EEVblog 1409 - The DANGERS of Inductor Back EMF

A practical demonstration of Lenz's law and back EMF in an inductive relay coil and how to solve it using a Freewheeling/Flywheel/Flyback/Snubber/Clamp diode. Also the downsides of clamping diodes, and switch arcing supression.
Also a look at an AMAZING potential phenomenon you probably haven't seen before!
Actually, two rather cool things you probably haven't seen before.
Along with transistor ratings, transistor storage current, and Collector-Emitter breakdown voltage, there is a lot to unpack in this video.

This is a follow-on from the DC Transients Fundamentals video:

00:00 - Recap of Relays, Inductors, Faraday & Lenz's Laws
02:30 - Relay Back EMF Explained
07:09 - The Flywheel analogy of Inductors
08:30 - Relay circuit demonstration
12:35 - 700V Back EMF!
14:43 - BJT Transistor Storage Time
17:03 - Back EMF Diode clamp demonstrated
19:06 - An AMAZING demonstration!
24:43 - Trap for young players
25:23 - DOWNSIDES of Back EMF Diodes
28:38 - BONUS cool effect of Back EMF diode DEMONSTRATED

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#ElectronicsCreators #Tutorial #Inductors

[johnwettroth]

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

[russellstephan]

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.

[avrolancaster]

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

[rogerfroud]

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.

[aiden]

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.

[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!

[mastertravelerseenitall]

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...

[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).

[nohagad]

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

[shodanxx]

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

[TommyBoyHeads]

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.

[johnh]

I blew so many mosfets during my undergraduate years before I figured this out.

Only wish you were around back then.

[danielnelson]

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!

[davidwillmore]

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. ;)

[kakugeiikkikomori-iko]

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!

[nwimpney]

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.

[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.

[lolfun]

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.

[nameredacted]

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!

[trevorvanbremen]

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.