#183

Today's "Back to Basics" tutorial topic - why flyback or snubber diodes are used around relay coils when switched or controlled by low power electronics. We'll talk about how and why dangerously high voltages can be generated from the collapse of the stored magnetic energy in the coil when they're switched off, and how the diode can protect the low power electronics from being damaged by these high voltages. Some voltage and current measurements are made on an actual circuit to see the real-world effects. A copy of the notes in the video can be found here:
www.qsl.net/w/w2aew//youtube/D...

[@MajorHenricks]

A common error: relative motion between a magnet field, and a conductor, does not induce a current in the conductor; it induces a voltage; then, if, there is a complete circuit, a current will flow.

[@w2aew]

Yes, a common error that I make! I'll pin this comment so others will learn from my mistake.

[@ricardothedardothebeastfro4747]

V = L di/dt

[@ernestb.2377]

@@ricardothedardothebeastfro4747 The Professor was talking about: e = N(dΦ/dt), I suppose.

[@NeilMartinsenBurrell]

Great use of the schematic, the circuit and the scope all together. Elegantly presented and very compelling for learning.

[@miket5506]

This video brought a perfect visualization to something I've only ever been told about--NOW I understand it. Thank you!

[@RexxSchneider]

Thanks for the very clear explanations. I thought I knew all there was to know about flyback diodes, but seeing the explanation about the current rating only needing to be equal to the current taken by the coil was a moment of revelation.

[@Roy_Tellason]

This strikes me as a good place to use up salvaged 1N4001s, which I would likely never use in any power supply application due to their low PRV rating... Way back in 1975 I was called in to help out with getting a bit of industrial machinery working. This was a *big* machine, had a whole room to itself. I was told that the original designers had spent $160,000 to get this thing built, and that since then between $300,000 and $400,000 had been spent to try and get it working right. One of the first things that I noticed was that every single fault indicator on the thing was indicating. These were driven by a panel of relays, which were in turn driven by some open-collector logic. (Look up Amperex "norbits" some time, those were some really bizarre parts.) Every single one of those driver parts was shorted at its output, because nobody along the way had bothered to put those diodes in. I ordered replacement logic, and put some diodes in, and that fixed that particular problem. It amazes me to this day that nobody along the way had seen fit to do that.

[@gerryjamesedwards1227]

Having the scope demo along with the theory makes it super-easy to comprehend. Great series of videos, all the 'back-to-basics' ones you've done. For people, like me, who are just getting into electronics it really helps when someone who thoroughly understands the subject takes the time to explain it fully, and including a well thought-out demonstration makes it the sort of tutelage you normally have to pay a lot for. Much appreciated!

[@user-iq5vr8uj7l]

This is the BEST video I found that details the test point locations to validate if a flyback protection circuit is working via an oscilloscope! Thank you!

[@gilmertugahan]

Just by watching these videos, my mind starts to shift from an ordinary fix-no-fix electronic hobbyist into an engineering-level enthusiast. This really encouraged me to buy an oscilloscope. I wish all schools were like this. Thanks so much.

[@thomasmaughan4798]

Excellent use of storage oscilloscope and actual breadboard circuits. Snubber diodes protect BUT slow down relay release. Closely related to all this is the presence of non-ferrous shim at the relay to prevent the armature from sticking. Long ago in a galaxy far away I used a scope to detect a bad shim in IBM electronic accounting machines. When the armature clamps shut, the velocity of the armature will produce a small spike of voltage (moving magnetic field) and at the moment of closure its velocity becomes very high. The stainless steel shim prevents full closure and that, combined with an RC snubber, greatly speeds up release. When everything is proper there's not much of a energizing spike but if you see a relay that has a voltage spike across its coil a few milliseconds after engergizing, that means the armature has clamped down and there's no shim. It is very difficult to see this with your eyes that the shim has broken off.

[@sanjayd411]

Great explanation as always Alan. Watching your videos takes me back to my university days studying electrical engineering. I have an MSEE degree and have been a wireless communications systems engineer for over 35yrs now, but these circuits related videos still teach me things that I have not done since a long time. Thank you so much for all your efforts and please keep ‘em coming. 👍👍🙏🙏

[@ww9l]

I've been playing with electronics for 40+ years and never had anyone explain this concept so well. This only proves the point that you're never too old to learn, Thank you!

[@jpol3808]

THANK YOU THANK YOU THANK YOU. I had a problem with a 20yr old industrial bake oven, the oven kept shutting off intermittently. It had a 5 pin 120v control relay, a 120v main power contractor for the blower and tapped from top of that, a 120v contractor for the 3ph heat elements. I changed an original stop button with weak spring, a noisy element contactor, an original control relay, and a original dual element thermocouple. Then by looking at the schematic, I figured out someone prior had changed a hi temp limit controller and during the rewire had removed the snubbers across the coils. Funny thing was it worked like that for more than two years before developing the problem which gradually grew more frequent. Now I fully understand why those are necessary, what they do, how they work and could probably diagnose the situation with a scope or analog voltmeter. THANK YOU. Your videos are complete and to the point. Keep up the great work. You made my day! Merry Christmas!

[@donpgibson]

Alan, Just a quick note of gratitude for all of your excellent videos and this one in particular. As I am educating myself on basic electronics and building circuits for my ham station, I come across topics I don’t fully understand. When I find that you have addressed one of these topics, I know I will be learning something valuable. In this case, I knew that a diode is necessary when you have to switch inductive loads and I could have just followed the “formula” and always included one in my circuits, but I wanted to know more of the “why” and “how” this works. I was a bit confounded on this one for some reason. This video nailed it down for me! Thank you so much for taking the time to educate us all. 73 de Don KJ6FO

[@onecircuit]

@5:09 My mouth drops. I had no idea... holy crap that's a massive potential from a little relay like that. Tons to be learned from this video. Thanks!

[@Observ45er]

+OneCircuit Yes. The voltage that the inductor generates is proportional to the rate of change of the current. Do something to get the current to drop rapidly and you get a very high voltage. If you pull a connecting wire away from the inductor really, really fast, the arc can have tens of thousands of volts in order to maintain the arc over the long distance and therefore keep the current flowing.

[@matey6760]

I remember my grandfather explaining this to me about 40 years ago when I was very young. Thank you for your lesson I actually think I get it now. Thank you so much.

[@wa4aos]

I knew what snubbers did and basically how they work but seeing it all displayed on the scope really nailed it down for me..Thanks Professor and thanks for taking the time to make your most excellent videos!!!

[@michaeljburt]

Excellent explanation!!! An old EE can even learn some more about this topic. Great work

[@spelunkerd]

What a great explanation.

[@RideGasGas]

Why in the world would anyone give this video a thumbs down. OK, a couple nit-picks as noted below, but the video was quite informative, the scope traces quite illustrative, and will really help the average hobbyist.

[@PlasmaHH]

The choice of a diode is important when you want to control the switchoff time of the relay. People use zeners, cascaded diodes, diodes with resistors etc. to dissipate the energy faster. Some people also just use a LED to even have a visual indication of this process.

[@jaredkusner1137]

Your use of the schematic, circuit, and test equipment made it fun to walk through. I really enjoy the back to basics. Thank you.

[@kd5ozy]

Aaaaaaah, your videos are always refreshing. There are things I forget about and take for granted. Your videos are by far the best and the most informative. Thanks!

[@dp0813]

Awesome video! I understood this concept in theory in my power Electronics classes for my electrical engineering curriculum, but never actually got to see it in practice like you've demonstrated here. Definitely very helpful and a must for any University teaching this type of material!

[@TheRedLava]

Amazing video! Even after 5 years of being posted it's gold! Thank you very much for your knowledge sharing :)

[@osvaldocristo]

Thank you. I was looking for if the diode type could be significant in the circuit performance and the peak current value in the snubber diode. You answered both questions.

[@stoneslice]

Outstanding and informative video. Thanks so much Alan for making it. The current probe is a great bit of kit!

[@jenko701]

Great video, I've been a TV tech for over 25 years and have seen this circuit all my life and never gave it a second thought , but it makes a lot of sense . Every once in a wile you will find this diode shorted ,but it's almost always because of lightning.

[@freddiemortos8519]

Another great back to basic tutorial. Looking forward to see more on this in your future video.

[@curtisbeef]

I use flyback diodes in brushed motor control. This video was extremely helpful to understanding why! Thanks! I love the Back to Basics videos.

[@VeryMuchBlessed]

Super helpful in understanding and seeing in real-time the wave-form generated with/without the snubber diode. I've been using them in my circuits but it's cool to see it on the oscilloscope which makes it much more clear. Thanks much

[@billybonewhacker]

Thank you so much. It's very helpful to see it all on a scope. Plus it forces me to remember how current flows through a circuit. Great job.

[@ericprice2033]

This was a great find. I like how you explained things. Exactly what I needed to get my electromagnet circuit going.

[@structure7]

I'm glad this was the first video that came up searching for this topic! Thank you!

[@SuperJetjockey]

I never tire of watching your informative videos. Bob

[@njnear]

I really enjoy your videos. The back-to-basics posts are always interesting, and great reminders of why I wanted to be an electrical engineer to begin with. Thank you for the effort you put into these. Also enjoyed your visit on The Amp-Hour.

[@w2aew]

Thanks! I enjoyed talking with Chris and Dave.

[@kristhetrader5029]

Excellent video as always Alan! As for my myself, for all of the signal dual latching relays I use in my designs (about 80 relays on one of them) I use the famous ULN2003D relay driver which happens to include that protection diode as standard, highly recommended for many signal relays. Keep them up!!

[@wk2w]

Very effective demonstration. I've recommended this video to others when the subject comes up.

[@spu3]

Excellent! Very clear and useful. Enjoyed the use of the oscilloscope. Really helps to visualize what's going on.

[@jluke6861]

What a great video and great explanation. Thank you for taking the time and sharing this with us.

[@copernicofelinis]

11:04 the sag in the exponential curve of the coil current is due to the change in the magnetic circuit. It's common to all relays driven in DC: there are basically two exponentials associated with the two different limiting values of the inductance: one when the magnetic circuit is open, and one when it is closed. The sag (or bump when you go from unenergized to energized) is what happens in between.

[@m1geo]

This was my guess too. That or the metal moving inside the relay.

[@thomaslanik7215]

Excellent subject, explanation and presentation- thank you! I like your video camera and editing, too. Good job!

[@ptianu]

Great tutorial and your illustrations were literally worth a 1000 words.

[@normandgallant8106]

Best basic tuturial sofar and I've been using KZbin for some years , thanks for your time

[@Radiowild]

I used to carry a roll of diodes in the back of my company car when I would find diodes missing from high current relay coils in equipment that was removed in the field! Nice Vid!

[@Yojimbonh]

Great explanation. I especially like seeing the waveform shapes to see what is actually happening.

[@krishnakhandelwal9466]

I was so confused. I heard people saying that when we suddenly stop flow of current through an inductor, it generates a reverse voltage. I was not able to visualize it. But now I understood. It's the increase in voltage at other terminal. Due to this diode gets forward biased and allow passing of this high voltage through it. Now, it's clear. Thank you so very much.

[@will.patton]

Excellent post - thank you! I was surprised to see the voltage so high (i.e. nearly 300v). Thank you for explaining that most any diode/rectifier will do, too.

[@hubercats]

Incredibly informative video. Thanks so much for making and sharing it!

[@tdumnxy]

Very clear and useful, thank you.When you show the circuit diagram, do the experiment and explain the resultant trace(s) on the oscilloscope, it really helps in understanding the theory. Inspired by your KZbin videos and by the efforts of a few others on YT I have been motivated to get a radio license. I am taking the Foundation course and exam this coming weekend. Wish me luck and many thanks for sharing your knowledge and enthusiasm.

[@w2aew]

Great! Good luck with the exam, and hope to catch you on the air in the future!

[@___xyz___]

Is getting a ham radio licence something you would recommend? I'm a physics freshman and electrical engineering enthusiast of a few years. I own a good Tek 2232 and always look forward to your scope videos. Lately, I've been exposed to a lot of applications for radio transmissions, especially the capability for spectral analysis with a SDR and satellite communications, with FM stations sadly being "shut down" across Norway. I looked into a licence this winter and realised there are some things I anyway ought to know about circuits. But whether it is still a worthwhile pursuit to acquire a licence is beyond me.

[@iddiadam857]

Hello steven i am from africa(kenya).

[@DonatoGreco]

I really enjoy this kind of educational with the scope to showcase the theoretical concepts. A big thumbs up 👍

[@robertcalkjr.8325]

Thanks Alan. There is so much to learn that it's easy to forget basic stuff sometimes.

[@conorpodonoghue]

Great video - comprehensive and to the point. Many thanks.

[@user-su5sq5ib3i]

This was a great lesson Allan, thanks!

[@sonnyfontes558]

You'll find the same thing on the primary of the flyback transformer of old TV's. Their horizontal output transistors usually have that diode built in along with a breakdown voltage of 1500v. There are also some retrace capacitors on it to slow down the collapse of the magnetic field. That is just an interesting application of flyback diodes.

[@jimadams2473]

Another GREAT video. THANKS!! This is the best video on electro-magnetics I have seen. Appreciate the time and energy you inject into these instructional videos.

[@famossfla]

Another superb explanation... Thanks again Alan.

[@redcatimaging]

Very nice visual explanation, I really like how you did this basics video.

[@jpopelish]

Couple nitpicks; At 11:44 "and dissipate it safely into the positive supply". No energy is going back into the positive supply, while the diode is conducting. The stored energy is being dissipated in the diode, but mostly into the relay coil resistance, because only one lead of the supply is connected to the current loop through the coil and diode. You keep saying the changing magnetic field induces coil current. I think a clearer way to think about this is that the changing magnetic field induces coil voltage, and the entire circuit determines how that induced voltage affects instantaneous current. Instant by instant, as that current changes, it alters how the magnetic field is changing and those magnetic field changes keep inducing resultant voltage. Current produces magnetic field. Rate of change of that magnetic field produces voltage. Voltage drives current. And round and round we go.

[@w2aew]

Yes, I miss-spoke about the energy going into the supply rail. Thanks for the correction.

[@hernancoronel]

Thank you for the deep explanation, it was kind of difficult to understand until I read this comment. Now I see how the energy moves and dissipates in that diode/coil pair. Thank you both for an excellent addition to the incredible video! Keep up the great work w2aew!

[@kakagaming7693]

@@w2aew hello i want to talk you.personaly

[@thomasmaughan4798]

Moving magnetic field induces a *current* in a wire. Whether that current produces a voltage depends on encountering *resistance* (ohms law). In a superconducting storage ring you will have zero volts but hundreds or thousands of amperes of current.

[@jpopelish]

@@thomasmaughan4798 What if this current is induced into an open circuit? How much voltage will it produce to drive that current through an infinite resistance?

[@stephenhearn472]

Awesome video! Thanks for taking the time.

[@hangfire5005]

Excellent as always! Thank you for this, I learned a lot that applies to my daily work

[@robytryall]

This was the best educative video, I have seen in a while!

[@schmittenhammer]

Thanks, very informative and clear. Appreciate your time in sharing this information. Maybe I will stop burning up transistors.

[@edwardvan00]

Excellent presentation and tutorial for a method to protect sensitive electronics used in many applications. I particularly found it interesting when getting advice from SilverLeaf Electronics tech support on protecting their circuits from a suspected issue with a White Rogers solenoid used as a bridging circuit for house and chassis batteries in our RV. Protection for expensive controllers is of utmost concern and importance. Thank you for the education.

[@mortenrolsing7137]

Wery nice video, thanks a lot, i am learning a lot from your videos, especialy the back to basics ones. Keep up the good work. Greatings from Demark.

[@jimfromcolo]

I've used diodes for this reason for many years, I've added them across relays on older American cars with relay operated components to make the switches last longer, it eliminates the transient spike across the contacts as the switch was turned off.

[@Observ45er]

+Jim Patterson That's a common example. When the switch opens, the inductor's high voltage causes the switch contacts to arc. This shows very well how the inductor will "Do what it takes" to the voltage, to maintain the current flowing through it.The high voltage arc that forms as the switch contacts spread apart will erode the contact material.

[@bigstuff52]

Enjoy your video tutorials. Also your use of graph paper. I picked up on using it also, when I started reading Forrest Mims in the early 70s from his Radio Shack tuitorials....thanks..

[@frequencydrive]

I loved watching this. I'm an electrical engineering student and although we do get experience in the lab with diodes, capacitors and transistors what we don't get exposed to is some of the practical things you have to do in "real life" to account for unexpected things like the "fly-back effect." You have really nice equipment too. That current probe cost thousands of dollars and you have a 1 Ghz 4 channel TEKTRONIX scope. Very impressive.

[@w2aew]

It certainly helps when you work for a company like Tektronix ;-)

[@headbanger1428]

Excellent presentation. Many thanks!

[@Good0Music0for0you]

Amazing video and excellent explanation , cheers from London .

[@Pwaak]

I very much appreciate your informative lessons...Thank You!

[@johannienel1]

Awesome explanation and even better visualisation with the scope. Thank you very much sir! God bless.

[@G4KDXlive]

Thank you. I am about to build an RF triggered switchover for a transmitter. I wondered why there was a diode across the antenna relay. All is now clear.

[@SkypowerwithKarl]

Thank you! You are so easy to follow. Never have I learned so much so quickly. I’m an old fart trying to figure out the diode polarization across a fuel solenoid on a boat. I wanted to protect the contacts of the relay from fly back voltage. Just the opposite of a LED. Your diagram showing (+) was great help. Thanks. PS I sure hope you are a teacher

[@w2aew]

Nope - not a teacher. I'm a Field Applications Engineer for Tektronix (Test & Measurement Equipment).

[@brucenguyen6527]

Very thorough explanation. My professor sent me here and I’m glad I listened…this time

[@kunalsonone4523]

Great video. Very well explained. Thank you

[@willyrivero470]

Wow thanks for this great explanation! Now I really will take into account this diode protection. Thanks a lot, thumbs up!

[@ernestb.2377]

You really explained it very good. Especially the part the current from the coil rushing into the switched off transistor that is high impedance and producing that high voltage. If there was a low impedance path there would be no high voltage.. Collector−Emitter Breakdown Voltage of 2N3904 would be 40V(min value). That is also the abs max voltage. So that has not "killed it" right away but it is not "healthy" for that type 🙂 In your configuration and that specimen it was more of a 60-70V. I was thinking (wrongly) that the diode should be of a high voltage type, but that is not the case, as the diode prevents that high voltage to occur in the first place 👍

[@44mod]

Great Video. you made the video in 2014 and today I am still learning from your video in 2020. Thank you so much. I saw a video and can not find it again with a compositor was used between the emitter and collector. I can not remember why it was used and I am going back to research why.

[@dancoz5477]

Bien expliqué avec démo pratiques appropriés et suivis. Bravo monsieur.

[@TradieTrev]

Well that explains it! Didn't think twice about seeing them in only some control panels

[@thomaslanik7215]

Very nice subject and explanation/presentation- with thanks. (nice 'scope!)

[@emilrodriguez5696]

Great explanation! Very helpful! I'm also with @James Wood, it would be great if you could cover how RC snubbers are designed and work. For example, for the case when an AC Motor or other inductive load is connected to the contacts of a Relay. I've noticed sufficient disturbance on the control circuit to cause microcontroller reset and other unwanted effects...

[@kalhana1]

Great video, I have never thought of actually measuring the voltage induced. I usually call them freewheel diodes.

[@washerdryeradvicechannel8041]

Thanks for sharing your knowledge.

[@deusexmachina3496]

that is so informative and descriptive video, really impressed

[@guillep2k]

I'm a simple man. I see a four channel oscilloscope and I upvote.

[@AB1Vampire]

Really good video, I learned much thanks for posting. I recently found some TE Application Notes that tried to minimize the coil collapse time of a NO DC relay by adding a series zener to the diode. Apparenly such an arrangement reduces the average 19ms coil collapse time for a diode alone. Zener & diode connected parallel to coil in K to K or Anode to Anode.

[@koffibanan3099]

Such a fantastic explanation! Thanks

[@paulmuff9883]

Nicely explained, thank you 😊

[@aslimlines3069]

thanks for taking the time to explain the function of the componet

[@pedrochirivella2473]

Thanks for such a great video

[@philipsilvester]

you said i hope you learnt a little, i learnt alot thanks for sharing.......thanks phil.....can i just say those scope shots were fantastic..

[@adriantorsvik8553]

Very well explained. Thank you!

[@anthonyheak3479]

Great explanation. Thanks!

[@josmunpav]

Great video, very easy to understand why the diode is needed. By the way, awesome Oscilloscope you have there, I checked it out and is about 15 grand!!!

[@nikhiume]

very helpful and clearly demonstrated, many thanks.

[@4lecsg]

Great lesson!

[@StrummerDave]

Best explanation I’ve ever heard. Thx

[@CurvedSlightly]

Great video. ~Even though it was quite technical, it was explained really well.

[@Purple__]

Very clear and interesting video, learned something today Thanks!