Snubbing relay contacts - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: June 21, 2018, 09:15:01 am
content: Consider some relay switching a non-inductive load. For example, 1 Ohm 10 amps.As the contacts part, the contact area gets smaller, with at some point - I assume - a phase during which lots of tiny surface bumps only just touch across the gap.Although the contact is a good conductor, say silver, a reduced area raises the resistance and helpfully reduces the current.So, a smooth-ish rise in resistance, no sparks? It does not work that way in real life.It is generally recommended to open the contacts quickly; presumably a slow opening actually ecourages arcing.I spent ages on Google, looking for a sensible way to calculate RC snubber values. I've not found anything convincing yet.I do gather though that the resistor is present to limit contact-close discharge current, and the contact voltage curve is dominated by the RC network.From a Matsushita handbook: Vcontact = Vo(1 - e^-(t/RC)), as expectedThen they giveR > V/(Imax - V/Rload); i.e "V/(max allowed current - current in load)"again that "looks OK".So then, it would seem, we want the RC charge curent to dominate any current flow over the time during which the contacts arein the process ofopening, typically 1..3mS.So, do I choose RC such thatVcontact/Rload > 10A during the turn-off time of (say) 3mS?

author: apblog
author_rank: Regular Contributor
timestamp: June 21, 2018, 10:29:39 am
content: I do not have the answer to your question.But I think this TE App note does:http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3236_AppNote&DocType=CS&DocLang=ENSorry I couldn’t be helpful...

author: 741
author_rank: Frequent Contributor
timestamp: June 21, 2018, 11:02:54 am
content: Thanks for that link. It might be one I was trying to find... Referenced in a similar TE app note "Contact arc Phenomenom", they have this little noteWe won’t get into arc suppression techniques here because that’s the subject of another application note titled “Relay Contact Protection.”I was never able to find that document, and it may be your link is the same thing but re-named.

author: Kjelt
author_rank: Super Contributor
timestamp: June 21, 2018, 11:18:15 am
content: I found snubbers of all kinds mostly "not working" satisfactory in DC or high inductive situations.I found some interesting relayfamily called "pre-make relays" which has two different contacts making contact apart from eachother.The first to make contact is the so called pre-make contact made of Wolfram highly robust but not low ohms, the second is a standard contact silvertin AgSnO2. On make the W premake activates ms before the AgSnO2 contact, on break the AgSnO2 Contact breaks first followed by the W premake contact. So the Wolfram contact takes the sparks the other one stays clean.The ones I found are rather small from Schrack the RTS3Txxx series and came across them in some industrial relay boxes.

author: JS
author_rank: Frequent Contributor
timestamp: June 21, 2018, 02:29:09 pm
content: I see a problem with your approach.Let's take your 1Ω 10A innyour example, you put a big enough cap for all tonwork. You want to limit the current while switching at those 10 amps, the cap presumably is charged with those 10V so you need 1Ω in series with your cap. When you switch off the load, right immediately after thebcontacts loose contact (uhu) you have half the voltage already. If the source is also resistive you are in an even worse situation as you need a higher R in the snubber as the voltage while open is higher than the voltage at the load when connected. With this in mind, the snubber could make things worse that not even being there abusing the contact at switch on times.JS

author: Miyuki
author_rank: Frequent Contributor
timestamp: June 21, 2018, 02:37:20 pm
content: I have here similar problemNeed to switch 10A at 200V DCAnd I have a dilemma of trying to solve it with relay and spark suppressor or just use huge DC rated contactor (which is bulky and expensive)

author: 741
author_rank: Frequent Contributor
timestamp: June 21, 2018, 02:41:22 pm
content: This is a really good App Note. One thing I do not get though, under the heading "Capacitor selection":Further assume an R-C network is needed that will result in contact voltage of perhaps 15volts 1 usec. after the contacts have separatedWhere does 1uS come from? Why not 0.5uS or 5uS?

author: 741
author_rank: Frequent Contributor
timestamp: June 21, 2018, 02:47:18 pm
content: "I found snubbers of all kinds mostly "not working" satisfactory in DC or high inductive situations."That is very interesting. It does not help that many relays are opaque. How can I check for arcing? How does a 'scope trace exhibiting arcing differ from a normal break for instance?The pre-make contact style sounds like good idea, I imagine it costs though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2018, 03:13:57 pm
content: Hmm, there would be a few regimes there, where different things might happen:1. Low current. The voltage remains low until the last contact point lifts off. For the first for nanometers, current tunnels across the gap. Resistance then rises, capacitance charges and inductance begins to discharge. If the peak voltage is small (under 20V, say),By the way, it is essentially impossible to ignore inductance here. Contact closures have rise/fall times in the subnanosecond domain. (Yes, the lowly relay, is actually one of the fastest, cheapest components you can buy! It's just incredibly sloppy...) So, a single inch of wire has enough inductance to matter, even in a high impedance circuit.Or more accurately, wires are transmission lines. When a step change occurs, there is not an unlimited rise in voltage, or change in current -- the ratio is fixed, and this ratio is the characteristic impedance of the line. The impedance of a typical contact circuit will vary all over the place, but the important part is it is perfectly well defined, physically speaking.The impedance might vary from, say, below 50 ohms within the contacts, to ~100 ohms over the springs/wires leading from the contacts, to 200+ ohms for loose wires traveling through space, and so on up and down, going along the rest of the wiring. But it is a perfectly knowable, bounded value -- just not a convenient one! (RF relays are basically made so that this impedance is nearly constant; they're also made with better shielding between poles.)2. Medium current. As contact points lift off (and some smoosh back together, because it's a messy sliding and rotating and flopping motion, not a rigid one way movement!), resistance rises, and voltage rises. A lot of heat is dissipated in some areas, but not enough to cause melting and sparking. As the contacts further separate, at some point the resistance shoots way up in a fraction of a nanosecond, and the step current change induces a step voltage change on the contacts. This is enough (~20V+) to break down what little air has entered the gap (reminder: the mean free path in air is 68nm, so until the contacts are about that far apart, they are effectively filled with vacuum!), and a spark is maintained until the voltage runs out (the transmission line discharges its current) and the gap opens wide enough to clear what voltage remains.This is the regime where EFT (electrical fast transient) occurs. The transmission line can act like a resonant circuit, and the arc has an overall negative incremental resistance. As the TL discharges, it ringing voltage "slaps" across the opening contacts, emitting a series of sparks. The sparks are rapid, on the order of 1ns, while the repeat rate is determined by the TL length. Each spark launches a wavefront of energy up the TL, where it couples into nearby wires, or radiates into space.A typical EFT model is described in IEC 61000-4-4: The pulse has an amplitude of 2kV (adjustable depending on what you're testing), the repeat rate is 100kHz (which corresponds to a pretty long equivalent transmission line -- which might be typical of a contact opening on facility wiring plus motor/transformer inductance), rise time 5ns, and an exponential decay (FWHM 50ns). The source impedance is 50 ohms and the signal is coupled onto EUT (equipment under test) cables with a "capacitive clamp", which is really a transmission line structure itself, which is important because it makes a single-peak waveform in one direction and a double-peak waveform in the other (because of reflection).These pulses would usually be generated from, hmm, I suspect a hydrogen thyratron really, but there's a couple of possibilities; but that doesn't matter. In the real world, any damn spark will do! (They wouldn't use actual spark gaps in test equipment -- too inconsistent.)3. High current. As contact points lift off, resistance rises, and voltage rises. A lot of heat is dissipated in some areas, causing melting. The hot points produce thermionic emission, and as air enters, it is ionized by the electron stream. (If air doesn't enter quickly enough, or the current is even higher, the material itself can vaporize, due to further heating (vaporization) and ion bombardment (sputtering). An arc will then be sustained on the ejected material.)Supported by emission, the arc can grow to quite long lengths before clearing. In the extreme -- when a lot of voltage is available -- a medium must be provided to extinguish it. Power line equipment has solved this problem with, for example: sand-filled fuses (the high purity quartz sand, melts and vaporizes without becoming conductive, absorbing arc energy and supplying more gas to snuff out the arc), gas filled switches (sometimes explosively triggered to be sure there's enough gas), or oil filled reclosers (the oil is vaporized and decomposed by the arc, generating gas; the oil also has a very high breakdown voltage, so less distance is required).(There are also vacuum reclosers, which suffer from the melting and self-arcing problem, but once enough distance is cleared, they go back to an open state of absolutely nothing at all. Downside: all that blasted-out metal condenses on the walls, eventually turning it conductive..)Incidentally, the same process in reverse, give or take a hearty dose of inrush current to boost things further, gets you welded contacts. Contacts normally weld anyway; it only becomes a problem when the coil/spring force isn't enough to break them apart!So in summary,0. Contact closures and openings are stupidly fast. There's always some final point of liftoff that makes a really fast edge (~1ns give or take).1. If there's not enough current/voltage to do anything interesting, that's that. You get the edge, but it just goes out wherever.2. If there is enough, then the contacts spark. The sparking makes multiple repeat edges, and they're bigger because they're supplied by what the line current was at, multiplied by the line impedance.3. If there's a lot of current, then the contacts melt and arc. (You probably don't get so much EFT in this case, but still one big spike when the arc finally goes out.)And so we come to snubbers.What are we doing?We are terminating the source end of the transmission line.That's all there is to it!So, R ~= characteristic impedance, and C ~= equivalent capacitance of the line. (If you don't have a direct measurement, use:\$ C = \epsilon_0 \times (\textrm{Length}) \times (\textrm{Impedance of free space} / \textrm{Characteristic impedance})^2 / (\textrm{Velocity factor})\$Ballpark values, \$ \epsilon_0 \$ is 8.84pF/m, impedance of free space is 377Ω, characteristic impedance is 50-150Ω (varies with geometry -- twisted pair is around 100, coax is down around 50, twin lead is up at 300, etc.), and velocity factor is 0.67 (coax) to 0.8ish (twisted pair) to ~1 (bare wires in air). This gets you the rough capacitance of any wire in space or insulator -- very handy, it's just a bunch of ratios.And you can always use more, up until the leakage current through the capacitance is excessive in the off state (since we're talking mains frequency, often). Or for DC, you have the problem of, when will it actually be "off"? -- you get the exponential tail of the cap charging.If you prefer to work in lumped inductances rather than transmission lines, that's perfectly fine, too! Then you work with the figure sqrt(L/C), which has units of ohms -- the characteristic resonant impedance. You can set this so that, at peak load current, peak voltage (Vpk ~= sqrt(L/C) * Ipk) stays reasonable. Then put R = sqrt(L/C) in series, and you're done!It's all about impedances. Impedance is just the ratio between voltage and current. Important stuff!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2018, 03:16:48 pm
content: Final addendum: so, what about contact closure, too? That R+C isn't doing anything if it's shorted across...Quite so. Ideally, we need an (L || R) in series with the contact, to terminate that action too. How do you calculate the values? Exactly the same! Just upside down, swapping L for C (according to dimensional analysis rules, that is).Both together, give the contacts a well defined impedance at radio frequencies -- a nice squishy resistance that absorbs noise, rather than letting it ring.A ferrite bead is the most basic single component like this, but you wouldn't actually use one because of other factors (a shame!).Most often... closure is just ignored. ¯\_(ツ)_/¯Tim

author: Kjelt
author_rank: Super Contributor
timestamp: June 21, 2018, 04:31:20 pm
content: Quote from: 741 on June 21, 2018, 02:47:18 pmNot extreme, i believe the named Schrack series where €4 a piece where the standard series are around €2.The big power relais I have not yet seen with his scheme but indeed they would be quite pricey

author: Seekonk
author_rank: Super Contributor
timestamp: June 22, 2018, 12:12:29 am
content: There is an interesting way to make the DC interrupting ability to close to that of the AC rating. Relay must NC/NO with cap across those contacts. Very high value resistor from NO to common. This introduces a reverse voltage on the contact that quenches the arc.

author: Belrmar
author_rank: Regular Contributor
timestamp: June 22, 2018, 09:47:18 am
content: Maybe using some kind of mercury wetted relay may help solve the arcing when the switch is closing, the biggest problem is when you open the contacts on dc loads, maybe you cold use a Mosfet on paralel that temporarily shunts the relay while it opens the contacts so no current flows trugh the relay while it opens

author: Kjelt
author_rank: Super Contributor
timestamp: June 22, 2018, 02:02:35 pm
content: Quote from: Belrmar on June 22, 2018, 09:47:18 amFor DC only why not use DC-SSR with fets?

author: Belrmar
author_rank: Regular Contributor
timestamp: June 22, 2018, 02:15:09 pm
content: Quote from: Kjelt on June 22, 2018, 02:02:35 pmfor some aplications mosfets are not ideal as they present somehow high on-resistance when they are designed for high voltage switching, dont provide direct isolation... that from the top of my head, also they can present some design challenges on cooling and controlEdit: also they are not good at blocking reverse current as they would need a series diode (or another comer complex design. yes, the paralel mosfet would need it too, but for small swithcing times might me ok with a small one witch means less cost overall).

author: xavier60
author_rank: Super Contributor
timestamp: June 22, 2018, 10:32:13 pm
content: I added RC snubbers to relay contacts which were switching 1.2A at 28V into hydraulic valve solenoids. The presence of the opening arcing could be noticed by observing the voltage waveform on a DSO.For the relays that I was using I found that the arcing became minimal when the initial voltage rise was limited to about 12V. The proceeding rise time could be fairly fast without causing problems, about 4V/us.I ended up using a 10400mW resistor and 0.33uF MLCC capacitor. This snubber is also fail safe. The resistor would not last long if the capacitor ever shorted out.

author: JS
author_rank: Frequent Contributor
timestamp: June 22, 2018, 10:55:57 pm
content: Quote from: xavier60 on June 22, 2018, 10:32:13 pmReal world experience, that's what was missing from this topic...JS

author: CopperCone
author_rank: Super Contributor
timestamp: June 25, 2018, 04:40:22 pm
content: Im starting to think to do this right you need like an infinite set of relays with varying resistances across them to keep the voltage difference between them small as you go to a full open or close.Anything else you are basically opening a portal to electrical hell.I suspect that in adition to thermal effects you get velocity related effects too i think this was mentioned. What an ugly problem you have here.I think esd guns use various heads to generate different waveforms.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2018, 08:22:37 pm
content: Quote from: CopperCone on June 25, 2018, 04:40:22 pmYou're actually very right!SPICE switches are modeled this way: a resistance that varies, continuously, from low to high resistance (depending on the control input).Not at all like a real switch, but as I covered above, real contacts are amazingly complex, so that can be forgiven!Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 27, 2018, 01:11:04 pm
content: If you want to see a real switch, look at this:(Photonic Induction, beautiful 100+ year old Whipp & Bourne 1600A (20kA surge) switch.

author: 741
author_rank: Frequent Contributor
timestamp: July 12, 2018, 08:14:17 pm
content: Coincidently, I now have a need to know about snubbing DC motor contacts. Is this the similar to the above discussion, but we're switching an inductive load?This is a motor to run from a car battery, about 10A.I can imagine that as the brush loses contact, the intial 'arc' gets suppplemented due to the breaking of the coil current. Quite how the dynamics of the moving rotor make all this differ from a static solenoid I can't put a finger on, but I think it might be less of a kickifthe inductance is dropping due to relative movements.Basically, would I need an RC snubber and maybe add some lossy ferrite choke?What component ratings do I have to be wary of in particular - eg is a 100V ceramic OK?How would I - practically speaking - go about optimising?Up to what kind of frequencies would troublesome EMI exist?

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2018, 08:19:47 pm
content: Quote from: Doctorandus_P on June 27, 2018, 01:11:04 pmPleasedon't remind me of that one - he disabled the very effective magnetic arc blowout system and then beat the shit out of it!It survived 100+ years in perfect condition, only to get vandalized for cheap laughs.

author: nctnico
author_rank: Super Contributor
timestamp: July 12, 2018, 08:32:01 pm
content: Quote from: Miyuki on June 21, 2018, 02:37:20 pmThe best way is to use a MOSFET (or more than one) in parallel but you might need rush-in current limitting resistors. Perhaps even surge rated ones. You can reach very compact solutions this way. I designed such a device a couple of years ago for a small electric verhicle. Spark / arc supression proved to be futile quickly.

author: heltonsouza247
author_rank: Newbie
timestamp: September 10, 2024, 12:19:28 pm
content: Hello. From which book the picture that Mr. 741 upload in the first post came from?

author: 741
author_rank: Frequent Contributor
timestamp: September 10, 2024, 07:39:53 pm
content: HiThe book was given to me by a Matsushita representative.Title "Relay Technology"Published: 1998Publisher: Verlag Moderne Industrie, D-86895 Lansberg/Lechwww.mi-verlag.de("First published in the series 'Die Bibliothek der Technik'Original Title: Relaistechnik)RegardsStephen

author: Kjelt
author_rank: Super Contributor
timestamp: September 10, 2024, 07:56:14 pm
content: Perhaps interesting for someone,Free pdf Relay book from 1986: modern relay technologyhttps://panasonicelectricworksuk.wordpress.com/wp-content/uploads/2013/02/modern-relay-technology_smaller.pdf

author: Zero999
author_rank: Super Contributor
timestamp: September 10, 2024, 09:29:00 pm
content: I don't bother with RC snubbers. If its AC, then I use a MOV or bidirectional TVS. If it's DC, then I go for the classic diode, or zener, if faster off time is important.

author: heltonsouza247
author_rank: Newbie
timestamp: September 11, 2024, 09:35:54 pm
content: Many Thanks I found it in Amazon:https://www.amazon.com/Relaistechnik-Grundlagen-und-neueste-Entwicklungen/dp/3478931886#customerReviews