Blow oscilloscope measuring coil Back EMF - Page 1
Forum section: electronics / testgear

author: ptluis
author_rank: Frequent Contributor
timestamp: August 24, 2023, 08:51:37 am
content: Hi, I've been testing some coils, and i wonder if it's possible to damage the scope measuring Back EMF from coils like relays? I got results like spikes of 400v up to 600v, with short durations in ns range. I'm testing on an old scope that allows a maximum of 300v on the inputs. His not damaged, yet, but I wonder if there's a proper safe way of doing it? 10x probe was used. I don't have a 100x probe, maybe i could create a voltage divider between probe and scope?thanks

author: RoGeorge
author_rank: Super Contributor
timestamp: August 24, 2023, 09:08:58 am
content: Yes, back EMF can permanently damage electronics, and sometimes the coil itself can get damage from its own back EMF.The voltage spike may vary wildly (for the same coil) depending on the initial conditions, I wouldn't risk measuring that.

author: MathWizard
author_rank: Super Contributor
timestamp: August 24, 2023, 11:03:48 am
content: People do make higher voltage probes, with resistor dividers, and caps. Dave has a video on it too, not much too it. You could solder up something on a protoboard, and then uses your normal probes on that.

author: Fungus
author_rank: Super Contributor
timestamp: August 24, 2023, 11:08:50 am
content: Quote from: ptluis on August 24, 2023, 08:51:37 amYes, of course it is!Quote from: ptluis on August 24, 2023, 08:51:37 amAny circuit with back EMF should have protection in it, it can also damage your relays.A $0.02 diode will do the trick. Try putting in an LED so you can see it flashing.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 24, 2023, 08:04:03 pm
content: If you divide the signal, you're loosing resolution. Maybe a simple clamp after the probe to limit the voltage. Recovery time may be a wash if you are over loading the scopes front end today and are happy with it.If you are just playing with DC, there are some very inexpensive 100X probes out there. Then again, I cut one open and wasn't impressed.

author: ptluis
author_rank: Frequent Contributor
timestamp: August 24, 2023, 08:53:13 pm
content: Quote from: MathWizard on August 24, 2023, 11:03:48 amI will try this one. Thank you.

author: ptluis
author_rank: Frequent Contributor
timestamp: August 24, 2023, 09:05:08 pm
content: Quote from: Fungus on August 24, 2023, 11:08:50 amI always use a flyback diode, but what I'm trying to capture is how high this EMF reach without any kind of protection on the coil.Talking about protection, Is there a real effective way to improve the protection against Back EMF besides using the diode? even with the diode those cheap voltmeter/ammeterhttps://www.aliexpress.com/item/4000310174925.html?search_direct=true&spm=a2g0o.productlist.0.0&gatewayAdapt=glo2brashutdown, the diode is not enough.

author: Fungus
author_rank: Super Contributor
timestamp: August 24, 2023, 09:29:52 pm
content: Quote from: ptluis on August 24, 2023, 09:05:08 pmIt's not a very useful measurement. There's no real limit if impedance is high enough and measuring it will usually alter the result.If you want to experiment you could measure the peaks into various fixed resistances that are orders of magnitude lower than your probe's impedance (eg. a 10x probe and 5k, 10k, 20k, 50k, 75k, 100k, 200k resistors) then plot a graph.When you have your graph you can extrapolate it and see if your 'scope would have died or not.

author: tautech
author_rank: Super Contributor
timestamp: August 24, 2023, 09:49:51 pm
content: Quote from: ptluis on August 24, 2023, 09:05:08 pmTo what end ?You're dealing with switched magnetics so back EMF will always be present and its level is always relative to the impedance of the return/discharge path.Place appropriate snubbers across it and move on with the rest of your design, whatever that may be.

author: ptluis
author_rank: Frequent Contributor
timestamp: August 24, 2023, 10:00:09 pm
content: Quote from: tautech on August 24, 2023, 09:49:51 pmI learn a new English word today "snubbers"

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2023, 10:01:54 pm
content: The answer is more easily found by modeling the coil and surrounding elements.First of all, we're generally not dealing with coils in the abstract, but flyback / inductive EMF is only a concern where it's an unintended/inevitable side-effect of a component, like a relay or solenoid.Coils, inductors, in general use, we aren't concerned with the EMF becausethat's entirely the reason we've chosen them. Whether as a resonant coil in a tuning network, or the active element in a flyback converter, the EMF is fully constrained by the circuit it is embedded within.So, assuming you're working with relays or solenoids here.Typically such a coil has an overall L || R characteristic, due to eddy currents in the core. This is exaggerated for solenoids (usually a solid piece of iron), significant for small solenoids and relays (often using pieces of stamped metal, less thickness = less time for field to penetrate the material), and optimized out of real inductors and transformers (using laminated sheets, or materials like ferrite that carry significantly less eddy current). For turn-off flyback purposes, we can assume the core linear, and therefore characterize it at signal amplitudes.The easiest way to characterize a coil is probably, using a sine generator and scope, connect the generator through a resistor to the solenoid, and measure the voltages at both ends of the resistor, with respect to ground. (So, scope probes, generator and solenoid are all common ground, and resistor connects to generator and solenoid, and both scope probes.)Use a calculation like this,https://www.seventransistorlabs.com/Calc/RLC.html#vecand tabulate values over a range of frequencies. Say 10Hz to 100kHz or more, taking a few points per decade, more if you find an interesting "kink" in the response.Likely, you'll need to switch resistors from time to time, or voltage scales; just make note when/where you're doing that.Then plot it: take abs(Z); compare Re(Z) to Im(Z); etc. Most likely you'll see Re(Z) ~ Im(Z) over much of the range (the signature of eddy currents in bulk metal), maybe you'll see a long asymptote where it behaves this way, or in fact behaves inductively (|Z| ~ F).The maximum impedance |Z|, at whatever frequency corresponds to turn-off rate, gives the peak flyback voltage for that device, when turned off at that rate.You might never be able to achieve such rate; for example a 1A solenoid driven by IRF520 will still be loaded by the few 100 pF of the transistor, plus some 10pF and kohms equivalent of whatever probe you connect to it. You would be able to infer, from these data, that something very fast and high performance, like an air spark (switching can occur within some ~ns; fractional ~pF loading is possible), could generate such-and-such voltages.For coils other than electromechanical ones, the answer is much simpler: don't use it that way! The EMF is not something god-given, it's an expression of the coil's response into the surrounding circuit. Simply don't let that voltage appear, and you won't see it! Hence clamping diodes or zeners being a common sight in coil driver circuits. An SMPS fully constrains the inductor voltage at all times (between switching transistors or clamping/rectifying diodes); even the boost/flyback converter (suitable for generating high voltages) can be constrained by, not just device characteristics (you can't go higher than the avalanche rating of transistors/diodes used..), but the control circuit as well.Tim

author: ptluis
author_rank: Frequent Contributor
timestamp: August 24, 2023, 10:12:15 pm
content: Quote from: tautech on August 24, 2023, 09:49:51 pmBTW to avoid opening a new topic, can you tell me what is the best Siglent website to download the latest firmware for Siglent scopes?For example SDS1202X-E on:Siglent.eu latest firmware is V1.3.26Siglenteu.com latest firmware is V1.3.27

author: ptluis
author_rank: Frequent Contributor
timestamp: August 24, 2023, 10:34:05 pm
content: Quote from: T3sl4co1l on August 24, 2023, 10:01:54 pmThank you very much for your explanation!What I'm trying to do is to design a portable module to protect the output of power supplies when I connect inductive loads like relays, contactors, electric egr solenoid, etc for testing purposes. When using digital power supplies sometimes the flyback diode isn't enough to stop the PS or the display from shutdown. No problems with analog PS with analog displays. The flyback approach isn't enough and that's one of the reasons I want to see how high back EMF goes. But I won't do it anymore I'm going to save the scope, I already see what's going onLike you write "Simply don't let that voltage appear" and that's what I'm going to do.

author: tautech
author_rank: Super Contributor
timestamp: August 24, 2023, 10:34:32 pm
content: Quote from: ptluis on August 24, 2023, 10:12:15 pmI always us the HQ website.Quote1.3.27 is the latest and there are some important fixes for SDS1202X-E.https://int.siglent.com/upload_file/zip/firmware/Oscilloscope/SDS1202X-E_V1.3.27_EN.zip

author: ptluis
author_rank: Frequent Contributor
timestamp: August 24, 2023, 10:39:31 pm
content: Quote from: tautech on August 24, 2023, 10:34:32 pmThank you! Link added. Firmware downloaded!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2023, 12:30:51 am
content: Quote from: ptluis on August 24, 2023, 10:34:05 pmThat sounds like a wholly different problem, and one that you cannot in general assist: highly susceptible (presumbly, cheap) power supplies.Tim

author: Fungus
author_rank: Super Contributor
timestamp: August 25, 2023, 06:37:46 am
content: Quote from: ptluis on August 24, 2023, 10:34:05 pmPower supplies will have a big capacitor across their output which will absorb EMF spikes, no problem.

author: ptluis
author_rank: Frequent Contributor
timestamp: August 25, 2023, 09:28:57 am
content: Quote from: Fungus on August 25, 2023, 06:37:46 amIt should, but the point is digital ps or displays shutdown randomly, and i want to prevent that. it doesn't matter how cheap or expensive the power supply is, I used to own a Rigol Dp832 and it happened sometimes, i sold it, and it was to heavy to carry arround. I normally test around 30-40 solenoids per day. I'm using an homemade ps with transformer and an old L200 regulator, analog voltmeter/ammeter. no problems. It only happens with digital ps supplies/displays.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2023, 01:34:56 pm
content: Quote from: Fungus on August 25, 2023, 06:37:46 amI built a homemade linear supply years ago which I damaged a few times from transients. Later I added a MOV to the output.I've also damaged both of my vintage linear HP supplies from transients as well. These both have a large TVS installed now across the connectors. Cheap insurance. The TO3 transistors cost a lot more.

author: Grandchuck
author_rank: Super Contributor
timestamp: August 25, 2023, 01:46:24 pm
content: Joe, why a MOV in one and a TVS in the other? Just curious.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2023, 02:07:28 pm
content: Quote from: Grandchuck on August 25, 2023, 01:46:24 pmGood question. I started my electronics design career in automotive. Back then we were using MOVs. This was before they became a fire concern and we ditched them. I wanted to learn more about microcontrollers and this power supply was the very first thing I ever designed using the MC68701. I wrote all the code, laid out the board and photo etched it at home. The keypad was from an old girlfriends phone that had died. There is an RS232 port to connect to my PC to control it and read back the current, voltage as well as a second voltage input.The transformer I dare say was something I salvaged from an old Sperry Rand UNIVAC terminal. Case is powder coated steel. The front plastic came from a friend who had worked for a plastics company.The HP mods were many years later.

author: HighVoltage
author_rank: Super Contributor
timestamp: August 25, 2023, 02:13:51 pm
content: The old FLUKE Combiscopes like the PM3394B seem to be immune to high voltage at the input. I had it several times that a 5kV spike jumped to the input and nothing happened to the scope.I would not try this on a modern scope.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2023, 02:16:37 pm
content: My love of the vacuum fluorescent display also came from automotive. The ADC is a whole 8-bits and supply still appears to pass cal.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 25, 2023, 03:29:42 pm
content: Quote from: Fungus on August 25, 2023, 06:37:46 amNot necessarily. Lab power supplies or programmable power supplies usually have a relatively small capacitor to avoid dumping a huge current pulse into a load before current limiting kicks in. It's plenty to absorb ESD or small spikes from smaller inductors, but not anywhere near enough to absorb EMF from a power relay coil (which remember is more than just an inductor, part of the spring energy is converted back to electrical).The output caps should give you some breathing room to let the protection circuit kick in. But a cheap power supply with bad or no protection circuit isn't going to be saved by a 10 microfarad capacitor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2023, 09:27:20 pm
content: I still see no mechanism for malfunction here. Say an inductor is fully charged, let's say fractional H at several A, and one lifts the wire to the power supply. In that instant, terminal current drops to zero in fractional ns; this launches a positive flyback up the PSU wiring, probably maximum amplitude of 20-100V as the initial (~nm) gap breaks down easily. The full route should be some 100nH to the input capacitor, if that. Very little energy. Could still be a fair amount of RFI around given the current and speed, but of note, this path is identical whether the load is inductive or resistive. If your supply can't handle a loosely-wired resistor, something else is wrong.On the coil side: terminal current is zero, so inductor current charges the stray capacitance of the coil. A few dozen nanoseconds later, it's ramped to the breakdown voltage of the air gap -- mechanical things move slowly, this might be a few 100 nm at this point, and breaks down at maybe 100V. The spark discharges stray capacitance into the PSU, and we have an EFT (electrical fast transients) scenario. Each pulse is roughly characteristic impedance (the coil doesn'treallyhave capacitance, it's a ball of wire, and that wire has characteristic impedance to itself), so, in the ballpark of 100V / 50 ohms = 2A. This energy dumps into the PSU wiring, recharging the output wires to coil current, until the spark cools too much, the connection breaks, and the cycle repeats.As long as the coil remains charged, sparking continues as the gap slowly widens; the EFT response is a rapid-fire chain of increasing amplitude spikes; the repeat rate slows as it goes, as the coil runs out of current to charge the capacitance up to an ever-higher breakdown voltage.In the process,no quasi-steady-state current greater than the initial coil current ever flows,and no peak current flows for longer than the characteristic length of the system (probably not the whole coil winding wire length, but a fraction of it; EFT is normally tested with 50ns (FWHM) pulses).The impedance is high, so a small capacitor will suffice to bypass the terminals. There is likely common mode coupling as well, for which caps to a metal enclosure would be desirable. Even 0.1uF will have significant impact (the equivalent pulse capacitance of EFT is around 1nF).It's far more likely to me that the EFT coupled upstream and blasted some control IC or transistor, especially onpoorly laidprotoboards with no ground plane. (Not pejorative, mind. I've made plenty of point-to-point and perfboard circuits. It's convenient. Susceptibility is simply a consequence of such open construction.)This also suggests that a MOV does more than a TVS -- strictly on account of having more capacitance; breakdown of either is probably never activated.Tim

author: AVGresponding
author_rank: Super Contributor
timestamp: August 26, 2023, 07:45:39 am
content: https://www.eevblog.com/forum/projects/when-to-use-mov-tvs-or-gdt/

author: ptluis
author_rank: Frequent Contributor
timestamp: August 26, 2023, 08:48:18 pm
content: Quote from: joeqsmith on August 25, 2023, 11:40:09 pmThis was the power supply I had before, and there were situations where the display blank, the ps freezes, and some reboots happened. I no longer own the PS, otherwise I would make a video and show you guys. But I was lucky and the PS never burned out. The power supply is very good but I'm sorry to say it is also not back EMF proof.

author: tautech
author_rank: Super Contributor
timestamp: August 26, 2023, 09:12:01 pm
content: Quote from: ptluis on August 24, 2023, 10:00:09 pmNow with more knowledge that you're working with solenoids.....Some years back when designing circuitry to momentarily switch a 24V 10A solenoid with gear I had on hand.....a whopping big free IGBT module and back EMF protection was required for its protection.It was for wireless remote release of solenoid controlled mechanical throwing arm of a claybird trap machine.After consultation with my EE mentor at the time, a reversed biased large diode and large Zener in series were used for back EMF snubbing.IIRC a 5A diode and 10W zener, maybe 12V I don't remember sorry.

author: ptluis
author_rank: Frequent Contributor
timestamp: August 27, 2023, 12:24:44 pm
content: Quote from: tautech on August 26, 2023, 09:12:01 pmThe IGBT and zener approach it's interesting, I need to do some testing. I've been collecting all sorts of information about the subject and making few testing because time is short, but i think I'll figure out the best solution. Thanks