Very good. At Maxim, I defined several families of fancy relay drivers for special markets like ATE (little COTO relays) and DSL provisioning (thousands of relays in a crosspoint). A few things that some of these guy do is interesting. 1. They will have all the snubber diodes for the relays return to a single zener that is reverse biased up from ground. Small transistors can generally take 50v, so it was common to use a 33v diode. The ULN200x series part makes the common diode terminal available for this purpose, you can just tie it to the high rail but for better speed, take it to a common grounded zener at a higher voltage- in 5v systems, this could give a 6x speedup. The reason that they like the return to ground is that often the high rail can't sink current (except into bypass caps)- ground is a better sink and probably stiffer. 2. In some critical applications where power dissipation of on relays could be an issue- they would have a two stage switch, it used the full 12v to pull in the relay and a lower voltage to sustain it. I defined some Maxim IC's that did all this- a 12v relay won't drop out until it gets down below 5v in most cases though generally you'd use 3/4 of the nominal to keep the contact pressure high and R low. There are several clever ways to do this if you look at app notes from Maxim and others. 3. The other cool thing is single coil and dual coil latching relays. Telcom guys would capacitively couple small single coil relay to a HCMOS output- when the output was energized, it would send a pulse to the coil in the forward direction, when the output went low, it would reset the relay. A .1 uF ceramic will create a lot of peak current with a CMOS rise time pushing it- you size the cap for the peak current and energy required for a good latch. This is a good topic. I talked to a lot of relays guys, relays are still used a lot but there are few drivers for them- the ULN2003 series goes back to Sprague in the mid 70's. We had some advanced development on MEMs relays, a whole 'nother very interesting topic that you might look at. Your discussion was well presented as usual. Regards

Don't assume that all designers know about these things. I was called over to look at a friend's electric gates and discovered that the DC motors were driven in either direction with relays, with no protection devices in the circuit at all! The relays had already been replaced once, and they can't have lasted more than a hundred or so activations before being destroyed by the arcing. I added a couple of large Zener Diodes back to back across the motors, and the relays have now been on there for years. I suppose the electric gate company likes replacing the whole circuit as a nice steady income.

an interesting demonstration of the back-emf current is by using an LED as the flyback diode

This is one of my questions when I'm interviewing circuit design engineers. I ask them to draw me the circuit they would use to connect a relay to a GPIO pin. They get points for knowing a GPIO pin can't source enough current and need a transistor of some kind, and extra points for including a snubber across the inductor. When I first wrote the question, I thought I was being too easy, but after a dozen interviews, I was wondering if I was being too hard. Interviews are stressful and make you second guess what color the sky is, but I get all sorts of answers I wasn't expecting.

Dave is an absolute legend! He is like the terminator of electronics and electrical theory! I would start my apprenticeship all over again if I could have Dave as my trade school teacher or my boss!

Beware that large high current relays need a zener type of diode (as said by dave in the end), this causes a larger voltage across the coil in off transient, creating a larger energy dump in the diode and a faster drop-off of magnetic field (resulting in a faster opening of the relay).

I first learned about this as a kid when I connected a relay and 9v battery with one hand on each terminal. Ouch!

Instructional videos like this are a gold mine, like that bit about BJT storage time, I didn't know that.

Another word for the diode: "MSRD - Magic Smoke Retention Device" If ya leave it out, then the 'Magic Smoke' is far more likely to escape.

As a young player, I got trapped by this one several years ago when retrofitting an old CNC machine with a modern computer. It took me forever to stumble upon the actual problem/solution. But once I did, all my little gremlins went away. Nice video, Dave.

Fantastic vid! Back in '96, my analog-electronics instructor demonstrated the back EMF phenomenon by having the entire class hold-hands while he stroked a 9V battery across a relay-coil. We all jumped during the shock. Gotta love the 90's! DiPaula was the best instructor ever...

I remember being a kid and using a relay to drive itself--put the coil in series with the NC contacts. Current will flow, the coil will open, current will stop flowing and the relay will close--repeat. Then put a neon bulb across the relay could and you just made a high voltage power supply. ;)

As a kid, I got a pretty good shock when, after building my own 2-way relay out of a hacksaw blade and some electromagnets, I decided that I might be able to configure it as a buzzer by having each direction's connection power the other direction's coil. 12v in, massive arcing and electrical shocks out.

Hi Dave. Good video. The most important is at the end! Often forgotten topic. I work with solenoid that are valves. In the valves you control the current by means of PWM, for that part you want to have a diode with as low as possible forward voltage. You do not want to dissipate energy, contrary, you want the energy to stay there, and keep turning on with PWM to keep the valve open. On the other side, when you what to shut the valve off, you want it to be fast, so you switch another part of the circuit with a Zener, because you want the time to be fast, but also well defined. Knowing the regulated current with PWM, and the zener voltage, you can relatively precisely tell the shut down time. Also for high power relays it is critical that the field goes down fast, moving the contacts fast and avoiding sparking that reduce the life of the relay. Exactly as you demonstrated at the end, is shuts down harder, but it may be a good thing after all. The harder it shuts down, the more life of the contacts, if the contacts in turn drive another inductive load. On the example of valves, sometimes you want to close them softly, to avoid seal wear.... so... it all depends. But it is important to know that a diode is low voltage and tends to "perpetuate" the current AKA free wheeling, and a Zener or something with high voltage will dissipate energy fast. I've seen even "active freewheeling" by using the diode of a MOS, and turning it on short after the diode starts to conduct, to allow for minimal energy dissipation, when you want to regulate a current in the inductor. This is made with a totem-pole or push-pull MOS stages, for example. It would be nice a video making focus on that topics!

I've come across this in real life, on a tractor using the horn would cause such a spike on the canbus the engine would stop from the ECU protecting itself. The horn circuit was not on the canbus but wires run alongside. Fixed the issue by putting a capacitor in parallel with the horn.

I have been at least 20 years wondering about the switching phenomna of relays Man, I have to say, you are a true legend

but i didnt see any oscilloscopes go pop!... ...oh wait..thats another channel! :P thanks Dave

While experimenting with pulse motors back in 2001, I managed to blow both channels on my Velleman PC scope rated at 600V. The supply voltage I was working with, was only 12V. As luck would have it, a friend gave me his Tektronix portable that was geared towards auto shops and ignition coils. It was a life saver sort of thing. It's still working like nobody's business.

I love your series of "fundamentals" videos Dave! This one on "Back EMF" is particularly well presented and enlightening. Top Notch!

I actually HAVE been shocked by a relay because of this! Before I fully understood counter-voltage spikes, I couldn't see why a little battery powered circuit was able to zap me!

The most interesting part of the demonstration is when lower breakdown voltage transistors were used. I never would have guessed that such a circuit had so much treasure troves worth of knowledge hidden behind such a simple form. Thanks again, Dave!

Hi Dave. Using the scope gives such clarity to your explanation. Brilliant and thanks.

Great video! Love these sort of fundamental type videos you do. Usually stuff we all/most are aware of, but the deep look on the oscilloscope and intricacies of it are fascinating when you present it like this.

You are incredible Dave, why I did not had professors like you or your knowledge and caliber, it really counts, thank you.

I knew the laymans version of this but you gave me the back story.. Thanks Dave.

A good easy to grasp analogy I think is water hammer, when a valve is closed rapidly with a high speed flow through it. U then get a huge pressure spike (because the water like anything that is moving can't just stop instantly) that can damage or even rupture pipes, valves and other stuff. This is sort of similar to that but for for electricity.

Like the way Dave explain things. He makes it simple and very easy to understand. Appriciate it. Thank you.

Again stellar tutorial. Good and useful for any power electronics engineer from novice to expert. I must unfortunately stress the importance of the basics again and again, over and over.

With the PN100 in circuit You created a source of QRM (man made noise) on the 160m amateur radio band. Shows just how important it is to get the little things right to avoid all sorts of unwanted 'magic' and unintentional RF noise.

Comments: 1) Although it does not solve the flyback issue, it's a good idea to drive a relay smartly these days to reduce static (resistive) power dissipation/waste: high initial turn-on voltage, and then decrease voltage to just enough to maintain hold. They have ICs for driving relays nowadays. 2) For motion applications: use a Voice-Coil Motor instead of a hefty solenoid. Their inductance is much less. 3) Use a Schottky for the flyback diode. Its breakdown voltage must be high! 4) MOSFETs have a built-in body diode which freewheels in this case, but I don't trust it and supplement with an external Schottky! 5) This snubber can get more complicated (RCD Snubbers, regenerative snubbers), but all of those require tuning to end application! 6) This is EXACTLY the cause of frequent posts on electronics-related forums of noobs saying "HELP my switching power supply project (or any switch project) transistor worked a few times and died..." Transistor is OK when it turns ON in a bad switch design/build, but is killed instantly when you ask the transistor to turn OFF. Well, besides proper design and proper build, you must also validate your design with simulation (which I don't care about) and real-world testing (which is what my job is all about)! Why relay is quieter with snubber is because the scope already showed us they relay turns off much slower. So, this actually can worsen (because contacts move slower) high-voltage acing across load contacts!

one of the most interesting practical video I've came across. Excellent job, thanks!

Fundamentals are always exciting. Thanks Dave.

This is one of the first principles I had to deal with in my profession. A horn relay can have one of the biggest pulses.

great! thanks for sharing this -- it is easy to overlook this as a problem in design!

Nice enthusiastic paresentation. We learnt about back EMF at college (millions of years ago) but have never really seen it in action. Thank you!

I like the way you wrapped up it all up in the end by mentioning another 3 snubber methods. Very useful stuff for electricians.

700 volts - ouch. Now I know why I blew so much TTL up as a kid.

Without the flyback diode, the energy stored in the coil dumps much more quickly because there are 90V at the end of the coil terminals instead of 0.6V, as a result the contacts slams really hard.

Brilliant, and thank you. I'm a complete ignoro-noob but can see the goodness and importance of this.

This was great. Very informative. I have been using my scopes more these days to look at details of general circuits I have built over and over through the decades and never thought to look more deeply into what was happening in more detail. Also have been helping a friend use a scope to view automotive electronics. It is amazing how much you can learn/troubleshoot about car electrical health from analyzing current and voltage wave forms of auto electrical components.

The best video on back emf suppression ever. Terrific video Dave. How about a deep dive into Hbridge design?

Fantastic presentation Dave, really appreciate that you've started to go back to these super educational pieces than just the Mail bag serious or tear downs - sure, I enjoy those segments to bits - but knowledge nuggets like this, with your style of presentation are hard to beat and look forward to a lot more in this direction! Now, how about we wire up a beefier inductor, to a transformer (more windings on the secondary) and a potato. Let's see if we can let the magic smoke out of the potato. Tee hee!

Thank you for the informative lecture professor Dave.

Awesome description and amazing demonstration. Thanks, Dave!

Here's a related "Trap for young players", and I saw this a lot when I did PLC work on industrial equipment, the back-emf diodes we had were on the output side of the relay that was driving large solenoids, plus of course the back-emf diode on the relay itself... These were circuits that had to perform millisecond sensitive operations, and the snubbers would not change how long it took for the relay or the coil to energize, but they'd significantly affect the time to de-energize and would limit the frequency and minimum reliable pulsewidths you could run because they'd hold the relay and solenoid open for longer Ideally (at least for our application) you would chose the maximum value Zener diode that still protects your circuitry, the higher you can allow the back-emf the faster things will respond

A good way to think about stored magnetic flux and the collapsing magnetic field generating EMF is that of physical momentum. If you roll a bowling ball and suddenly try and stop it, the force pushing back on your hand to stop the ball is the back EMF force trying to stop to momentum of the forward current flow. Or another analogue is of a spinning bicycle wheel and tire. Spin the bike wheel up while you have the bike elevated and then suddenly try and stop the wheel with your hand. Everyone, from child through to elderly, knows EXACTLY what will happen. This is exactly the same thing but in the physical world where people can more easily understand it. Great video, Dave!!!!! PS 14:00 That poor poor BJT is getting flogged!!! LOL

Excellent video! Clearly explained with great visual aids from o'scope making this one of the most practical explanations of the laws governing back emf.......without all the math!

That was a good demonstration, a very nice use of your osciloscope and probes!. I had to figure this out the hard way years ago, when trying to accelerate relay switching for a UPS circuit. Very good work!

28:40 My guess is because the diode has slowed the switching action slightly, the contacts don't slam shut as forcefully. This makes the relay operate more quietly.

Dave: You are a blessing to a guy who never finished high school and loves electronics. Also, I wish more teachers would use "E" for EMF instead of "V" for volts.

Best demonstration I've seen of this, thanks Dave.

Excellent demonstration of a very necessary protection for inductive circuits. I have made many pulse width modulation power control circuits, mostly with multiple BUZ11 mosfets in parallel, for speed control of electric trams on my 7.25" gauge garden railway. In every case, I fitted a substantial flywheel diode across the output to the motors (which could draw up to 60 Amps). Even on lower current applications where relays were involved, diodes were a must. One point that should not be overlooked is the use of fast recovery diodes to handle high frequency switching.

In junior high I bought a plastic sandwich box, put 2x 9v batteries and a relay (wired to oscillate) inside, on the outside put screws as terminals and a red button. It was fun to test for how long people could withstand the shock, and buzzing of the relay made even more hilarious. It taught me that people have vastly different electric shock threshold.

I remember in my basic electronics lab course we used a transformer, a neon lamp (80V indicator neon lamp as I recall), and a push button switch to demonstrate flyback voltage. Someone in my class hooked up the transformer backwards (and used 120VAC wall voltage instead of our 12VDC power supplies) and ended up getting a large enough flyback to explode his neon lamp (and push button switch). After that we joked with him that he left his shadow on the wall behind him from the flash.

That was an eye opener. When I started watching, I was thinking to myself, I know all about Back EMF, but I decided to carry on watching anyway, because Dave is always one step ahead of me. So you start out with this little power transistor, and I am saying to myself, Yes, I know all this. But when you put that HF transistor in, I was fascinated. I have never seen that before in all my years in my career. What a great lesson. You are never too old to learn.

Every time i have a problem Dave comes in explains the thing and saves the day.

14:40 Automotive ignition coils use this to generate the spark that fires each cylinder. They can turn 12 volts into something like 50, 60, 80kV.

I love the sound of relay to the point that i treat it like a musical instrument, made a relay board for some effect production

That was a lot more fun than I expected. Being able to monitor reverse EMF was interesting, didn't expect such a high voltage from such a small coil. Being an 'old school' motorcycle mechanic I don't get to use oscilloscopes and have to use a peak voltage adapter in an ordinary meter which tends to average out max voltage. I have only seen 318v on a 12v system but now I'm sure it was much much higher. Thanks for an interesting vid. I'm off to see the Fluke meters next.

Spinning flywheel reference... I describe the issue like a pneumatic air hose. If you're working on your car in the garage with air tools, when you plug a line into the compressor, the line expands a little like a balloon as it takes on the working pressure. And, while your using air tools and such, no problem. However, disconnect that pressurized line and you'll get a nice blast of air in your face, a hose end that whips around and a loud pop of escaping air. The longer/wider the hose, the bigger the issue. That magnetic field is just like the expanded rubber of the air line. It wants to keep squeezing things along until fully dissipated.

I've built a few linear power supplies and ended up putting some high voltage ceramic caps across the diodes to help smooth away the spikes caused by the switching action of the does in the rectifier. Wrt the microphone, I reckon there is electromagnetic noise that is suppressed with the clamping diode, and that the mic is picking that up. Also potentially some arcing in the relay.

I've always had hard time hearing things like "the voltage must rise to obey Faraday's Law". No. It does not rise because of Faraday's law but Faraday's law perfectly describes what is going to happen. That is the order of "status", the laws we have FOUND, not made. Nothing follows our laws, our laws follow what is happening. I remember having this been a problem all the way the first year of EE, as our teacher approached everything from that angle, "X happens because of Y law". In the second year i got a different teacher and he approached everything from the opposite angle: X is going to happen and this law is formed to explain it. He was actually a lab teacher. The first teacher didn't do anything but theory so very single equation was presented in the FULL form, then simplified to the thing we will use. He caused so much confusion in a subject matter that is at the core of understanding electricity. He was a bad teacher anyway, each question was answered "it is in the book".

Perfect Perfect explanation as always, wish I could ever have you as my instructor Dave. Thank you so much.

Great video and very detailed explanation. The simple flyback diode can saves a lot of trouble in relay driving circuits. I destroyed a 3 phase 25 kVA generator winding by arcing out some old MOT's in a three phase arrangement. as soon one of the MOT's primary winding went open circuit, flames shot out of the generator winding vents. Hindsight, I should have used a load bank in parrallel or a snubber/surge protector across each phase. Expensive lesson learnt.

You should have fire up a MW radio, and tune to 1.5MHz. Ahhh.

So that’s why big DC contactors have a varistor in parallel to the coil! Thank you so much! BTW, isn’t it the way old school gas engines points ignition works?

That's fascinating! Thanks for that, I'm sure I've used relays in the past and not bothered to prevent back emf!

Superb tutorial on the effects of back emf. Those expanded scope traces are amazing to actually see. A bit surprising the transistor didn't get destroyed by almost 3x the max rated voltage, even though the current is so low.

Coils are still somewhat mysterious for me. You store energy in a magnetic field, that expands into the universe with the speed of light. How can you retrieve that energy almost instantly?

Just yesterday i was talking about this with my boss, he told me that every professor explains back EMF on inductors wrong, he had difficulties to understand it for weeks until he reached a explanations and when talked with his tutor about it, the tutor explained that it was true but people wouldn´t understand it that way as it was a basic level class. The explanation says that the Inductor doesen´t magically generate a reverse voltage on its own, its the current trying to continue flowing forward that tries to find a path, as the path is not found the voltage rises and rises until a path is found, (it might even be the impedance of the air ) then the current is dumped trough that path and thus you now have the inductor working as a current source powering a impedance( whatever is working as the path for the current) so now the voltage across the coil is reversed since it was a load and now its a source. i still don´t think i fully understand it, it might be from the years i thought of the magical voltage veresing explanation but it makes complete sense.

When I was starting my electronics experience ( after uni, because they didn't teached us that), while designing my first prototypes of a AHU control PCB I got a such strong kick back it restarted the microcontroller on the board. took me a while to to realize that I need a diode on every relay and that I have those in the uln2003, just needed to connect the com pin to relay drive voltage. And thinking of it now I remember that my father told that the spark on the motorcycles ignition coil sparks when the contacts on the crank position sensing thingie opens. Took a good 15 years to wrap the experiences and data together.

No wonder I have shocked myself with the relay switch - I figured it would have to do with the inductance of the coil. Good explanation!

In the 70's Spencer Gifts sold a prank cigarette lighter which used a small relay to give you a pulsating AC shock. I recreated the circuit for other pranks.

we used to call it the MF Diode... like in m0ther *** .... because a student didn't want to put it in the design (replaced it with a resistance ) ... so the team leader just said ... put the M*F* Diode

THIS is why I watch EEVBLOG.

That answered a question or two about flyback diodes and learned another bunch of stuff too. Thank you Dave

26:10 the current probe is not in the right place on the schematic. It should be in the loop made of the inductor and the diode...

I enjoy your presentation's, They are always educational. I have been in electronics, sense the early 60's, first as a T.V. tech, then in later years, as a land mobile tech, for Motorola, and at 80 Yrs. i still find that I don't know every think, so thank you for the knowledge you share.

Finally a video where we did not saw the brymen bm786 toy. Great content, thank you!

Thanks for the informative video. I wasn't aware of Back EMF. There are a couple projects I did that need modifications now.

Ah, finally a KZfaqr who knows how to properly draw a resistor.

Pretty cool you can see the blip of the armature movement. The back emf voltage, goodness so high!

Excellent presentation and for everyone understanding !

I did this exact thing when building my super cap spot welder, i put in a single diode across the coil, and a diode and tvs diode across the welding leads to prevent the contacts from welding themselves together, this combination was a compromise on the voltage spike across the contacts and the time it took for the solenoid to disconnect, around 11mS and +/- 20V. I also had soo much energy from those super caps i had to increase the length and decrease the cable cross section, actually using the cables themselves as a current limit because otherwise it was too much energy from those caps and just blowing metal apart and creating holes in 1mm aluminium. I also had a timer feeding the solenoid so i could adjust the on/off time for the spot welder.

This video was awesome. This is one of those things that I know to do but I don't really know the consequences of not doing. I loved the demo and seeing how high the peak voltage was during the field collapse.

I have been looking for an explanation on this topic for quite some time. Thank you sir!

I remember this lesson in college... This is also how some old school automotive ignition systems work... in this case, the back EMF is used to generate the voltage needed to drive a spark plug. I've also seen capacitive snubbers for AC only use... that's how Omron's low voltage (24V) relays seem to be offered for AC. DC is a diode, AC is a cap. Most of the stuff at work is 24VDC, but one application (off-the-shelf container hydraulic lift tables) use 24VAC directly from a mains transformer. Since normally they're just some switches and a contactor, no need for DC...

Back EMF can be a valuable safety learning tool. My collage instructors son didn't think a plain 9v battery and a "coil of wire" could possibly hurt him, the shock he received was a very valuable learning experience...

I've had to deal with back EMF when building my DIY spot welder. The solution i came to was to connect a zener diode between the gate and drain, and by choosing the zener voltage accordingly i could make my MOSFET assembly turn on whenever the gate voltage exceeded the supply voltage. A schottky diode was added in series with the zener to prevent the MOSFET from dragging the gate voltage down. Handling over 900 Amps of back EMF is no easy task but this solution did it beautifully. In your case you could omit the second diode so that the transistor would limit its base current and prevent saturation, which you mentioned can slow the switching down of the BJT. Doing it this way you only need to worry about the energy dissipation and supply voltage, because you know that your switching transistor can handle the current already. :)

Brilliantly demonstrated. Kudos!

This is a really great educational video Dave. Seriously!

We dealt with this during my apprenticeship as a colliery electrician, although electronics was just entering the industry in the mid 60's. BUT relays were common in gate end boxes, (flameproof contactor units). All low voltage control circuits needed to be intrinsically safe so as not to produce sparks that could ignite methane. Early ideas were capacitors, then placing the relay in a selenium rectifier bridge circuit, then as silicon diodes starting coming off the production lines, diodes across the coils.

Most circuits I'm familiar with usually use a 1N 4001 for back EMF relay clamping rather than signal diodes like 1N 914 or 1N 4148. Thanks for sharing. 😎👌🏼

Great information. In industrial settings, RC networks are used mainly in AC circuits and almost always a diode in DC circuits. Not using suppression can be very problematic in CNC machine tools.

We always called that diode a counter-EMF diode. Learned about that little guy in the Navy electronics school AV-B. And that magnetic field collapse will zap you a little if the diode goes bad. Learned that the hard way too. 😁. I think the US Navy had the 1N914A designed just for this purpose, because we had thousands of those diodes all over the aircraft. But, that’s supposition on my part. 🤔 Outstanding video!! Thanks Dave!!

Nice video! I have preference on MPSA06 and MPSA56 for low voltage, nice gain and low saturation voltage, MPS42 and MPSA92 for 300v transistor. 2N7000 are also very nice mosfet for this type of job. I use transorb for fast recovery relay coil in automobile application.

An interesting demonstration of the back emf concept.

Flyback can also be your friend if you need to an easy, inexpensive way to create a voltage - like a for a spark plug or a CRT. The technique is also used for boost converters to avoid using a transformer.

Great video, no bs and easy to understand. Thanks

Lovely demonstration! 👍🤓