductor is being shunted through the light bulb. That limits the magnitude of the inductive voltage spike. The other thing that limits the voltage spike is the size (value) of the inductor.
@@AxcelleratorT ok that makes sense. The bulb filament is still red hot, so it provides a low resistance shunt.
@@clownhandsthe light bulb takes takes the decaying current in the reverse direction as the switch is opened.
Yes, I connected LED indicator light across a solenoid coil on a truck. LED lasted about 12 on off cycles before dying. I gave a bright flash each time system was powered off. Fly back diode connected in reverse bias across coil solved problem
This is a great video. I am an aviation maintenance instructor and I teach this in my magnetos class. The collapse of the magnetic field of the primary winding (a few hundred turns of thicker wire) moves the field across the secondary winding which has several thousand turns of very thin wire and this is able to generate tens of thousands of volts that get applied to the high tension leads which lead to the spark plugs inside the cylinders of the engine to provide ignition of the fuel air mixture. The amazing thing about this (amazing to me at least) is that this is all done with a relatively small magnet that spins to create the initial current flow through the primary and the entire device is self contained (requiring no outside electrical input).
This is a great introduction into how switch-mode power supplies work.
Thanks for posting and keep up the good work!
This used to be a huge problem in the electronic security industry. We would warn installers that they HAD to install a kickback diode, parallel to the big maglocks or door-strikes, but they wouldn't, because the locks would "hang", very slightly (you would have to tug on the door, if you immediately tried to open it). Result? A lot of system boards went to an early grave, with pitted/burned relay outputs.
Excellent video. Back EMF can be mitigated with a reversed biased diode, to protect the circuit upstream. You would need to measure the spike to get a handle on the size and spec if the diode. Also, keep in mind, that other tings like transformers, are coils. This is why there are flyback or reversed biased diodes across the output and input to handle the back EMF when say a transistor is switching off and on quickly in circuit to say manage a PSU.
Professor Sam Ben Yaakov, you the best on explaining the stuff, thank you for this walk-through , cheers from Tanzania , I appreciate your infomative and intuitive videos
Dude- the algorithm just introduced this video to me… Freaking awesome and looking forward to binging on your stuff
Great Hands on demonstration 👌👍😁 will try this on my oscilloscope also connecting the inductor in series. Thank you
Thank you for a very nice Lenz's law demo. 😎
The name of the video wrapped it all up for me.
👍
It's called 'matte top coat nail polish' if you're looking. The gloss nail lacquer has some electronic applications.
i did wonder about that
I play bass and flamenco guitar. Thanks for noticing!
Do you slap tho?
@@dotslashsatan
No, he djents!
@@michaelmasuda7096 I wonder if Paco de Lucía knew how a SEPIC works. You can guarantee John Deacon does.
Great explaination!
Thanks for your sharing
Very nice coil.
I like that demonstration coil.
Thanks a lot this video really helped for my school project
You didn't mention, that coil also affects rising edge on circuit close. It makes rising edge slightly rouded - stretched in time.
Howdy. Yeah.
The "free wheeling" diode over the coil or an inductor is a proven remedy. But, as one commentor pointed out, it makes the turn off of magnetic valves and locks sluggish.
One may consider using 5 diodes in series over the coil. This will allow the backfiring emf to rise to 5 x 0,7 V = 3,5 V. The dissipation of the magnetic energy will be faster. There will be contact arcing, but way less.
A smarter way is to connect a diode and a zener diode in anti series and connect this over the coil. One may choose a suitable zener to which level one will let the backfire emf to rise. There is one problem though. Zener diodes are slow. The backfire emf slew rate must be moderated. This moderation may be done connecting a capacitor too over the coil. Suggested values. A magnetic lock, 1 uF. A DC contactor coil, also 1 uF. A relay coil or a small magnetic valve coil, 100 nF.
Regards.
Very nice demo of a basic physics concept that powers so many things today. Just subscribed - will have to look at your many other experiments and demos when I have time!
very intereting, though I am well aware of this and lost many good chips and FETs for this very effect. and yes, it is called back EMF, but I personally call it beatch
Uggh... Thank God for little coils!
With relays I know that often to counteract this back EMF sometimes a diode is placed across the coil in the reverse direction of the main current flow and that captures the back EMF and allows it to dissipate in the coil instead of going back into the circuit where it might cause damage.
It might have been more obvious to see the negative going spike if you set the scope to trigger on level change and used a faster time base sweep, since you're using a digital scope.
You could also put the coil in series with the circuit and put the scope across the coil.
Or instead of the light, use a relay as the coil but still scope the coil of the relay.
If you understand the physics, the the same blip you see will be missing at the beginning of the on , the same stored energy, is lost, it s called reactance, same applies to capacitors
An important part that 'back EMF' experimenters pay close attention to is the delta T in the denominator - see the (dPhi/dT) formula of Faraday's law at time 5:00 in the video. The complete formula is
E = dPhi / dT, Voltage (EMF) equals changing magnetic field / change in time. The "E" in this case is the back EMF voltage spike.
As the time of the change in current tends to zero - ie., as dT drops to zero in the denomintor - the voltage "E" grows inversely.
This is why the huge number of 'back EMF' experimenters on youtube and elsewhere seek out MOSFETS with the fastest switching times possible - to cut current flow to the coil in the briefest time possible.
*_A PHYSICAL ANALOGY_*
- a large water tower is filled with 1 million gallons; a release valve is opened a tiny amount and the water drains out slowly. You can stand beneath the tower as it drains with a trickle and be in no danger.
- the time span for the draining is shortened immensely: the entire bottom section of the water tower is removed instantly and 1 million gallons falls to the ground. It's not safe to stand beneath it
As the time to interrupt current flow to the coil tends to zero, the voltage spike grows inversely, and with a very steep rise time and large magnitude, like emptying the water tank quickly.
Faraday's Law.
.
Points condenser ignition coil from old days on motors used this up until 1970s.
If the lamp is not connected the bigger is the "effect".
Several years ago, I decided to measure the resistance of a microwave oven transformer's secondary on the bench using a DMM. I noticed a small arc when I pulled the probes away from the transformer. The measured resistance was rather low, less than a K ohm, so I decided to repeat the measurement and touched the metal tips of the probes with both hands as I made the measurement knowing that error due to my body's resistance would be minimal. This time I got a jolt as I pulled the probes away. The high voltage due to the collapsing magnetic field caused the DMM's measurement current through the secondary to deliver the shock. The DMM wasn't damaged because the current forced through it was no greater than its measurement current to begin with. If you remove the lamp and oscilloscope, the snubbing effect of the lamp will disappear and you will get a momentary small arc as you open the switch. This current collapse may create enough high voltage to damage the scope. That's why it's best to remove the scope for this experiment. If you hold your fingers across the closed switch, this circuit is likely to give you a shock as you open the switch if the inductance is great enough and the coil's resistance is low enough to allow sufficient current while the switch is closed. The bulb, when in the circuit, absorbs the coil's current when the switch is opened resulting in the relatively low negative voltage spike across the bulb seen by the scope.
This explanation of yours opened my eyes even more...
Current is Lorrentz. Magnetic field is lenz. Lenz is with a coil is inductively coupled, with a magnet on rotor on magneto if you want. The coil field change the polarity → ← or ←→ always stop the magnetic field pole and reverse for this with charge by the field of the magnet. The Back EMF on a coil is the discharge capacity of the wire and impédance on the system...never the capacity in the coil are on the maths for this... . On a coil you have R L and C if timing is introduce is another history, capacity of the coil Discharge...Tesla work on this, Best Regards
You should have done a capture on the scope!
its called the flyback voltage , specifically the reverse flyback , so the inductor flips polarity when it fly back
Why you didn’t use the trigger once on the oscilloscope to freeze the signal upon the opening and closing of the switch?
It would have given a better image that can be zoomed to show what did happen. A missed opportunity.
If you float the system, powered by batteries and do not ground it through your scope (use only the positive leads by the difference of two positive channels leaving negative disconnected), it is more impressive (max out probe ratio, because it's hundreds of volts, mine was 150 to 400, quick and harmless though). When the field collapses, it is a massive volt/current noisy chaos and then a ringing resonant ordered thud at the end as if there is momentum in the coil that must be stopped. Put a core in that coil, and it separates the resonant thud across a voltage. I cannot make sense of it. Is that familiar to anyone, and can anyone recommend a reference to what I describe? This is not my area, so terminology is off.
it sounds like you understand it just fine. before you cut the power, the DC current through the coil will have reached a saturation point with magnetic flux, preventing it from storing further energy in the intervening magnetic field through induction. when you cut the power, that energy very suddenly has nowhere to go but back through the circuit and will cause back EMF due to Lenz' Law. For a second the inrushing current would be trying blow the loops in the coil apart and inducing an oscillating magnetic field, hence a ringing or buzzing until the hysterisis dissipates.
the reason this doesn't happen with the core is that when the coil has something to magnetize, it will "choke" more magnetic flux lines, to a far greater density than before; the surrounding magnetic field no longer reaches saturation because the coil's inductance is now orders of magnitude higher. therefore, it can now react to those same resonance conditions without trying to dissasemble itself.
@Michael Masuda
Interesting Lenz law demonstration. Quick question are you in any means, trying to harness the spikes generated when the light bulb/coil is cut off? Well you may harness the spikes when the power to bulb is cut off, while harnessing the power while power to bulb and coil is cut off, then basically your are spending some energy to energize the coil, therefore this arrangement does not create any free energy.
as the magnetic field collapses it induces current to flow, the inductor resist changes in current, so if you pass 1 amp for instance it will attempt to maintain that level until its magnetic field has collapsed. this is the important part, it will try to maintain the current flow even if the current path resistance increases, and it will pump up the voltage to do it, do your same experiment except remove the bulb, and place a resistor across the switch, and watch your scope when you close then open the switch. its kind of how the coil in a car works, to take the current from a 12 volt source and generate like 50kv to make the plugs spark.
I have a coil like that, where did you get yours. I can't remember where I got mine.
can you stick a magnet on back of the coil? And test it in both directions of the magnet (N/S) and let us see the results? I'm thinking if that back EMF Spike will be greater or smaller. (but you need to stick the magnet so it won't move)
Shouldnt the oscilloscope read max volts when the switch is open?
capturing that back emf and using it to power the same light, by pulsing the switch at about 200 hertz.
What is the R of the coil? is it not negligible?
The Bedini engine uses the "Back EMF" phenomenon
How can I buy those mini incandescent bulbs with alligator clip hookups? Not blocked by the general incandescent ban?
BEMF. Back Electro-Magnetic Field.
Use a Compass on the front of the Coil and watch the deflection of the needle.
Put the Battery inside the Coil, they'll last about 18% longer.
Good treatment 👍
;)
EMF stands for electromotive force.
Well, ok, if you say so.
;)
@@snakezdewiggle6084 electro motive force, turkey.
and magneto motive force.
another uncomprehending parrot?
arrk! polly wanna cracker!arrk!
@@paradiselost9946 you sure deserve a cracker
John Bedini used that effect to charge batteries with a negative voltage if you connect it to an earth ground you will get a much stronger negative voltage .
Why would you think an earth ground will change anything? It’ll just reference that point to earth potential.
@@stargazer7644 strange question, i thought i had already answered that, because it would give you a greater negative potential? of course ground topology, depth of your ground and whether your above a water source such as a well or aquifer are also factors.
@@stevengibbon I think you're rather confused about what an earth ground is and what it does. Connecting to a ground cannot increase a DC potential. All it does is allow you to measure potentials in your circuit relative to earth ground. The main reason we ground things is so that exposed metal is brought to earth potential so that you who presumably are standing on the ground and are also at earth potential cannot receive a shock when you touch them.
@@stargazer7644 i think your the one who s confused i never said you could increase dc potential i said it would increase the amount of bmf your getting. either try it or don't it does not matter to me but you may learn something if you do.
@@stevengibbon You literally said "if you connect it to an earth ground you will get a much stronger negative voltage". You don't have a clue.
A tv repair man told me one of his trainees had quite a bad shock from a part of a tv, which was unplugged. Could have been something like this?
That could be due to a residual charge built up on the TV tube itself. I remember not grounding my wrist as a dumb kid and getting a terrible shock from trying to replace RAM into a first generation Macintosh computer. It was painful. Always use a ground wire on your wrist to avoid having current pass through your chest.
These are a big issue in relays. Use of snubbers / kickback arrestors are needed to stop this from destroying some sensitive components.
Back EMF is such a bad term. It's actually the current carrying forward in the same direction it already was... Like if you put a ferrite on a wire it suppresses the ripple because any increase in current generates a larger magnetic field but then if the current to the wire decreases then the field collapses a little and allows the current carry forward. If that was in some way opposite to the current that was in it then you would amplify ripple rather than filtering it out. And you'll find that the voltage going negative is just the current continuing to pass forward in the same direction that it was before but now there's nowhere for it to go so it registers as a negative voltage. You know the electrons stack up in the same direction that they were already going. There's nothing going in an opposite direction.
It was a missed opportunity to not end the video with a diagram of a diode in anti-parallell to deal with back EMF :)
So...why doesn't the 'back emf' light up the bulb, momentarily? Furthermore how is the magnitude of the 'back emf' calculated...just curious. Subscribed.
Not enough current. Also it only lasts a millisecond after the switch is closed.
I don't get why it produces a back emf. It's resisting the reduction in magnetic flux by inducing a current such that it re-establishes the magnetic field; the induced current travels in the same direction as the initial current, thus the E-field is still oriented in the same direction. Why would it not be a forward emf, and a positive voltage spike?
> It's resisting the reduction in magnetic flux by inducing a current such that it re-establishes the magnetic field
Well, so that it *maintains* the magnetic field. Inductors resist changes in current; they generate whatever voltage is necessary to achieve that.
> the induced current travels in the same direction as the initial current
Yes, he got that wrong.
> Why would it not be a forward emf, and a positive voltage spike?
Good question. To create the magnetic field, the voltage applied by the external circuit forces current through the inductor in a given direction. When the field collapses, the inductor tries to maintain current through itself in that same direction. To do so, it has to act as a supply: instead of being driven by the external circuit, it now drives the external circuit, forcing current around it and through itself. That requires the voltage at the inductor's terminals to be of opposite polarity to the battery. If the battery polarity's positive, the spike generated upon disconnecting the inductor must be negative.
Explained by E = - dA/dt
This is a simplified version of how flyback Transformers and tesla coils produce super high voltage and generate massive arcs.
That would explain why breakers are so hard to flip.
Seems as just as an ignition coil works in a car.
nice demo, shout out to your manicurist!
why don't you show how to capture that and feed it back to the batteries? capture the spike and gain energy.
It is the rapid collapse of the magnetic field that creates the back EMF not Lenz's law.
If there was an iron core the lamp would have blown out from switch-off
Counter emf
Sir, When i teach this subject it is important to let people understand that the magnetic field does not go away and it is not a ghost voltage. Those analogise are in poor choice. It is the collapse of the magnetic field that causes the electrons to flow back into the circuit hence back emf. That is the reason when using coils or anything that causes a magnetic field a diode is place in line to protect the circuit. Please correct your video.
Put a diode across the coil and call it a day.
Put a big honking slice of ferrous bar stock in the center of the coil and what happens with the back EMF? Maybe a stupid question but one day someone like me will try it with larger coils fewer turns.
Pleaase tell me it's stupid possibly deadly so don't do it.!!!!!
From what I can understand, it would be like putting a dielectric in between two plates in a capacitor. This will induce a greater amount of magnetic field flux density inside the coil when current changes, so more magnetic energy can be stored temporarily during that time.
I'm betting that you are a guitar player?
According to my research this is a direct conversion of dielectric tension (voltage) to amps, by collapsing the voltage and letting it release into magnetism in a coil, AKA the Bloch wall, you're accessing the zero point. This was why Tesla was adamant about using high voltage and high frequency resonance.
The early electrical engineers could not figure out what was causing their transformers to blow when they would cut a circuit in large transmission lines back around 1910, what was happening was voltage rebound. With AC the electrical wave oscillates back and forth and when the circuit was cut, it would SNAP back or rebound with tremendous force, much higher than their components could handle. This would result in an explosion, thereby dissipating the energy through thermal radiation or heat.
What ended up happening was C. P. Steinmetz was brought in to fix this problem. He called it "back EMF" or emf that flowed backwards with amazing force/pressure. We call it amps. Get it in resonance and you'll be converting the voltage directly to amps every cycle.
I recommend starting with 50 volts and going up from there, 3 volts will get you barely anything as seen from your test results, since voltage multipliers can be used for this getting a high voltage power source is easy, and the implications of that are obvious. For AC input you'd want to use tesla's Bifilar coil to get around the disruptive aspects of this.
I'd love to see a video showing some test results of this.
you got me confused... the coil in parallel to a bulb in DC should just short the bulb...? why isn't this happening? diodes?
why is the coil not in series to the bulb??? when in series, the coil produces the same effect
The coil has resistance
actually it does shorts but a very little short because the coil has resistance. Why does the coil have resistance? because resistances depend on how much the length of the wire is used.
it has resistance and it has inductance?
when the coil is first applied, it has no magnetic field.
the current starts to flow through it, but its inductance opposes the change (back emf... this video, like 99.9999% of others, is incorrect in what Back EMF actually IS)
the inductance is the increasing current due to an applied EMF, causing an expanding magnetic field that cuts the conductors of the coil and induce an opposing EMF to the applied EMF. that is back emf and inductance is a measurement of it.
once the magnetic field has reached the maximum size the applied EMF will create in that coil, there is no longer any opposition to the current, as there is no change of magnetic field or current. this being an air-core coil, that isnt terribly large, will take fractions of a second to "stabilise".
at this point the coil is a simple resistance, determined by what sort of wire its made from, its cross section, and how long it is...
and from there, the current supplied by the batteries must flow through the entire circuit. the bulb, and the coil.
the bulb just happens to have a lower resistance and hogs most of the current...
I am not sure that is what back EMF is. Secondly, a coil essentially goes short after it is fully magnetically charged and thus should short out the globe as it is in parallel. WTF????????????
YES YES YESYESS
I HAVE VISUALLY UNVEILED THIS
GHOST CURRENT!
Okay Okay I have a way to see this!
And Without*** a monitor...
my video called "Laptop
LCD's reveal this?!"
You can actually SEE the liquid crystal magnetically polarizing, it's like a shock pulse with delayed fluffy clouds, then it actually fizzles and "sparkles" (for lack of a better term) away until neutral again!
I implore you to take a look! You will most definitelyyyy find it fascinating!
Would love to hear an educated opinion!
-just want to share my very strange phenomena l've happened upon in my mad scientist experiments to the science community more than anything- it’s hard man! 🤪
those fingernails
Bass player, intermediate flamenco guitarist. And drummer but you don’t need nails for that. 😁
yo dawg y yo nails look shiny?
Only on my right hand. I play flamenco guitar and electric bass.
just put a diode and will stop this ...
Why are your nails so shiny? What the... :D
I’m a bass player and an intermediate flamenco guitarist.
Very cool, just like the ignition coil.
Please lose the ladies nail polish
I have brittle nails, especially on the pinky nails and the ring fingers, so I put nail polish on them. Is there a mens nail polish?
@@franzliszt3195 Never thought about it, but that's a good idea. I do happen know that there are clear matte top coats, that should do the trick i guess.
@@marshad82 So far I have not found one that is invisable. I know that people are looking at me as I'm a weido, so I try to put it on just at the very end of the nail but that's not perfect also. No one has ever said anything, but I think they are coming to conclusions that are not me.
@@franzliszt3195 By virtue of you using it it is now a mens nail polish
Lenz law does not produce anything. It's the intrinsic character of a coil that is described mathematically by Lentz law.
Fun guy at party
@@omnirath dumb guy at making sentence.
Nice nails honey! ❤
Purty nails🤩
You should see his lipstick !!
@@merlin5476 no lipstick. I play bass and intermediate flamenco guitar. I’m also a gigging drummer but you don’t need nails for that. 😁
dude why are your fingernails so long and why you have nail polish on them?