EEVblog 1409 - The DANGERS of Inductor Back EMF

[@johnwettroth4060]

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

[@davidwillmore]

Great comments. This reminds me of the work we did at Eaton when designing oven controllers. Given the longevity requirements and the high temperatures involved, relay life *and* power dissipation were important design concerns. The normal way to power relays was to use 18v relays in a 24v circuit. The uC would drive the relay at full voltage for a few small time and then PWM it to get a lower effective drive current--but above the hold limit of the relay. Turn on/off was generally synchronized to a zero crossing to minimize current through the contacts when they made/broke. Depending on how the drive circuit was designed and the behavior of different relays, the microcontroller would be programmed with different timings to get the actual make/break exactly on the zero crossing. Pitty the purchasing manager who tried to substitute a different relay in a design without engineering approval. ;)

[@Rx7man]

LM1949 is one such chip, it's primarily for low impedance injector drivers, putting full power into the injector to pull it in, then throttling the current to about 1/4 to hold it in, making for much less power draw, less injector coil heat buildup, and MUCH faster turn-off times.. One of the great things about them is they can also power high impedance injectors just fine, since they will just never draw the current needed to trip into current limiting mode

[@johnwettroth4060]

@@davidwillmore - I worked on white good stuff some. You have some clever designers in that field. Getting the high volume cost down to be competitive took a lot of smart engineering. Zero crossing and cold switching is the same thing that a lot of ATE guys do. They have control of the Source/Measure unit or PMU driver so they can keep everything cold while swtiching. The ATE guys wanted to get 10^11 cycles out of those little Coto relays- and they could do it if you switched cold.

[@johnwettroth4060]

@@Rx7man - exactly- injector drivers are very sexy. They actually use active clamping like a synchronous rectifier to get things to really move. Modern direct EFI, does tiny little timed squirts of 100 uS etc.- amazing- Bosch was/is king. I went on after managing the Standard Products definition to start Maxim's automotive group, I setup the target markets to keep margins high, defined the first 50 or products and hired a bunch of auto industry guys to get it going- a lot of fun. Its now a billion dollar group inside Maxim, soon to be ADI- amazing. We did some interesting injector driver parts and other automotive inductive drivers- modern 7 and 8 speed automatic transmission use very sexy drivers to move hydraulic proportioning valves to make all the shifting buttery smooth. Incandescent lamp Drivers are actually challenging too because the cold resistance is a lot like a short and the hot resistance is much pretty high- took pretty smart drivers. The customers also wanted these to work with LED's with no changes (sense the load profile). LED's have a completely different set of problems. Infineon makes all kinds of really cool smart drivers for automotive and they sell them cheap- at least to the automotive guys. There is a lot engineering in the smallest details everywhere.

[@Rx7man]

@@johnwettroth4060 That's really cool.. I found the LM1949 when I took a Holley Commander 950 standalone controller apart.. it was built in about 2000 or so I can only imagine the new diesel injector drivers have gotta be really nifty to get the response and resolution, and drive piezo injectors which have gotta be very different from coils

[@aiden1015]

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.

[@sencillamentecharles4359]

Awesome fact, could you share more of those "tricky" you questions you ask?

[@stephanweinberger]

Not too hard at all. That's really basic stuff. Easy to forget, especially in a stressful interview situation, but basic stuff nonetheless. I hope that most at least get the transistor part right :-)

[@aiden1015]

@@sencillamentecharles4359 They're all pretty basic EE101 to gauge their working knowledge. Aside from the relay question, I do a equivalent resistance circuit using values that makes calculating values in your head easy (like two 2k resistors in series, which is in parallel with a 4k resistor, and one 1k resistor is in series with the others). Another question is about ideal op-amp characteristics, which leads to a question about why you would use an op-amp configured as a unity gain. The other is just a basic bit wise operations. This does have one trick to it to see if they recognize the overflow that occurs and if their result of the 8-bit unsigned integer is 0x08 or 0x108.

[@aiden1015]

@@stephanweinberger the transistor is 50/50. I've even had people connect the SPDT switch to the GPIO even through I drew the relay symbol with the coil on the left, and the uC right next to the coil.

[@Wingedmechanic]

ULN2003 Is the answer. It has got some 8 Darlington arrays, diodes for freewheel....works nice till the magic smoke escapes.

[@esepecesito]

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!

[@mastertravelerseenitall298]

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...

[@godfreypoon5148]

None of my teachers did that one to our class. But I did.

[@DeanTheDoctor]

Experience is one of our greatest teachers! 😊🌎✨

[@rogerfroud300]

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.

[@nameredacted1242]

Actually, if that was a brushed motor, you just switched the failure mode from driving circuit, to now motor brushes wearing out eventually! (Brushed motors would not appreciate sudden discrete ON/OFF voltage polarity inversion, especially under load!) A much more complicated H-bridge transistor circuit with braking in the control algorithm would be required!

[@gblargg]

@@nameredacted1242 Dave didn't cover it but you can also use an RC snubber, suitable for AC situations. I'd assume it has a softer clamping action.

[@cambridgemart2075]

@@gblargg RC snubber or a varistor would be better than a diode snubber in this situation.

[@JeffGeerling]

@@cambridgemart2075 to be fair, he did touch on it briefly in the middle. I think the main point is you should validate your circuit in general and try to minimize surprises.

[@Blox117]

i like big motors and i cannot lie

[@russellstephan6844]

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.

[@DannyBokma]

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).

[@Thats_Mr_Random_Person_to_you]

I've seen blogs with scope outputs that show how different flyback configurations can have dramatic effects on switch speed. Quite cool. Rule of thumb I've seen when driving with a transistor is to use a zener value that is as high as possible, but less than the transistor breakdown voltage (with headroom). Not sure if that rule is valid.... I've seen come fylback suggestions with diodes and caps and resistors, but not sure what benefit, if any, that has...... sometimes I wonder if adding extra BOM is worth it when we all know fairly simple diodes work just as well.

[@Konecny_M]

Used for fast PLC relay outputs for the very same reason.

[@daleburrell6273]

...TWO ZENER DIODES BACK TO BACK IN SERIES IS EVEN BETTER-(!)

[@rainmakerscustomsrainmaker8985]

A good peace of info. this is a new take that I had not known about. Using a Zener across the coil dose make sense on some of my applications.

[@daleburrell6273]

@@rainmakerscustomsrainmaker8985 ...remember that the 2 zener diodes are connected 'cathode to cathode' or 'anode to anode', in SERIES across the relay coil-!!!

[@shodanxx]

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

[@basaltnow]

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.

[@objection_your_honor]

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.

[@henkdeleeuw251]

Yeah, Dave said you wouldn't blow up the input of your oscilloscope because there is so little energy in the voltage peak. This is true for the resistors in the input, but the frequency compensation capacitors or a (MOS)FET input can certainly be damaged even with a short overload like this.

[@johnpekkala6941]

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.

[@s_s-g4d]

this is yet another one of a multitude hydraulic analogy working very well to explain electricity.

[@joeds3775]

Yeah, i did that once..... Then i had to explain water hammer... I gave up after trying to explain how echoes work. Went home, got drunk and cried.

[@brjplummer9415]

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

[@douglasdeboer2259]

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

[@ct92404]

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!

[@jasonkey8513]

Yes I shed to feel it as a kid messing about and always wondered what it was lol

[@Arthur172B]

OK, how 'bout listening to a legacy one-tube radio with earphones? The 22.5 V B+ lead went to the phones, and I got zapped when I opened the phone connector. SURPRISE! Some phones actually had EXTERNAL screw terminals! Duuuh?

[@trevorvanbremen4718]

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.

[@jdarst100]

Hate those devises

[@007floppyboy]

@@jdarst100 All electronics work on captive smoke, the amount of times I have let it out and the damned thing then stops working....

[@californiakayaker]

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.

[@avro683lancaster7]

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

[@galfisk]

And you can demonstrate the high voltage issue by using a neon lamp as a clamper.

[@theantipope4354]

@@galfisk I actually did that back in the early 80s when I was learning about this stuff.

[@Ozzy3333333]

@@galfisk yes and no, the yes is it will lite, the no is there is not a lot discharge, so depending on the about of stored energy in the inductor, the low power neon bulb might not clamp it much.

[@amitrajput6904]

Yes But it will damage the LED .

[@Ozzy3333333]

@@DavyOneness that video is pretty long, about where in there is the charging off the cemf?

[@hairypaulmm7wab195]

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.

[@StreuB1]

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

[@FFXIshibaa]

Water hammer

[@EEVblog]

Yes, I used the flywheel analogy in the video. No transistor were harmed in the making of this video.

[@RODALCO2007]

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.

[@atmel9077]

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.

[@bsodmike]

How badly can this slamming damage the contacts? The clunk-clunk sounds was really cool. Thanks Mr. Dave!

[@Basement-Science]

@@bsodmike It doesnt. Instead it REDUCES contact wear because any arc gets extinguished faster. Saying to just always add a diode is not the best advice. For example just a Resistor, an RC snubber, or Zener diodes can all be better options than a flyback diode. Plus you cant use a normal diode for AC relays.

[@bsodmike]

@@Basement-Science Thanks appreciate your response :)

[@Magneticitist]

Recovery snubber is the way to go. It reminds me of playing with old pulse motors and how guys were using the back emf to recharge batteries etc. The high rate of change produces the high transients which could do things like weld reed switches together after the bit of plasma in the discharge created enough heat at the contacts.

[@mysock351C]

You can even see that the throw of the relay changes. If you look closely the armature actually goes up until it contacts the top of the enclosure.

[@1crazypj]

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.

[@jdsmedley]

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.

[@nameredacted1242]

But its contacts would now arc more if HV inductive load is switched!

[@ElectronicsExplored]

Yep, that would be my guess as well.

[@gblargg]

EDIT: apparently it's not electromagnetic, and actually making the relay louder. Fascinating. The high-voltage pulse generates RF, which the mic picks up due to imperfect shielding. Basically a small EMP. If you hook a relay up so that its contacts open when energized and power the coil via this, you get a simple oscillator that emits wideband RF, with the spark occurring in the relay contacts. Look up spark-gap transmitter.

[@snivesz32]

My guess is that the high voltage coils magnetic field is coupling to the microphones coil.

[@SamGossner]

​@@snivesz32 That would be a good theory, but the mic used is a Rode NT5, it is a condenser (capacitor) type with no moving coil or transformer, as is more common in studio applications due to higher sensitivity and linearity compared to moving coil (dynamic) designs. The diaphragm itself functions as a capacitor which varies capacitance and thus voltage due to the change in distance between the flexible diaphragm and static backplate when air pressure impinges on the former.

[@gandalf87264]

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.

[@nutsnproud6932]

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

[@Rx7man]

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

[@Speeder84XL]

In an application where fast switching times is needed, mechanical relays is probably not the best choice anyway - physically move contact still takes ages, compared to using semiconductors. But if just a freewheeling diode is used, that also means the current gradually goes down, which could cause slow contact separation and that could be a problem (especially for bigger relays that switches high loads). Even if turn off delay is not a problem, arcing at the contacts may be. I got quite impressed by that "RF transmitter". My experience is that the transistors just fail - not that they "break down" and recover repeatedly like that. But it's apparently not the high voltage on it's own that causes them to fail - but rapid heating from huge internal losses. In a low power circuit like this, where the energy stored by the inductor is very low, as well as the average power dissipation (due to the very low switching frequency) - there is no significant heating anyway. Interesting with the mic at the end also - the RFI-clicks could clearly be heard. Funny that it was a Rode mic - those seem to have bad shielding against RFI in general (they pick up a lot of noise from cellphones and other stuff)

[@Rx7man]

@@Speeder84XL yeah, relays are a bad choice, but they managed with what we needed from them.. I'm sure that now they're using solid state or better.. this was 15 years ago

[@Speeder84XL]

@@Rx7man Yeah. I was also just thinking that even if the switching can be made faster, it will probably still take 10 ms or more to switch on or off - which feels like an eternity when it comes to fast switching in electronics. But I can think for example an elevator that uses relays. If it travels for example 2 m/s and the switching time is shorted from 50 ms to 15 - that makes 7 cm less motion before the motor is disconnected (which makes it significantly easier to set it to stop in the right place). Same applies for other machines moving as well. So, even for applications that don't need to be switched in micro or even nano seconds (as they could using semiconductors), 10's of extra ms can still have quite a significant impact. And for the last part, mechanical relays are not always bad - for high power stuff that doesn't need to be switched very often and very fast, they may still be the best choice. Because metal to metal contact have almost no forward voltage drop and very small losses at on state. The down side, is mainly slow switching and mechanical wear in applications where they switch on/off very often.

[@Rx7man]

@@Speeder84XL In my case, it was a piece of packaging equipment that needed to spray a glue pattern down on a moving piece of cardboard.. the most important thing is consistency, and the longer the delays on switching, the more inconsistency you can have with varying input voltage, temperature, etc.. the relays we used were small 24V ones and they were really quite fast, I'd say probably around a couple ms to turn on at most, and maybe double that to turn off.. As long as the box was moving slow, or was a large box, it wasn't bad, but when you were trying to crank up a small box to 60/minute, timing gets super critical or you get glue all over the place where you don't want it Here's one of the big machines I worked on kzbin.info/www/bejne/o4LYnKtoft6sgpI

[@Speeder84XL]

@justan idiot Ok, that was quite interesting. But I can think that the rise time may in fact be very short when using physical contacts. Once the conducting parts come close enough, current quickly start flowing. (even at a very low voltage the air will break down when the gap is small enough and form an arc/spark, that can allow for a very rapid rise in current flow) Most of the "switch on time" in a relay happens before the "rise time" though (the time it takes from applying voltage on the coil until the circuit it controls is fully on, is whats's interesting for most applications). Also, it's much harder to "turn off" when using physical contact - because there is always some arcing (which can cover much larger gaps when the contacts is moving apart) before the current stops. The fall time, is always very much longer than the rise time. 440 Hz is quite impressive for a mechanical circuit though - much faster than what could be possible with a solid state relay in fact, since those can only turn off at a zero crossing of the AC current. Since an AC half wave on mains frequencies is 10 ms for 50 Hz (or 8 1/3 ms for 60 Hz), that's the time it may take to switch off those (so theoretical maximum repetition rate is twice mains frequency - 100-120 Hz)

[@RicoElectrico]

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.

[@thefatmoop]

I did this in 5th grade and wired it up to the 3v side of a 120->3v transformer. What a messed up childhood

[@abhijitrashinkar]

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

[@Sembazuru]

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.

[@festeradams3972]

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...

[@Ozzy3333333]

Hurt as in EGO?

[@mindaugasvaskevicius1818]

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.

[@rabbithazel3034]

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

[@ct92404]

Homemade metronome!

[@Herby-1620]

I know of one computer (HP2114, from 1970's) that used a "clicker" open frame relay to indicate soft button presses. I suspect it didn't have a full snubber (maybe a zener) to maximize the sound generated.

[@briannahoff8589]

Wow, as an engineering student, I would have learned useful stuff in circuits if you had been the professor. You incorporate the theory into the practical with great visuals, demonstrations, and clear explanations. Thank you!!! This should be a tutorial on how to make a teaching video!

[@bertblankenstein3738]

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.

[@ericwilner1403]

Mic (or the associated circuitry) picking up the electrical spike is my guess, too. Bonus reason: it's the drop-out side of the cycle that gets louder, and that's where the spike happens. (Of course, the pull-in side of the cycle shouldn't be in any way affected by the diode.) Mostly, though, the extra noise sounds more electrical than mechanical, somehow.

[@josephguida5432]

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

[@rollbot]

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

[@beardymcbeardface69]

I inadvertently discovered back EMF when I was a kid, after realising that I could configure a relay to act as a buzzer, passing one of the power lines to the coils first through the normally closed switch pair. Thought it was fun that I was able to make a buzzer, then realised that it was even more fun to touch the coil contacts to get myself zapped! Was high enough voltage to cause my finger to spasm.

[@BigDaddy_MRI]

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!!

[@appelGeek]

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?

[@AnnaVannieuwenhuyse]

Yep! And that's also why timing on them is so so important.

[@nunamvseravno]

The "points" are actually on the Transistor's place in the car's ignition....so no, not the same thing.

[@AnnaVannieuwenhuyse]

@@nunamvseravno what does points mean on this? I had assumed that was a typo.

[@piconano]

@@nunamvseravno Points were used before electronic ignition (transistor) came along. It had platinum contacts. Before your time.

[@yoramalon5273]

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

[@derekloudon8731]

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

[@aicisha]

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

[@squidcaps4308]

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".

[@EEVblog]

Whatever floats your boat.

[@mkepler5861]

hey Dave, excellent video, OF COURSE!!! thank you for your time and talents!!! mike

[@andreasu.3546]

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

[@poptartmcjelly7054]

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. :)

[@JFirn86Q]

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.

[@connecticutaggie]

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.

[@anunaccountablescience6464]

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

[@Mrdibzahab]

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?

[@0MoTheG]

Because the vast majority of the energy never goes far, it stays in the air gap between the ferrous particles. Radiation only occurs when the field is changed over a distance that is long compared to the time it takes.

[@piconano]

The universe is a magical place. there should be nothing. A void. Yet here we are!

[@janami-dharmam]

Only a part of the magnetic energy is radiated off that cannot be recovered; that happens when the magnetic field is changing. The stored energy that is recovered comes from the DC magnetic field and when that collapses part of it also gets lost in the form of radiation.

[@absurdengineering]

Energy storage and energy transmission are separate concerns. Energy transmission (electromagnetic wave propagation) happens when things change. So each time you charge or discharge a capacitor or an inductor, the dynamic part of the electric and magnetic field induce corresponding changes in each other, and those propagate out. But energy storage is a static phenomenon - once a steady state is reached, a certain amount of energy is stored as long as that steady state persists. Most coils and capacitors are very poorly coupled to the free space (by design and happenstance), and thus most of their energy is typically retained in spite of dynamic changes. If you couple a capacitor or an inductor well to free space, a lot of the energy can be radiated out the antenna rather than flowing into other parts of the circuit. This can be seen easily when you have an LC tank with high Q corresponding to long oscillation decay time constant. Once a matching antenna is attached, the Q can be lowered by an order of magnitude or more - more the wider the bandwidth of the antenna and the matching network, since at low Q the effective frequency also changes quite a bit. In circuits that are reasonable to construct, the Q can be lowered to low single digits that way. If the matching network and antenna have low losses, a large fraction of energy stored in the tank can be radiated out :)

[@amancu1963]

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

[@AlejandroLopez-qd3xm]

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!

[@Ureallydontknow]

TE has an application note about this for mechanical relays. They say the diode decreases relay force because it decreases opening force and opening velocity. Both necessary to break the dendrites that grow on the contact point. If you put a flyback diode on control side you have only the spring force to open the relay. It will open slowly with diode but it will slam open without the diode. The problem is you need 80V NPN for 24V relay and that is slightly out of spec. 90V collector absolute maximum rating would be ideal for 24V. It depends on proprietary specifications of your exact relay part number though. This is what it was for my TE part. It also depends on your requirements for relay cycles before failure. I follow the application note from TE to get 1,000,000 cycles guaranteed and certified on the datasheet.

[@acoustic61]

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.

[@ratdude747]

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...

[@LG-xp6fn]

Good illustration of just how different real circuits behave vs a basic circuit design. Looking just at the basic circuit it actually breaks Kirchhoff's current law a few times, e.g. when the free wheeling diode is conducting. No current in from the power supply, no current out to the transistor, yet current flow is decreasing... all the hidden parasitic resistances/inductances/capacitances of the physical circuit are not shown on the basic circuit diagram but they are the key to where that current is going.

[@flmmaz]

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

[@michaelmiller641]

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

[@glasslinger]

Nice description for most people. Note that even theoretically the voltage will not go to infinity. (there is ALWAYS distributed capacitance so the voltage will rise until the distributed capacitance absorbs all the inductive energy.)

[@eDoc2020]

The transistor also doesn't turn off instantly.

[@Vilvaran]

For those wondering about the loudness of the relay increasing; it is clearly the microphone picking up a little of the RF mentioned earlier... If it was a physical phenomenon, I think the relay would have been louder in the rest of the video...

[@cmuller1441]

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...

[@EEVblog]

I mentioned that several times. I didn't want to have to redraw the whole thing.

[@derpythecate6842]

The BJT breakdown reminds of of this cool demonstration by a fellow youtuber, LOOK MUM NO COMPUTER, where he used a broken down transistor to create a reverse avalanche oscillator! The exact same looking sawtooth like curves were created which is useful in analog music and synths. As an additional note, my guess with the flyback diode lowering the sound amplitude is simply because of the increased duration for the energized inductor to lose energy in its magnetic field, so this results in a dampened collison with the contacts.

[@yauwohn]

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.

[@maximusmcfire]

Some relays, especially for industrial application, have already built-in various protection circuits like: rectifiers and voltage reduction(for ac) so relay can be dc but socket for ac(!), filters, emf protection; some protection can be built-in into sockets or can be as additional mini-modules. Actually, it's more convenient and reliable than building your own protection and adding filters

[@flaplaya]

I enjoy HV stuff. Been nailed off of 12VDC through an inductor many times. As field collapses... 90 volts easy all day long. Smart video sir

[@jjmcrosbie]

Good. Interesting. It deals with a much overlooked subject, and the comments on flywheel time are good. I liked the demonstrations of the effects of spring energy in modifying the waveforms. I should like to add a little detail however: 1 - The back-emf at a perfect switch-off would aim for V=I×squroot(L÷C) where C is the total capacitance, ie coil self-C plus stray C. So no infinity, sorry. 2 - Replacing the perfect switch with a transistor and nothing else will cause the transistor collector V to rise until it breaks over the transistor's collector-base voltage which acts like a zener diode. Like the man said. 3 - As another commenter has said, a zener diode can be used to clamp this flyback voltage to a value somewhat below Vceo. The higher the flyback voltage (eg high Vceo transistor with high voltage zener) the shorter the flywheel time, because there's a fixed energy in the inductance (plus contact spring etc if present), and the rate of energy dissipation is proportional to this voltage. 4 - For really heavy-duty flyback limiting. connect the zener cathode to the collector and anode to the BASE. This creates a fast, high-power zener. The transistor must have sufficient power rating, but it beats trying to find a high-power zener. 5 - I've had fairly large telephone relays driven by unprotected transistors pulsing away for hours on end with no apparent problem, but any reliability engineer will tell you that even a single break-over of the c-b diode permanently compromises a transistor's reliability. 6 - In 1967 I was required to adapt an echo-sounder for shallow water readings. The instrument used a single audio transducer for transmit and receive. It had a post-office type relay to switch the transducer from the power driver output to the high-sensitivity receiver input. The flywheel time was so long that the bottom echo was not received: it had arrived during the flywheel time. I cured it by omitting the flywheel diode and used the highest voltage approved transistor available to me - around 120V. And this was my first introduction the this kind of phenomenon. 7 - A factor ignored in this video is current induced in the iron. Some energy is dissipated by the iron core acting as a shorted turn. This effect will actually prolong the flywheel time as it represents a low-resistance, low voltage path just as the simple flywheel diode did. I found this effect with iron-shrouded solenoid valves to completely take over from the electronic circuit at switch-off. They took over ¼sec to close after the end of their drive pulse. This was 1970. I'll be 80 on Thursday!

[@TheNCGardener]

Pretty sure this is what is going on in my irrigation relay board that is causing the solenoids to short out and the flow meter to read erratic flow rates at closing and opening. I could not figure out why it was counting pulses and displaying a false reading. Now I know it is oscillating. I knew adding a diode across the solenoid would probably cure it. Will have to wait until the end of the gardening season before a add the diodes. Thank You So Much!

[@KozmykJ]

I Do like a good bit of scope work. They taught us back in 1970s TV Repairman school, (think Stanley Tweedle, Technician 3rd Class), that the term Flyback came from the action of returning the scanning electron beam to the beginning of its scan. The thing wot lights up yer phosphors to make the picture ... Lots of lovely Back-EMFs in the back of the good old CRT tellys.

[@007floppyboy]

The down side to any coil snubber is the longevity of the drop out, this causes premature wear in the contacts. Best circuit I have used is a diode and resistor in series, then you can fine tune the circuit by altering the resistor to get the least drop out time and least voltage spike. Nothing else has been better, TVRs VDRs Zeners, non as good. The other thing to watch is collective dropout, if you have a series of relays, say three or four the delay can run into many 10s of mS. When controlling pneumatic valves its good to have the same use as well, although then its usually ok to just have a diode, as the difference for a valve of a few mS is not as critical.

[@KyouTGD]

Forgot to put these in and blew a couple PLC outputs. Same issue with trying to run a motor directly off a PLC output. Use relays to power inductors, and use diodes on those relays. You didn't have to call me out specifically about the 1N4001s I use though.

[@sirousmohseni4]

Fundamentals are always exciting. Thanks Dave.

[@erikdenhouter]

When you placed the mike to listen to the relays, you could even see that the relays mechanism released harder and with more distance from the core. It seems rejected / pushed off harder.

[@ketutwiadnyana2610]

In one of the sections of the avionics standard RTCA DO-160 (sect 19-5), the humble relay is harnessed into a transient generator (for susceptibility testing). It is very eye-opening to see that the spike generated is not just a single spike, but multiple transient spikes. So yes, it is a simple device (that we can easily ignore) but knowing its inner working really opens up new horizons. And do not forget, the old computers, before we tricked rock (silicon) into thinking, were made of your humble relays.

[@patbaptiste9510]

I learned this formula in college back in 1994 - VL = di / dt * L. That's the only formula in Calculus I remember where VL is actual V sub L This formula allowed us to calculate the Back EMF induced in the coil, which as you said is many times higher than the applied voltage.

[@jtveg]

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. 😎👌🏼

[@macfixer01]

Another way of demonstrating back EMF is the way that a transformer will convert pulsating DC on the input side to AC on the output side. Such as on the old Ford Model T spark coils, or demonstration induction coils made for science classes. The input voltage feeds through a vibrator on the end of the device, which contains a set of normally closed contacts. When the primary coil is energized the transformer core attracts the metal flapper with one of the contacts mounted on it, which opens the circuit allowing it to drop back out again. The flapper goes back to it’s rest position where the contacts are closed again and the process starts over. Meanwhile the secondary coil is producing high voltage AC from the pulsating DC input.

[@GoldSrc_]

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

[@rmendes2mendes915]

John Bidini says that back EMF negative energy spike is actually a different form of energy. He says it is Energy that is drawn in from the environment. It’s a new energy source of a different frequency and behaves different from conventional positive energy and that you can use this energy by capturing it and storing it in a battery which somehow over doubles the batteries capacity. What would be really cool is if you could make the SSG Badini circuit and show us how it works since you are very good at explaining and showing us on the scope take that negative pulse and use a diode to send that spike to a lead acid battery and show us what it does thank you for your videos

[@CollinBaillie]

THIS is why I watch EEVBLOG.

[@Johnnybytheway]

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.

[@jeffryblackmon4846]

I needed this as my Associates degree is 42 old and I'm retired. Can't allow my brain to behave like a certain US President. It's interesting content, Dave, and I appreciate it.

[@apesmavliet9377]

Let's go Brandon.

[@beaviswashere8009]

My first inductive spike learning lesson was with a 12 volt 2 amp car battery charger, I was about 12 years old( in the 70's) and was hold both leads and touched them together by accident! Holy cap that hurt and scared the crap out of me. I knew you wouldn't feel 12 volts. But now I learned about that new friend, coil collapse , or whatever his name is ...lol

[@danbrit9848]

I think this is the best place to say my most recent vacuum tube build...it's a vu meter the tube is a bar display ...well I hook it up and it's maxed out so I add high value resistors and still no change....then in frustration I unplug the audio lines...boom I have a perfect signal and not maxed out dancing away....after some fiddling I found my audio source a blootooth pcb was sending audio thrue the power USB plugs on it threw the other power plug and into the tube ....my build is literally relying on back emf as it's input lol.....I found it cool just thought some one else might too :)

[@fredmitchel1236]

Love the Australian humor...the 50 ohm signal generator termination...I thought the emitter current wasn't exactly the same as the collector...lol I just played with some littlte Omron PLC switching a 7 watts DC solenoid...we had 10 systems returned for failed closed contacts...the solenoid had no snubbler...I tired to cycle and repeat the failure measured -375 volt reverse spike when opened...relay cycled 1.5 x 6....I was advocating a snubber...& 1N4002 clamped it good enough...it was their design....that valve might be opened 4 times a day in its life...so probably the contacts were being eroded..but not the reason for failure on install for PLC relay Q3. What could it be? Ah...there is another solenoid..a N2 purge...that had 8 foot cable from the Q3...and is also was cycle tested unclamped...two 7 watt solenids ran 600,000 cycles without a failure...humm...then found if the 8' cable to N2 was shorted..I got the relay contacts to weld closed in 5 cycles...the little 24 VDC switcher could supply 5A continuous and lots of fat filter caps...before it foldedback...to save itself... At the end of the day...a i am almost sure a new hire was installing the unit and was powering up before terminating the N2 solenoid..and we didn't always do a good job separating the wire ferrules after final test on that cable...before shipping.... Problem went ...quietly away.. No snobbery clamps were installed...let it just radiate... Good job...on BACK EMF

[@victordesanderobledo4522]

Back in high school our teacher demonstrated that in a particular way. He made one of us touch both contacts of a big choke while he connected them to an AA battery. And when he broke the circuit, the other guy holding the wires got a mild but annoying shock. Apparently he did that every year until an offended student complied to the principal.

[@johnsheppard8396]

Mad dog mazlin

[@redsquirrelftw]

That's really cool. When you had the mic close and no diode you can really hear the high voltage pulse interfere with the mic. Like a "tick" sound along with the relay closing.

[@prathikprashanth2932]

Connect two wires across the relay (without the catch diode) and keep the open ends of the wires close to each other to see if an arc jumps across the wires. That would be cool to watch! Heck, you can probably make a taser like that lol!

[@МихаилЦымбал-д9м]

And this is the basic working principle of flyback converters, btw they are actually used in tasers

[@ridefast0]

I blew up a micro-ohm meter measuring the dc resistance of a microwave oven transformer primary. Yes it was the back-EMF did it. Kelvin clips interrupted the test current without isolating the voltage measurement. BTW it was about 6 ohms for a 240V primary! 😢

[@frankbose544]

its cool i just did a physics 1 class I want to do ee, and its awesome you can represent anything with math

[@VoidHalo]

I'd love to see you do a video on maxwell's equations. This was a nice teaser, though.

[@P25AES]

Awesome demonstration Dave!

[@kalli71]

very cool! someone once told me imagine a coil to b e like a big water wheel. you put craploads of energy in there to get it to spin and then you cut off the water - it will not just suddenly stop -- that finally makles sense to me - thx!

[@Vintaronica]

The reason the relay noise changes when you add and remove the diode is because as you already said, the relay switches slower, so the diode will make the relay action softer as the voltage/current dissipates then this translates to a less vigorous click as the contacts hit each other more softly..?

[@nightshadelenar]

07:15 this arrangement is used in many HE (High Efficiency) LED lamps, where not only the current being drawn through the inductor is used, but also the energy stored in the inductor is used as well. Bigclivedotcom has taken apart many a lamps with this configuration with buck or SM PSUs

[@Apple.Repair]

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

[@jkbrown5496]

Induced currents are interesting to consider. Mostly because they flip things from the standard training on electronics of a voltage source when applied to a resistance creates a current. But a current when applied to a resistance creates a voltage. And how in older car points and condenser ignitions the capacitor (condenser) was used to delay the voltage rise and thus arcing on the contacts (points) as they opened and closed.

[@Aileron90]

Ran into this problem! Was fitting PLC’s to drive 24v relays which drive devices, fuse protection kept blowing and it was the back emf of the relays! They had no diodes and it was dumping its coil current back and smashing the fuse modules, ended up wiring diodes just like shown here, only problem was we had over 1000 relays hahah.

[@bill-2018]

An excellent demonstration. It's one thing reading this in a book but it's much better when demonstrated like this. Perhaps if you had a radio and we could listen to the r.f. produced as well it might be interesting. When I was about ten or twelve years old I had a transformer and connected a 9 Volt battery to the primary. I held the wires on the primary and got a shock and didn't understand why. You don't get a shock from a 9 Volt battery. Perhaps if I had asked the physics teacher at school why this happened I might have known. We didn't do electronics at school then, how different it is now and I might have taken a different route in the jobs I had. In the mid 60's I got the impression teachers were not interested in much more than just doing their job. I made an H.A.C. one valve radio from a kit at 13 years old but never told any other kids because I'd probably be laughed at. It was football, football, football and nothing else. I wasn't interested in football. When I was 19 I studied for the radio exam and I found out about back e.m.f. and have since always put a diode across a relay. I use BC109's for the switching transistors and 1N4004 diodes for switching frequency bands in my home built transceiver because I had bought loads of them cheap. I love making and modifying radio stuff. G4GHB.

[@moienm8802]

This is a really great educational video Dave. Seriously!

[@byronwatkins2565]

If you need to stop the inductor current quicker, increase the diode voltage with a series Zener or R||C snubber (inset at 8:15). Keep in mind that the V_CE (or V_DS) will also increase.