Snubber Circuit capacitor question - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: March 06, 2023, 03:09:50 pm
content: In the circuit below the R1-C1 snubber is to prevent false triggering and protection of U2, the relay coil current is only 20maI don't believe C1 needs to be a X-type safety capacitor (it's not across the line and if it were to fail K1 would simply pickup)Agree / Disagree?If the current was higher a PP or PE capacitor would be suitable, however because of the low current and my preference to use a SMT capacitor, I'm considering this ceramic capacitor:https://www.digikey.ca/en/products/detail/w%C3%BCrth-elektronik/885342208021/9345890?s=N4IgTCBcDaIBxwKwGYAsYwAY6bARhAF0BfIADoes anyone see any problems with this snubber?

author: Terry Bites
author_rank: Super Contributor
timestamp: March 06, 2023, 08:50:33 pm
content: Buy a readymade snubber.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2023, 09:19:44 pm
content: The capacitor's voltage is limited by the breakdown voltage of U2; no approvals necessary, as far as I know. Ceramic or film are available and will do fine.If you need less leakage current, consider a bypass between lines (H-N) (whichwillhave to be 'X' rated) then snubber across the coil. Note this serves to limit turn-off transients, but does not filter mains surge transients (which could still trigger the thyristor). Mains dV/dt could be limited in part by a line filter, but maybe that's getting to be too much bother for just a relay driver.Also note that MOCxxx aren't rated for continuous loads, IIRC; check the datasheet. You may want to use a TRIAC (even if a small one) with it, or an SSR (which is basically the same thing already packaged).Tim

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2023, 09:34:50 pm
content: An X cap is only strictly needed for a capacitor that is directly across mains L-N. You should rate R1 for maximum dissipation or use a fusible resistor to protect against the capacitor failing short circuit (the relay coil will helpfully limit the current in this example).

author: Jester
author_rank: Frequent Contributor
timestamp: March 06, 2023, 10:14:16 pm
content: Quote from: Terry Bites on March 06, 2023, 08:50:33 pmFor example?

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2023, 10:17:38 pm
content: Quote from: Jester on March 06, 2023, 10:14:16 pmAnd how should this off-the-shelf snubber be specified?

author: jonpaul
author_rank: Super Contributor
timestamp: March 07, 2023, 12:24:57 am
content: R1 C1 IS the snubber.Nothing more needed for a simple triacJ

author: Jester
author_rank: Frequent Contributor
timestamp: March 07, 2023, 12:26:01 am
content: Quote from: T3sl4co1l on March 06, 2023, 09:19:44 pmTim,Thanks for your thought provoking response. Can you please expand on the following:a)If you need less leakage currentare you referring to avoiding latch up associated with leakage current and high temperatures ie. T > Tj max?b) I actually have a "X" rated capacitor in this circuit (not shown), it's used as a capacitive divider to generate the non-isolated power supply for the MOC drive etc, directly from the MAINS. I suppose this would actually help mitigate line transients, even though that's not its primary purpose.c)The capacitor's voltage is limited by the breakdown voltage of U2, Vdrm for the MOC3063 is 600V, so I interpret your comment as if the voltage exceeds 600V the Triac will start conducting so that is about the maximum voltage the snubber capacitor is expected to see, hence my choice of a 1kV capacitor, so I think that should be enough margin (voltage wise)?Hopefully the combination of the "X" capacitor in the power supply, the TVS (also not shown) as well as the R-C snubber will be enough to prevent exceeding Vdrm breakdown.d)Also note that MOCxxx aren't rated for continuous loads, IIRC; check the datasheet., I don't see this in the datasheet

author: floobydust
author_rank: Super Contributor
timestamp: March 07, 2023, 03:34:29 am
content: Is this project commercial (approvals matter) or hobby?You posted the relay coil is 3k ohms and 8H, is this right? Coil inductance is much higher with the armature in.Is it acceptable for the snubber capacitor to fail short, relay stuck on?I don't see this snubber doing much of anything with the small capacitor. I know you want it cheap and tiny but the coil inductance is large and makes quite a voltage spike.Common failure is the relay coil having shorted turns, so coil current goes high and causes things to burn up. You do have a suitable fuse, right?Note there are special high-duty cycle relays required if the relay coil (mains) is on all/most of the time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2023, 04:40:22 am
content: Quote from: Jester on March 07, 2023, 12:26:01 amLeakage to the load, to be more specific. The snubber has some impedance at mains frequency. For a relay, this probably doesn't matter (the coil impedance is low in comparison). It will for example cause LED lights to dimly glow, or flash.QuoteBut it's in series with a resistor (it is, right..?). You would also need a TVS on said supply, to handle inrush and mains surge (see IEC 61000-4-5).QuoteBingo. I'd be fine with 630VDC (250VAC) or higher. It should have an AC rating -- capacitors don't automatically handle AC as well as they do DC, so having the rating is prudent. Plenty do, so it's hardly a problem.QuoteNah, you will get breakdown from time to time -- mains surge is quite low impedance, it won't be stopped by a couple tiny caps.More important is to be able to ride through such events. The surge will cause breakdown, hopefully causing only temporary turn-on, and the coil only sees (up to) one half-cycle of voltage. In ~8ms, it probably doesn't even pull in!TRIACs can fail in ways other than overheating (excess current), and I'm not real clear on the mechanisms, but overvoltage is one of them. (Perhaps avalanche leading to uneven and rapid turn-on, violating dV/dt or dI/dt?) It may be that it eventually fails from such service; the only prevention would be a MOV, preferably at the inlet (to directly shunt the surge -- also makes things easier on your C-dropper supply). A clamping voltage of 800V peak is easy enough to get (while handling up to 250VAC input; better still if 120V only*), which wouldn't be the happiest for the MOC3063, but 800V TRIACs are in ready supply.*Which, I mean, you'd have to change the relay coil to run anything else, so...The surge also rises in a couple microseconds, so can easily cause turn-on by dV/dt.There's also EFT and ESD (same standard, sub-parts 4 and 2), with pulses in the nanosecond range, but these are however easy enough to filter.QuoteSeehttps://www.onsemi.com/pdf/datasheet/moc3163m-d.pdffor example.Absolute maximum ratings:I_TSM: peak non-repetitive surge current. One-time rating (whether that's one timeever, or once every "until the device returns to Tj = 25C", is less clear), not applicable.I_TM: peak repetitive current. Presumably short duration, but repeating say twice every cycle.I_T(RMS): not found.Page 6 note: "This optoisolator should not be used to drive a load directly. It is intended to be a trigger device only."Toshiba may rate theirs for continuous:https://toshiba.semicon-storage.com/info/application_note_en_20210607_AKX00767.pdf?did=63136Their symbol is I_TP instead of I_TM, but an RMS value may also be given.The nearest equivalent is probablyhttps://toshiba.semicon-storage.com/info/TLP3062A_datasheet_en_20191118.pdf?did=30346&prodName=TLP3062Aor thereabouts, which does give 100/50mA RMS (derated with temperature). Similarly for the compact TLP265J (at slightly less current), and probably others too. (Only one part is showing very much stock at DK though, so I would likely shy away from Toshiba for a design-in.)Most of the info in the appnote is applicable to other manufacturers, and the symbols are probably from an international standard (that they never tell you about because only semi mfgs need to know about them; JEDEC, JESD, etc. come to mind), give or take differences like I_TM/I_TP seen above. Such standards probably also provide test conditions (which as you can see, some leave out; shame on onsemi for not specifying pulse width/waveform!).Whether we can apply Toshiba's 50mA+ RMS rating to the onsemi part, is another matter. Strictly speaking, no. You can likely get away with it -- but for how long is the question.Alternately, if onsemi has additional supporting information (an appnote for the part, or ask an FAE directly), perhaps they do in fact permit continuous load current. Again, for design-in purposes, I would avoid it. A suitable TRIAC is trivial anyway (maybe a cute lil' SOT-89 one?), or use a full proper SSR....Which may be one reason they prefer not to rate components that way: if they put an RMS rating on them, they can be sold as board-level SSRs instead of TRIAC drivers. You might shop under that category and see what shows up. It's entirely possible that a small enough AC SSR is just the bare photodevice, and just isn't rated exactly as one (but has enough parameters in common that you can guess what it really is). Whether that gets you better specs or cost or availability, I don't know, you'd have to take a look.Tim

author: floobydust
author_rank: Super Contributor
timestamp: March 07, 2023, 12:43:45 pm
content: It is confusing. Datasheet:"Recommended for 115/240 Vac(rms) Applications:• Solenoid/Valve Controls • Static ac Power Switch• Lamp Ballasts • Solid State Relays• Interfacing Microprocessors to 115 Vac Peripherals• Incandescent Lamp Dimmers• Motor ControlsThen a bombshell: "* This optoisolator should not be used to drive a load directly. It is intended to be a trigger device only."Toshiba Triac couplers—basic properties and applicationApp noteandVishay App Note 34says it's all fine. Could not find Mot/Fairchild AN-780A.I did a bit of research and find:2.2 Phototriac limitations STControlling a Triac with a phototriac AN5114"As specified in many phototriac datasheets, the opto-isolator should not be used to drive a load directly. It is intended to be a trigger device only. Phototriacs are very rarely used as AC switches. There are several reasons why phototriacs are not intended for standalone service.Some are obvious and can be directly deduced from the datasheet. For example, maximum power dissipation specified is only few hundreds of mW at 25 °C while peak surge current withstanding is around 1 A.Commutating dI/dt, also called (dI/dt)c, is a dynamic parameter somewhat difficult to apprehend, and represents an important phototriac weakness that makes this device not suitable to drive a load directly.This parameter corresponds to the phototriac/Triac behaviour at turn-off. If the slope of the decreasing current is too high and/or if the slope of the reapplied voltage is too high, the device may trigger and the conduction may continue after the zero current crossing point. This non-destructive event is troublesome since the load becomes unmanageable. To get a fair idea of phototriac (dI/dt)c capability, four references of phototriac are measured.Results are collected in Figure 2"

author: Jester
author_rank: Frequent Contributor
timestamp: March 07, 2023, 02:20:02 pm
content: Quote from: T3sl4co1l on March 07, 2023, 04:40:22 amUnderstood, this application is powering a small motor, so okay on that front.QuoteYes, small series resistor, and yes I have a 130Vac TVS, as this is a 120V only circuit.More important is to be able to ride through such events. The surge will cause breakdown, hopefully causing only temporary turn-on, and the coil only sees (up to) one half-cycle of voltage. In ~8ms, it probably doesn't even pull in!I can live with that.QuoteSeehttps://www.onsemi.com/pdf/datasheet/moc3163m-d.pdffor example.Interesting this is the part I was consideringhttps://www.digikey.ca/en/products/detail/liteon/MOC3063S/3199531Same 1A peak current, and they do specify On-State RMS Current ID(RMS) at 100mA and for this circuit the relay only requires 20mA .My assumption (never a good thing), is that many TRIACs require gate current of that magnitude but to your point they don't really specify continuous current but they imply it with On-State RMS Current ID(RMS), so this part looks more like the Toshiba part you reference.QuoteAnother good point, and interestingly the AQH series of Triac SSR do have a specified continuous RMS rating and use almost the same footprint (two additional unnecessary pins that create creepage issues on my small board)https://www.digikey.ca/en/products/detail/panasonic-electric-works/AQH0213AX/4757797?s=N4IgTCBcDaIIIEUASAGMBGAzHAGiAugL5AAnother possibly better option would be to use a FET based SSR, like the TLP170https://www.digikey.ca/en/products/detail/toshiba-semiconductor-and-storage/TLP170J-F/3056586Three obvious advantages:1) Smaller package so no creepage issues on my small board2) Less drive current only 1mA required3) Turn off time is a bit quickerDownside?Thoughts?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2023, 06:29:04 pm
content: Quote from: Jester on March 07, 2023, 02:20:02 pmThe relay is? Why not SSR that directly then? I would've assumed a 120V relay coil being like a contactor, say 240V 30A at least...QuoteHmm. You might want to post the whole circuit (or at least the stuff touching mains). TVS have to be...rather large to withstand mains surge. MOVs are many times cheaper. (The downside is, MOVs generally experience wear under such conditions; when you need extremely high reliability (decades in service, or very high frequency of surges (mountaintop facility?)), the TVS is worth.)QuoteRight, it should be fine when on, at least. Turning off is hard to say, though. They don't give (dI/dt)c, or t_q or anything. Turn-off as a driver is irrelevant, because the main TRIAC shunts the driver to a couple volts as soon as it pulls in, and it's given tons of time (~ms) like that to return to an open state.They do hint that it should be used as a driver, by putting in only one suggested use-case circuit -- but that's only a hint at best.QuoteThere's also,https://www.digikey.ca/en/products/detail/ixys-integrated-circuits-division/CPC1972GSTR/1277128and maybe,https://na.industrial.panasonic.com/products/relays-contactors/lineup/semiconductor-relays/series/12439/model/12464Panasonic's database/documentation is weird sometimes... The product page here says 1.5kV isolation. The datasheet says 5kV. But the datasheet doesn't enumerate a non-H type part. So where's the AQV214EAX?! Anyway, if you only need 1.5kV, that's still fine.I feel like a little higher current/power rating (or lower resistance) than the TLP170J would be nice, but it will do. Turn-off is actually slower, in that the internal gate capacitance has to discharge over some ~ms, whereas the TRIAC can turn off in 10s us (at least, regular ones; not sure about the photo- kind). This is still short enough not to burn a huge amount of power, and the soft on/off reduces EMI. You lose zero-crossing, but it kind of wasn't right in the first place (the coil should actually be switched on a peak, I suppose, but it also has enough resistance not to care), and it won't make any difference for slow on-off applications.Mind you probably need 2.5kV isolation between mains and SELV, if SELV is ungrounded. If it's not touchable by fingers, it probably doesn't matter (is it triggered by the C-dropper circuit?)....Which, if the latter is true, you can just use a TRIAC straightaway, use a half-wave doubler circuit with a too-small series capacitor at the input (same as the current C-dropper) and run your control circuit common-"ground" on the neutral side, then put the load between hot and TRIAC. Drive TRIAC like it's any bipolar transistor -- this will trigger in quadrants I/II, which is almost always as good as the I/III a photoTRIAC gives. (Or is it quadrant IV, not II? I never remember which way is which. IV is generally poorer, but just find a device rated for it, and drive it with a little extra current.)Speaking of EMI, you probably want a snubber across the relay contacts, too. Having that micro-EMP bomb go off nearby some control circuit may not be desirable -- mechanical contacts can generate waveforms in the sub-ns region, they're disturbingly nonlinear(!!!). A snubber gives some time for the contacts to open while load inductance discharges, reducing or eliminating arcing, which can otherwise be a machine-gun burst of fast pulses (modeled as EFT, see IEC 61000-4-4).Tim

author: Jester
author_rank: Frequent Contributor
timestamp: March 07, 2023, 07:59:34 pm
content: Might help if I explain what this is. I have power tool with a 120V, single phase 1/2HP motor (horizontal bandsaw). From time time to time the blade jams and bad things happen quickly. Nominal current is about 8A, start-up current is about 30A, and locked rotor current should be at least 30A. Circuit breaker is too slow for anything other than protecting the motor. I need to trip quickly in locked rotor condition.QuoteThe relay ishttps://www.digikey.ca/en/products/detail/cit-relay-and-switch/J115F11A120VACS6/12502987?s=N4IgTCBcDaIFIEYEFYBiSCCCwAYBqGAwgMoBsIAugL5AQuoteNo SELV in this circuit, just a manual start and stop button rated for 250VacThese locked rotor events occur rarely, but the gearbox design does not tolerate these well and the replacement gears are $100 a pop plus the down time to repair. I think if I can trip in a cycle or two it will save me a lot of trouble.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2023, 09:11:38 pm
content: Hmmm check your quoting there.Ah neat, fast(er) circuit breaker. Mind that startup looks the same as locked-rotor so you may need a startup ignore process, or some kind of soft-start circuit (in addition to this), or a relatively high threshold. You may need a lower-gain device than ACS712.And the relay will be opening under load (depending on how close to zero the contacts actually open at -- assume random) and it's an inductive load so it'll be hot and ready to arc. Snubber definitely a good idea, or even a small MOV (across the relay contact).An SSR could handle that, but it won't break during a cycle, as you're well aware. I suppose the relay has just that little edge -- but it does still take ~10ms for the contacts to open. You trade one half-cycle in conduction for a bit over one half-cycle of delay. It's kinda whatever, I suppose. That said, the TRIAC driver would still incur that trailing half-cycle -- putting an advantage to the photoMOS SSR + relay.Advantage to an SSR (instead of the relay) is, if not a zero-crossing type, you could phase control it. Maaaybe not a good idea (doesn't work well with induction motors, and doesn't work at all with single-phase + start winding type induction motors!), but it would be one way to limit current, anyway. I'm... not quite sure of the algorithm you'd use there, actually, since you only measure current when it's on, but then you're stuck on for the rest of the cycle; you'd have to use some sort of limit-approaching-then-shy-away method (compare to anti-knock methods in car engines, optimal* timing is at the onset of knock).*For some optimization anyway, power I suppose. Not necessarily like emissions or efficiency.Downside to the SSR, it needs a modest heatsink (~20W continuous, I suppose?). The relay is probably cheaper in terms of overall build -- no heatsink and hardware needed.So, I don't disagree with your overall design.As for surge, the two resistors (R13, R9) must be pulse rated. Note that the fuse can be saved by using a fusible type resistor for R13.Assume 1.5kV mains surge: R13 limits the current to ~36A. Most of this current flows into Z2. Probably an SMDJ180CA would be adequate: it's rated for 377V, 51A at 8/20us surge waveform (the combined-wave 8/20 / 1.5/50us generator will be closer to 1.5/50 in this high-resistance condition, but that's still in the ballpark). C15 is large enough compared to the series resistors that it doesn't have much effect on the surge (R*C ~ 87us) so we can assume the resistors and TVS/zener are doing the work here.A peak voltage around 300-350V on Z2 drops across R9 into Z1, so Z1 also needs a high rating. Probably a 3 or 5W zener would do. (Zeners aren't usually rated for surge conditions; conversely, TVS aren't usually rated for voltage regulation purposes, and have wide tolerances on Vz. There's not really a good solution here, but AFAIK, this type of TVS is just a large zener, and a large enough zener (per se) is likely identical to a tighter-selected TVS.) The 5v1 rail may be subject to high peak voltages in this condition (8, 10, 12V?) so I might avoid running logic from that supply, but the 3v3 should be fine (AP2205 has a 24V input range so it's fine).Also, if we're talking say a ~50us pulse at 300V into R9 into C10, that's about 6A peak and a 1.5V rise; C10 by itself -- if its ESR is low enough (< 0.2 ohm say?), will do a pretty good job already, even without the zener providing extra clamping.So the zener + large cap should do well enough. Still depends a bit on what's powered by 5v1, but most logic will tolerate that alright. (Like, 74HC family is rated 7V abs. max., but it'll break down at more like 8-10V. So it can tolerate such conditions without blowing up; it won't exactly be happy, but it can survive. Parts rated 6, 6.5V max. I'd be a bit leery of.)Hmm, suspicious of the AP2205. It's probably fine, but it's missing these points that an LDO (especially PNP type) should mention:- Stability: only one mention ("compatible with low ESR ceramic capacitor"), no range given, no plot of capacitance/resistance/load.- Ground pin current: no plot of Ignd vs. Vin. Normally Ignd goes way up in dropout; good ones limit this wasted current, bad ones can dump like 50mA. The "low ground current" is highly touted, but they only specify it at high drop (Vin = Vout + 1V). No idea what it does overall.CMOS LDOs are usually free of the Ignd problem (for obvious(?) reasons), but stability is still a concern.Going from 5V to 3.3 isn't a big deal; a medium-dropout regulator like LM1117 (which I wouldn't recommend per se, it's more current than you need here, and older/crustier enough that I would shop for a newer type) would be good enough....Hmm, maybe even 1W zener is fine. Consider:https://www.onsemi.com/download/data-sheet/pdf/1n4736at-d.pdf1N4733A says 890mA peak reverse for 8.3ms square pulse. If we assume Ipk ~ 1/sqrt(t) (a typical thermal diffusion response, but depends wildly on device construction!), that's 8.9A for 83us. It's in the right ballpark, but close enough that these assumptions might be wrong.In contrast, here's one that does have pulse ratings,https://docs.rs-online.com/21f5/0900766b80de2a4e.pdfthough the series doesn't include 5V1 and it's definite overkill (the 6V2 is 9.3V at 34A, 10/1000us surge -- beefy!). 1W zener might well do the job, it's just hard to say for certain due to lack of data -- even this one doesn't provide transient thermal curves, or a peak power curve below 100us.Anyway, SMDJ180CA should be fine for Z2, Z1 should be a fairly beefy zener, and the resistors should be pulse rated, probably something metal-film, metal-oxide or wirewound; and at least the one, flameproof / fusible. The rest should be okay. 3v3 reg suspicious but probably fine. PhotoMOS SSR probably better than TRIAC. Probably need lower gain current sensor. Snubber on relay contacts. And that's about it!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2023, 09:25:42 pm
content: Speaking of MOV, just to check -- the ~cheapest SMT, B72580T0131K072 would clamp around 400V, notthat much worsethan the SMDJ, and would be good for ~hundreds of surges (which is probably fine, at least a few years, maybe over a decade lifetime in average conditions). The cheapest of any type is a 7mm disc, such as MOV-07D201K, which will probably last a long time, oh what the heck Bourns doesn't publish derating curves; the next in line seems to be Yageo 201KD14 (14mm disc, so cheap!) which will last basically forever, and could be used for both Z2 and the relay contacts.TVS are generally sharper/stiffer than MOVs, but this goes to show you just how aggressive mains surge is -- the impedance is low and the voltage is high, it takes a big TVS to handle that kind of power. Even with the 39R in series, it's really pushing a fairly large (SMDJ, "3kW" class) TVS, but the MOV doesn't even flinch -- while offering similar clamping voltages. TVS excel at low voltages of course (where MOVs simply stink), or when you can afford to use big beefy ones (15kW+ class).Tim

author: Jester
author_rank: Frequent Contributor
timestamp: March 08, 2023, 12:18:12 am
content: Tim,When I started this thread I was actually hoping you would chime in, I appreciate the time you have taken to share your expertise. The 5V1 node is only used by the 3v3 regulator so no worries there. I'm going to tidy things up and order the board. I'm leaning towards the PhotoMOS SSR to drive the relay, what would you recommend for protection of that device; just a snubber or TVS or some combination?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 08, 2023, 01:22:54 am
content: Quote from: Jester on March 08, 2023, 12:18:12 amSure, a MOV there again, I suppose. Can replace the snubber with it. (The MOS will have some avalanche capability, but not very much. Maybe it survives in series with the coil anyway, maybe not.) Could also put a MOV across the inlet (probably a 14 or 20mm disc would do this best).Tim

author: Jester
author_rank: Frequent Contributor
timestamp: March 18, 2023, 12:08:39 pm
content: UPDATE......Circuit board arrived yesterday, so a little soldering and a page of code and it appears to work like a charm.