Resistor type for mains snubber - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: September 22, 2020, 11:06:52 pm
content: Hi,I have been looking up snubbers and the best type of resistor to use. Some application notes like this one say it should be Carbon Compsition type:https://www.cde.com/resources/catalogs/Ap-Guide-Snubber-Caps.pdfWhile plenty of other posts say that there is no need for the resistor to be non-inductive or non-anything-else.The PCB has limited space so using carbon composition would take a lot of space and would rather not use it unless really needed.Also there is a MOV across the snubber too. Perhaps that helps relax the constraints of the resistor?Thank you

author: Manul
author_rank: Super Contributor
timestamp: September 23, 2020, 12:43:16 am
content: I will just comment about resistor. Any thin or thick film resistor will be good. Carbon resistor is somewhat archaic and almost in every way inferior. Any modern resistor will be just fine, they will have negligible inductance in your case, we are not talking hundreds of Mhz or Ghz RF. Your PCB layout will likely have more inductance then a typical resistor.

author: Cerebus
author_rank: Super Contributor
timestamp: September 23, 2020, 02:22:35 am
content: Don't use any old resistor, use one for which you actually have a data sheet and moreover a data sheet that explicitly calls out a "limiting element voltage" or "working voltage" that exceeds your requirements, and it wouldn't hurt to check that you've also got with an acceptable specified voltage coefficient of resistance (the latter is highly unlikely to be a problem with a snubber - for other uses it can be a critical factor). For the record, you won't find an SMD device that's acceptable until you get bigger than 2512, for which package the limiting element voltage is in the 250V region. A 1W carbon film through hole resistor will typically have an operating voltage in the 400V region. Just like you'd use specified X and Y class capacitors across the mains, this is an area where you need a properly specified part, not merely something generic.Be generous in deciding what voltage to set as 'acceptable', as well as allowing for normal peak mains voltage of (sqrt(2) * nominal * tolerance), also take into account whether your load is inductive and can generate voltages in excess of the mains peak voltage. So for the UK I'd personally set a minimum of 1.414 * (250Vrms + 10%) => 390V for a well behaved, well characterised, purely resistive load (I pick 250Vrms because I still regularly see that as an average mains voltage in some parts of the UK - some places don't seem to have got the message about 240V, let alone the nominal 230V -6%/+10% that we theoretically work on nowadays). It never hurts to err on the side of caution and overspecify in safety critical situations. I'd be looking at a minimum 400V working voltage specification and an overload withstand voltage of at least twice that. That's for something going into a domestic socket to control a resistive load. If it's somewhere industrial, or anywhere near mains switchgear like a distribution board I'd be at least doubling everything. Reactive loads need actual calculations before arriving at acceptable minimums.Carbon composition may be 'old fashioned', but they behave very well with gross voltage overload, and that's why many people still recommend and specify them in situations where this might be an advantage.On a final note, if your PCB hasn't got sufficient space for a 1W carbon comp resistor, then it probably doesn't have adequate clearance or creepage distances for mains operation either and a redesign to respect minima for those may be in order.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 23, 2020, 05:58:41 pm
content: Thank you Cerebus for the detailed explanation!The load is "only" light bulbs. I say only because I assume they might want to connect all sorts of types (incandescent, halogen, dimmable LEDs) although the guy I am doing this for said that he can limit the types if that design really needs it.Assuming it is only (or including) incandescent, that would make the load inductive? What additional practical guidelines could you give me about such case?Thank you

author: Cerebus
author_rank: Super Contributor
timestamp: September 23, 2020, 06:54:32 pm
content: Incandescent lamps are almost pure resistive loads, quite non-linear, but purely resistive. No inductive characteristics in the region of interest at all.Modern LED and CFL lamps are completely unpredictable loads because they contain active circuitry. They might look resistive, inductive or capacitive, or anything inbetween.Anybody actually measured any of the above and have any insights to offer?

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2020, 02:38:08 am
content: Carbon composition resistors are suitable because they have high peak voltage and power capability, and they tend to fail open.Modern ceramic composition resistors are suitable as replacements for carbon composition resistors. Some film resistors are "fusible" and also suitable. I suspect metal oxide resistors would also be a good option.

author: srb1954
author_rank: Super Contributor
timestamp: September 24, 2020, 06:20:33 am
content: Quote from: Manul on September 23, 2020, 12:43:16 amA thin film resistor would not be a good choice here.In this application there is a very high peak power dissipation if you connect to the mains supply at the peak of the waveform and C16 is initially discharged. In this case the peak resistor power could approach 500W and a thin film resistor might be damaged by such a high peak power. The peak power is even higher if fast high-voltage transients appear on the mains supply: the bulk of the transient voltage will appear across the resistor as the capacitor will have a low impedance to the fast transient. Under these conditions the MOV helps limit the peak voltage applied to the resistor but you are still looking at several kW of peak power dissipation in the resistor.Generally the peak power capability for a given physical size resistor can be graded as: wirewound > composition (carbon or ceramic) > thick film > thin film.If your application can't tolerate the inductance of a wirewound resistor then a carbon composition resistor is often the best choice in pulse applications. A ceramic composition is a good substitute for a carbon composition if you can find them and don't mind paying a higher price.Unfortunately, these days not many resistor manufacturers publish comprehensive datasheet details of a resistor's peak power capability so it is hard to determine the suitability of a particular resistor for surge conditions.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 27, 2020, 09:04:04 pm
content: Thank you all!!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2020, 09:51:36 pm
content: Why the MOS SSR? It's using about 12 more components than a TRIAC (or either kind fully integrated) would use.Also beware of the leakage path: the rectifier diode into one of the MOSFET body diodes. Might not matter for the intended load, but could lead to surprises.The bias network can at least be greatly simplified, using a charge pump (half wave voltage doubler, with the series capacitor being rather small) and just the one zener, on the gate, to limit voltage. The opto is wired in shorting mode either way (i.e., gate voltage is default on, and shunted off when the opto is energized). Very little actual current draw should be needed here, assuming turn-on time doesn't need to be anything impressive (under 1ms I suppose would be "fast").Also, the fuse won't do anything, and fusible resistors can be specified, solving that for double.Oh also also, put a gate resistor or ferrite bead in there; the capacitance of the zener diode, or of the pair of MOSFETs, is likely to make an oscillator during commutation. I'm not sure if Ye Olde IRFP460 is hot enough to do that, but I guarantee a more modern SuperJunction type will.IRFP460 is only rated 500V; the MOV has a breakdown of at least 390VDC. A typical 275VAC MOV has breakdown Vv = 430V, and maximum clamping voltage Vc = 600V at 1A for example. I would expect those MOSFETs to avalanche, and eventually fail shorted. A good rule of thumb is to use semiconductors of 2-3 times the MOV voltage rating, i.e. 900V would be fine here. Which, transistors that size aren't super expensive or anything, it would be an option.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2020, 01:56:50 pm
content: Wing getting into detailed advanced topics like this, a good starting reference is The Art of Electronics and the companion x-chaptershttps://x.artofelectronics.net/the-book/table-of-contents/Be prepared to lose several weeks of your life, profitablyYou should read the resistor manufacturers application notes (and data sheets) paying careful attention to what one specifies but the other does not.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 29, 2020, 03:51:42 pm
content: Thank you Tim and tggzzzTim, as usual thank you for the detailed explanation and suggestions, much appreciated and very useful info!! I implemented the mosfet related suggestions. I placed a 4R7 gate res and I can tune it in the proto. And changed the MOSFET to a IXFH22N60P3.Could you please explain your charge pump solution? It seems very interesting because the space is very limited. But I don't understand the configuration you suggest.BTW, the reason for using mosfets is that the guy asked me to copy a ref design he came across, built and tested and is happy with it.Ref the fuse, are you referring to the schematic in my OP or the modified one in more recent replies (also attached here)? I rewired it as in the attached sch to protect the load.Thank you again

author: Cerebus
author_rank: Super Contributor
timestamp: September 29, 2020, 04:14:21 pm
content: Quote from: tggzzz on September 29, 2020, 01:56:50 pmAnother useful book is "The Circuit Designer's Companion" by Tim Williams (ISBN 0 7506 6370 7) which covers a lot of the practicalities and oddities that other electronics texts don't, such as types of resistors and their characteristics and much more besides. Ought to be on everybody's bookshelf when they're starting out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2020, 05:24:45 pm
content: Quote from: Cerebus on September 29, 2020, 04:14:21 pmWhich, no relation in case you were wondering...(Nor to the nearby (in Hamming distance) Jim Williams.)Which I've not actually read, but I hear good things about it so I'm not going to disagree.Quote from: ricko_uk on September 29, 2020, 03:51:42 pmIt would be like, well, let me see, I think I can actually reason it out by substitution. Bear with me:Change:- R11 to a small value capacitor (say 10nF >=250V Y1), and R12 to a lower value, say 1k (or, it's fine where it is, really; anywhere will do)- Move D4 to between D5 and C14- Remove C15 and R10- Reduce C14 to 0.1uF say- Remove R13, R16, D6- Add another R?, so there's one for each gate (so the gates don't try to fight each other). Any value from 4.7 to 1k should be fine.So, there's a capacitor bleeding charge from LIVE to a pair of diodes, one which shunts from "GND" to the cap, and the other from the cap to a larger bulk cap (C14 || MOS gates). This delivers pulsating DC, which is clamped by D5 when the opto is off, or clamped to ~0V when the opto is on.So, to make that clear, the opto is wired across C14 and the gates, and acts to short out the supply. There's resistance between C14 and gates just to keep them from oscillating. There are two diodes, wired in series from GND to cap to C14, the bottom one being a zener which clamps the maximum voltage.Also note that LIVE is always, well, live; take precautions. This might not pass as a home light switch (read up on The Code, I don't know offhand), but it's fine within equipment.QuoteThe OP of this thread. Yes, that's good.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2020, 11:35:19 pm
content: Quote from: Cerebus on September 29, 2020, 04:14:21 pmAgreed.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 30, 2020, 01:06:01 am
content: The snubber components should be properly designed, one way or another.The design is based on a damped R-L-C circuit.In the circuit above, L will be in the load.The solutions (or simulations) will be optimised to give the peak current in the snubber,to meet the needed max dv/dt on the switch devices and the voltage overshoot (safe repetitive V peak).Also the switch-on di/dt has to be safe, which is related to gate pulse rise time etc.The use of a mov across that snubber is a bit unusual in my experience.The "average" power in the resistor will come from the power per pulse * 2 * frequency (simple snubber).If the capacitor has fusible links ( "weak points" to meet safety standards) , those may cause early failure in snubber duty.I can mention that there are other considerations in high power, high frequency water cooled snubbers, not expected at 50 Hz.One is the inductances of the R and C. Another is the "skin effect" in the conductors forming the non inductive resistor.The resistor can be de-rated to offset the skin effect.Failed resistors and capacitors cut open after years of service can show ablation of the conductors.I don't intend the above to be too depressing, 50/60Hz snubbers are of course, very reliable if properly done.

author: TimNJ
author_rank: Super Contributor
timestamp: September 30, 2020, 04:38:45 am
content: Quote from: T3sl4co1l on September 29, 2020, 05:24:45 pmAre you advocating for fusing the gate control circuit only? Am I interpreting you correctly?Isn’t it good practice to fuse an SSR’s load path?

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 30, 2020, 01:43:08 pm
content: Is this controller the chopper type where the fets turn on then off in their half cycle?Or is it the old type ( like triac) where the fet commutates off at zero crossing?If the former then a snubber is needed for switch-off overvoltage control.If the latter, then I think the snubber is not necessary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2020, 02:41:51 pm
content: Quote from: TimNJ on September 30, 2020, 04:38:45 amMy initial comment was in regards to the OP, and the newer version is good.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: September 30, 2020, 02:54:49 pm
content: Tim, I think you're sewing confusion by saying 'OP'. Some people seem to be reading that as Original Post, and some as Original Poster.I have to admit that I'm beginning to lose the thread now, but that may be down to me having recently been trying to pickupthe thread in another topic where one poster seems to have lost the whole plot. Dazed and.

author: Renate
author_rank: Super Contributor
timestamp: September 30, 2020, 03:03:58 pm
content: I'd use a rubber snubber.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 30, 2020, 10:32:50 pm
content: Thank you all...!!A bit confused about the snubber/MOV after all the above feedback.Which is the safest option? To keep the snubber OR the MOV OR both?Tim,I modified the left side of the schematic following your description. See attached screenshot. Not sure I implemented your description correctly so few questions (also inserted in the schematic):1) is that the correct implementation you meant?2) Doesn't it need a resistor to ensure mosfets are kept off?3) Doesn't it need an antiparallel diode in series to D5 to protect when forward biased?4) Can I invert R12 and C42? Or are there some safety issues in doing so?5) What power rating for R12?Thank you again

author: Cerebus
author_rank: Super Contributor
timestamp: October 01, 2020, 12:20:38 am
content: Quote from: ricko_uk on September 30, 2020, 10:32:50 pmThe job of a snubber is to control the transients you expect to see all the time (every cycle usually). The job of a MOV is to handle infrequent overloads and clamp the voltage to an acceptable level.MOVs wear out, they take a little bit of damage every time they fire, it may be miniscule or it may be significant depending on the size of the overload. The datasheets for MOVs will give a rating for the life of the MOV in terms of number of overloads tolerated as peak overload current versus impulse duration, like below.So, at the extremes, one of the 14mm Littelfuse MOVs that this chart is for will take a single 6 kA amp* 20 us overload (120 mA-seconds), or a million 50 A * 20 us overloads (1 mA-seconds). Keep the peak current below the bottom line (~ 600uA-seconds, or if you prefer 600 microCoulomb) and the device will last indefinitely. The damage, by the way, is roughly related to the heat generated by the overload, and the ability of the device to dissipate it before it can do any damage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2020, 02:05:54 am
content: Quote from: ricko_uk on September 30, 2020, 10:32:50 pmClose. Short out between the pins of the opto (where it is now), and move the opto to parallel with C14.QuoteNo! In fact it's being used intentionally for forward conduction. While LIVE is dropping, D5 charges C42. While LIVE is rising, C42's charge is pushed ("pumped", you might say) into C14 via D4.Which... let me see, Q2 body diode allows forward charging, but blocks it in reverse. So "LOCAL GND" can get negative bias on it this way. Which means Q2's Coss acts in series with C42, significantly reducing charging current. Which also puts a small amount of negative voltage on the "LIGHT OUT" terminal (via leakage).So, maybe this would need a bit of capacitance in parallel with Q2 to charge at the expected rate. Otherwise, one of those things I should breadboard before recommending...QuoteEither way is fine. R12 should be fusible (just in case) and rated for peak voltage. Which isn't hard, pretty much any metal/oxide film resistor will do I think, 1/2W or higher.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: October 02, 2020, 02:55:46 pm
content: Thank you Cerebus and Tim!Tim,I am not sure I understand the "Short out between the pins of the opto (where it is now), and move the opto to parallel with C14."Do you mean as shown in the new screenshot attached? If not, I also attached the Altium file in case it makes it easier for you to mod that (I had to attach it as ZIP because the forum does not allow me to upload SchDoc files).Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2020, 03:22:22 pm
content: Move the gate supply to the top of C14 and you're done.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: October 02, 2020, 10:33:04 pm
content: Thank you Tim,like this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2020, 11:20:56 pm
content: Yes.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: October 03, 2020, 08:57:44 pm
content: Thank you for all the suggestion Tim!

author: ricko_uk
author_rank: Super Contributor
timestamp: November 19, 2020, 04:08:45 am
content: Hi,with ref to the snubber (schematic in previous post) the capacitor I came up with for the snubber is this:https://api.kemet.com/component-edge/download/specsheet/R46KI310000M1K.pdfBut it is too tall for the enclosure at 11mm. The maximum available space is for 10mm and preferably less.Few questions:1) Does the snubber capacitor need to be X2?2) what happens if I decrease the snubber's capacitor value? And do I have to increase the resistor value?Thank you