MOV Questions - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: January 07, 2021, 12:12:43 am
content: I'm planning to design a power supply and use MOVs on the inputs (maybe the outputs too - surface mount).I'm looking at the MOVs on Littelfuse and see many normal (physical) sized are rated for <200 Wtm. I realized a power strip I own is rated at 1000 or 1200J, so I disassembled it to read the MOV part number(s) to see what was used and maybe incorporate them into my design.Unfortunately I can't find the datasheet (see attached MOVs pictures) to find their value. My questions are: why are six MOVs used? My understanding is that only three are needed: one from hot to Earth ground, neutral to Earth ground, and hot to neutral.Why are the same (or similar) sized MOVs used in this power strip rated for much higher than the MOVs on Littelfuse? I'm guessing there is some fabrication that the power strip company uses.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 07, 2021, 12:59:02 am
content: How can you say those MOV's are 'rated for much higher' than Littlefuse MOVs when you haven't identified a part datasheet - is the rating you refer to (1000 or 1200J) from some simple device spec or marketing blurb (as it doesn't identify the test conditions for a J rating)?Multiple MOV's could be used instead of one MOV to increase a 'rating'. Two MOV's in parallel may not degrade evenly when subjected to current or power limited pulses (ie. within each part's capability), depending on the size of individual pulses, and how the degradation from one pulse then modifies the sharing of the next pulse. Note that the fusing is not a simple design process, and may have a number of design requirements that may not be easy to identify from just inspection.Are you planning to design for your own DIY use and local conditions?

author: bostonman
author_rank: Super Contributor
timestamp: January 07, 2021, 01:33:55 am
content: QuoteThe power strip states it's rated for 1000J.I plan to design this for personal use, and, since MOVs are cheap (or a sample order for free), I figure, why not make the power supply a bit more robust.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 07, 2021, 02:21:46 am
content: "Robust" in that it could trip your upstream breaker or fuse if a MOV fails short for long enough?

author: bostonman
author_rank: Super Contributor
timestamp: January 07, 2021, 02:25:25 am
content: Robust enough to pull any lightening strikes away from the power supply; and of course trip the fuse if they short.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 07, 2021, 03:23:50 am
content: Yes it is a can of worms when some protection widget's aim is to restrict the voltage rise between conductors, and divert current driven by the voltage rise, and whether that alleviates collateral damage to some powered equipment, or re-directs current in to the feed conductors because all of a sudden that is now the lowest impedance path in the building, and the feed impedance than raises the risk of EPR safety.

author: arcfour
author_rank: Newbie
timestamp: January 07, 2021, 05:02:23 am
content: MOVs are a bit...er...controversial these days because of the fire risk they can pose. Just a few weeks ago,IEC 62368-1 replaced the old IEC 60950-1 standardwhich tightened the safety requirements for MOV surge protection - it seems now that it's a requirement to use a thermal fuse with the MOV (great idea regardless!)Is this for personal use? If so, why not cough up the extra cheddar for a beefy TVS? Or you could use a mov+thermal fuse and a TVS...double the protection, double the fun, right? Perhaps one of those fancy active crowbar circuits on the output? It all depends on what you plan to do with it, but I wouldn't use just a MOV alone.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 07, 2021, 07:20:38 am
content: arcfour, the OP answered your main question in the third post.

author: jonpaul
author_rank: Super Contributor
timestamp: January 07, 2021, 11:06:29 am
content: As SMPS and electronic ballast designer, since 1970s, we had many MOV blowups and one serious fire caused by similar MOVs in 1986.When most needed they often fail open after repeated transients.Most modern designs avoid them.Transient Absorbing Zeners need careful specification and are NOT good directly on power mains.You should consider the level of risk and transients expected and the level of protection for the circuitry.In an industrial environment with many inductive loads switching eg factory, welkding shop, the transients are very bad.In a home or office, much less so.Bon Chance,Jon

author: bostonman
author_rank: Super Contributor
timestamp: January 07, 2021, 02:39:40 pm
content: The power supply will be for a C64 (and maybe another for the disk drive). A few years ago I built a linear supply that's really junk because it overheats and became a complete mess.In any case, I incorporated MOVs (using free samples) last minute, and thought, if I can still get free samples (or even just spend a few bucks), why not stick in heavy duty robust ones that can handle several thousand Joules of surge.Everyone is correct, I planned to use the MOVs that have thermal protection included (I just didn't think it was necessary to include this as my questions were pertaining to other factors). After disassembling the power strip, I became curious why six were used, and why the power strip is rated for 1000J (I need to look at the package again to confirm, but know it's the very least 900J).As for Zeners, I've seen these back-to-back devices on designs at my company, but wasn't sure how well they protect or really how to properly select ones.At one point I wanted to include a soft-start circuit because I was seeing small voltage spikes on the 5V DC output, but, after talking to my friend, he told me the CMOS chips in the C64 could overheat because of the slow turn on (i.e. the totem pole transistors would both be on simultaneously and damage the chips). At that point I became cautious about incorporating anything too fancy in the power supply.My current thought was high energy MOVs on the AC input mains, and some chip MOVs across the 9VAC and 5VDC outputs (mainly just for bragging purposes - I know they probably won't do much good).

author: CaptDon
author_rank: Super Contributor
timestamp: January 07, 2021, 03:18:01 pm
content: I am a bit confused by the huge numbers here. I thought a 130V10 was 10 joule ratedand a 130V20 was 20 joule rated. They are about the size of two U.S. mint quarters ontop of each other. 900 to 1000 joules?wouldn't that MOV be the size of a coffee saucer?I had one of those rack mounted multi-outlet power strips with built-in LED voltage indicatorand it always had a 'hot electronics' smell from day one. Well after just a couple of daysof use the thing blew up and flamed out in a big way until the mains tripped. The built-inbreaker which was part of the power switch welded in stuck closed fashion and the MOV'sexploded to bits and blew right off of their wire leads. They were apparently the 130V10part and there were lots of them. I suppose they were factory rejects??? I think there were12 total, 4 L to N, 4 L to G and 4 N to G. I think the 4 from L to G burned. The switch wassingle line only break, the neutral was never broken. I replaced them all with GE 130V10and it has worked well ever since. Added a rear panel slo-blow fuse!!!! Even with that additionit could probably burn for a while before the fuse tripped!!!

author: bostonman
author_rank: Super Contributor
timestamp: January 07, 2021, 03:30:07 pm
content: Let me look at the box later and respond.I'm talking the spec shown on the box on what it is rated for.Here is one on Amazon for 4,360J:https://www.amazon.com/12-Outlet-Protector-Charging-Mountable-Protection/dp/B07W5MYBFJ/ref=sr_1_1_sspa?_encoding=UTF8&c=ts&dchild=1&keywords=Surge+Protectors&qid=1610033320&s=aht&sr=1-1-spons&ts_id=761520&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzU1FVUzhKSkNESlpLJmVuY3J5cHRlZElkPUEwNzk1MzEzUTlUMUxGQlNIUTFRJmVuY3J5cHRlZEFkSWQ9QTA3MDk0MjNHQklBUE1RVVlFWlgmd2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

author: madires
author_rank: Super Contributor
timestamp: January 07, 2021, 03:53:47 pm
content: Quote from: CaptDon on January 07, 2021, 03:18:01 pm10 and 20 could be simply the MOV's diameter (in mm).Quote from: CaptDon on January 07, 2021, 03:18:01 pmThat's the marketing department's creative interpretation of the datasheet.

author: bostonman
author_rank: Super Contributor
timestamp: January 07, 2021, 04:08:08 pm
content: QuoteIf you think 900J is high, then look at the one in the Amazon link I provided above.

author: madires
author_rank: Super Contributor
timestamp: January 07, 2021, 05:15:30 pm
content: The calculation is:<rating from datasheet> * <number of MOVs> * <wank factor>

author: SeanB
author_rank: Super Contributor
timestamp: January 07, 2021, 05:27:46 pm
content: Yes, the number after the voltage is a diameter. I needed some mains MOV's, so went to the local supplier, and asked for 275VAC MOV's, the cheapest price. Funny enough those were the biggest diameter ones, 32mm, and I bought the entire stock of them, because likely nobody bought them, wanting 30mm, so i got all 36 of them cheap. Useful when installing LED lights outdoors, as otherwise they tend to get detonated by the mains surges. Yes they do go up nicely when they fail, had a pack of brand new old stock GE ones, and had a few that were faulty out of the pack, not surprising as they are very old zinc oxide types, not the more modern variants that have a much sharper knee.Had to educate a few technicians as to the difference between a 150VAC and a 275VAC type, as they were surprised that the 150VAC type would blow up after 10 minutes. A lot of lift controllers still use 115VAC for controls, along with 60VAC and 48VAC for lamps and buttons. especially those not based in Europe, where there is a love for 230VAC and 24VAC, and lamps are not, while still fitting in the same socket, interchangeable.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 07, 2021, 11:37:26 pm
content: A MOV's spec sheet shows the max Joule rating typically based on an 8/20us, 10/1000us, or 2ms transient test scenario. The J spec rating of a MOV typically increases with the Voltage rating of the part.If marketing used the number of MOV parts = 6, and multiplied that by a per part 150J rating, and if the MOV parts were 20mm disks, then that would align with standard 220VAC mains protection that typically have MOVs with a 275V rating. If higher voltage rated MOV parts were used, then the joule rating would increase.As this is for DIY, and you appear to have a house that is prone to lightning strike transients coming in via the mains, then I'd suggest looking at how to place a MOV 'device' either on the incoming mains switchboard, or on the first 'powerpoint' for the mains feed that you use for special equipment connections. There are devices that have fused MOV modules with indicators (either visual indicators or alarm contacts or both), and so you can manage any degradation of the MOVs as and when transients are shunted as close to the switchboard as possible. That doesn't mean not including MOV type protection at your equipment connection or within your equipment (behind a fuse and power switch), but could well suppress and manage frequent lightning strikes in a much more robust manner.I would always suggest slipping a sleeve of high temp, UL94 rated type tubing over a MOV for DIY.

author: helius
author_rank: Super Contributor
timestamp: January 08, 2021, 12:19:02 am
content: Quote from: trobbins on January 07, 2021, 11:37:26 pmIs it better to install a secondary surge arrestor at the service entrance (in the meter panel itself), or at the main fuse panel?The arguments I can see are that the former is "blocking the surge closer to its source", whereas the latter is "closer to the load". The latter also protects the MOVs by the main fuse.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 08, 2021, 12:45:15 am
content: I would suggest that adding a MOV assembly to the main switchboard (fuse panel) is for an electrician. Not even looking at regulations or DIY competency, just the sizing of cables and fuse ratings for a module at the switchboard requires competency as the fault current level is higher.The service entrance may require electric utility approval/electrician, depending on the dividing line, and may be more environmentally exposed and not so visible to periodic inspections (if arrestor health indicators are available).

author: helius
author_rank: Super Contributor
timestamp: January 08, 2021, 12:52:56 am
content: Secondary surge arrestors are a COTS item and have already been tested to industry standards.

author: bostonman
author_rank: Super Contributor
timestamp: January 08, 2021, 02:31:32 am
content: I don't believe we are prone to lightening strikes.As I mentioned, I was doing this primarily because it's inexpensive to incorporate.As for marketing surge protectors, is that what companies really do? They take the number of MOVs, multiply it by MOV rating, and that's the marketing number? If so, that's horrible because that's not a real number.I'm uncertain about lightening strikes, but my opinion is that it would only occur on one line (most likely the hot because neutral is tied to Earth ground and any strike would dissipate into the Earth). If so, even a hundred MOVs in parallel would still be the number of a single MOV as the single MOV would decrease in resistance and therefore all the current would go through that one.Getting back to my design (because this thread deviated a bit), will incorporating three MOVs with internal thermal protection, and the best number of Joules be a good plan? Also, place some service mount on on the outputs for added fun/protection?

author: trobbins
author_rank: Frequent Contributor
timestamp: January 08, 2021, 02:58:43 am
content: Quote from: bostonman on January 08, 2021, 02:31:32 amIt is a real number, it is just that it is not explicitly defined, and you would need to be experienced enough to appreciate what the number implies for your specific usage situation.Quote from: bostonman on January 08, 2021, 02:31:32 amUnfortunately the effects of a lightning strike are not that simple. You could make an assumption that your house has a local ground that is equipotential to the neutral mains feed, but that starts to go wonky when a large transient current flows along any utility cable, or house feed cable, and certainly is not the situation if the lightning strike is near to your house. With large transient currents flowing, a particular cable point may experience a large voltage difference between the conductors (hence the reason for MOVs between the three conductors). Conductors can have large voltage drops along them when transient current flow is high (which may not occur just along the path of a MOV, but can easily occur along any path where equipment is operating, or even as flashover when equipment is not switched on). The effective resistance of a MOV when conducting is but only one impedance element (of many) that defines where and how much current will flow.

author: bostonman
author_rank: Super Contributor
timestamp: January 08, 2021, 03:05:40 am
content: QuoteI'd imagine they are real numbers, but real in the sense that, say, in a controlled environment capable of producing a 4,360J (per my Amazon link earlier), actually save equipment plugged into it?

author: trobbins
author_rank: Frequent Contributor
timestamp: January 08, 2021, 03:41:23 am
content: Your Amazon product link is likely to use a similar marketing equation: # of MOV multiplied by MOV rating. You would have to crack open the casing to see if the MOV's are spread across the conductors and in what number. You would have to check the MOV marking and hopefully find a datasheet, or interpret the marking for a voltage rating, and use a generic datasheet to indicate the joule rating.Even then, there are so many ifs and buts as to whether one product is 'better' than another product, or whether a product may 'save' a specific type of equipment plugged in to it.If you were keen on the topic, then you may benefit from assessing the most precious item of equipment you want to protect, and work through what internal parts could be stressed or fail as a transient over-voltage is applied to it. Such an assessment also includes you defining specific transient waveforms or swell characteristics, and working through each one, and cross-referencing each part in your equipment as to how it may perform. Then you could work through the 'what ifs' about adding some protection part to some point in your equipment. Endless fun ahead.

author: bostonman
author_rank: Super Contributor
timestamp: January 08, 2021, 03:46:31 am
content: I forgot to look at the Joule rating for the power strip I disassembled yesterday that had six MOVs, but believe it's up around 1,000J (I sent pics of the MOVs and PCB in my initial message).I'll have to look at the power strip later and post the Joule rating to provide an "accurate" number.

author: Gyro
author_rank: Super Contributor
timestamp: January 08, 2021, 01:01:19 pm
content: Quote from: trobbins on January 08, 2021, 12:45:15 amSurge Protective Devices (SPDs) were added as a strong recommendation in the UK 18th Edition wiring regs. They are basically biggish diameter Din rail dual 275V MOVs with a thermal cutout (spring-open contact, with low melting point solder joint and red flag). They appear to be rated for surge current, eg. In=20kA, Ipk=40kA rather than total energy. They claim 25ns clamp time - originally they were specified to be connected directly across L-N-PE in the consumer unit (fuse panel) with tight limitation on lead length.More recently however there has been a requirement to fit them in series with a 16-32A MCB, but no requirement to retrofit (clearly there must have been 'incidents' with them relying on their own thermal failure protection and the supplier fuse).To my mind, this now makes them pretty pointless. The metal cored trip winding in the MCB is going to add significant series inductance, and in the event of a substantial duration surge, the MCB will just trip and the SPD go and hide in a corner, rather than protecting the installation up to its thermal capacity. Added to which, N and PE are tied together on the most common TN-C-S installations so most of the time, one half of the unit is effectively unused.

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2021, 02:16:17 pm
content: I have found that most electronic meters do have a massive MOV in them, though with a 480VAC rating, and this is there to protect the meter itself from high voltage transients, as it is included with a thermal fuse that breaks the measuring circuit power, and has a sense lead that will likely trip the meter off in case the thermal fuse operates. Fails sort of safe, though the latching relay is pretty small for the 80A ratings they have, but I suppose the massive busbars do allow a good amount of contact cooling, for what is likely to be a rarely operated switch. Also have current shunts built into the relay, along with a signal contact.

author: bostonman
author_rank: Super Contributor
timestamp: January 10, 2021, 04:45:40 pm
content: Attached is the box for the surge protector I originally mentioned along with the circuit board and MOVs (which I already posted in my initial message - just posting again for reference).I couldn't locate a datasheet for them, but guessing the 20 is the size in mm. As for the 1200J of protection, if the marketing department multiplies the total number of MOVs and their individual J rating, then I assume each MOV is 200J, correct?As for the actual protection, it's really smoke and mirrors. I assume this is using two MOVs per line (hot to neutral, hot to ground, neutral to ground), and the fuses () between are to open should the MOVs short (one per each line as listed in my previous sentence)?

author: trobbins
author_rank: Frequent Contributor
timestamp: January 10, 2021, 10:02:23 pm
content: MOV markings can be difficult to cross-reference sometimes. Search for Maida and D6521ZOV131RA20 - a 150J rated part.http://static6.arrow.com/aropdfconversion/a3797fc7dce144ce3d772b5216f64974753143cc/maida_standard_46141.pdf

author: floobydust
author_rank: Super Contributor
timestamp: January 10, 2021, 10:53:49 pm
content: Surge-protector energy rating is a marketing numbers game.You have six 20mm MOV's made bymaidaMDC Z131 20UL means 130VAC 20mm these are 150J each. Their 250VAC/20mm parts are 300J.A long time ago, APC power bars MOV's started fire and after a couple buildings got damaged, class action lawsuit and product recall.APC surge protector recall (all models before 2003)As MOV's age their leakage current increases until the part gets hot and burns up. Newer MOV's have an optional built-in thermal fuse.

author: bostonman
author_rank: Super Contributor
timestamp: January 11, 2021, 12:44:23 am
content: QuoteAt 150J and six, that would only be 900J of protection. Seems that stating 1200J of protection is false advertising.Is it safe to say that this is ideally only rated for 150J; or maybe 300J since two are used for each line?Ignoring that it's 900J or 1200J, what gives these companies the right to promote them at 1200J and only be rated for 900J or the fact that not all six MOVs can simultaneously work together?

author: trobbins
author_rank: Frequent Contributor
timestamp: January 11, 2021, 01:06:10 am
content: Perhaps step back a bit and firstly ask yourself if the Joule rating means anything tangible to you, or are you just using the joule value as an ill-defined benchmark to compare different products?Also note that not all MOV's fail in a way that is noticeable (by smoke, or smell, or indicators), and so how are you going to manage the device's capability over time? Are you going to open it up to see if fuses or MOV parts look ok? Are you going to test each MOV? Your device may lose its capability for OVP overnight, or not at all after 20 years of use.

author: bostonman
author_rank: Super Contributor
timestamp: January 11, 2021, 01:11:39 am
content: QuoteI'm using it as a benchmark to compare different products, but, also, I could wire a bunch of capacitors, inject a few hundred Joules, and, when it blows up, show that their advertising is false.My point is: these companies are advertising they'll protect equipment up to thousands of Joules, providing a warranty card showing they'll insure your product up to X dollars, etc... Seems to me that advertising something that can't meet such a rating is a hazard.

author: floobydust
author_rank: Super Contributor
timestamp: January 11, 2021, 02:24:05 am
content: From LeGrande manual forOP's 77000Nthey've changed their song:WHY DON’T WE SPECIFY JOULES?"The Joule rating does not provide an accurate indication of the robustness of a surge suppressor. The Joule rating is energy over time and it is very easy to artificially inflate the Joule rating by changing the time base. The Institute of Electrical and Electronics Engineers (IEEE), as well as the National Electrical Manufacturers Association (NEMA) both warn against using Joules to compare surge suppressors.""A much better indication of the robustness of a surge suppressor is the Nominal Discharge Current (In). The Nominal Discharge Current is a rating of the current surge current the surge suppressor can handle for 15 surges and still remain functional. This rating is independently verified by Underwriters Laboratories (UL) and the rating is required to be marked on the surge suppressor. In fact, UL requires that the surge suppressor must have an Inrating of 3kA or it is not suitable for installation closer than 30 feet  from the service entrance. All of our surge suppressors have an Inrating of 3kA and can be used anywhere in the building. When comparing surge suppressors, don’t be fooled by the Joule game. Look for an Inrating of 3kA to be sure you are getting the best in surge suppression."

author: trobbins
author_rank: Frequent Contributor
timestamp: January 11, 2021, 02:35:48 am
content: Another interesting assessment from 2005 is in the link - including the warranty's back then, and a delightful categorisation scale of 'crispyness':https://www.nist.gov/system/files/documents/pml/div684/TOVs_on_SPDs.pdfAlso, you can't say they don't meet their rating - if the MOV has a rating for a specific applied transient, then one could apply a transient that forced all MOVs in the device to their rating.

author: coppice
author_rank: Super Contributor
timestamp: January 11, 2021, 02:37:49 am
content: Quote from: arcfour on January 07, 2021, 05:02:23 amThe big problem with MOVs is them exploding with enough force, and jagged enough corners, that they punch through the equipment case and hurt people. That's why so many MOVs now have a thick heat shrink sleeve over them. Restraining the fragments as they crack apart in usually enough to damp the explosion enough that the equipment case is not breached.

author: bostonman
author_rank: Super Contributor
timestamp: January 11, 2021, 02:39:14 am
content: Wow, this is amazing.Initially I planned to order an MOV with the highest Joule rating with an internal thermal protection.This power supply can really be a basic 5V DC and 9V AC to power a C64. I figured, why not add a few extra parts (most likely free samples) to practice making a robust power supply.Obviously adding the components helps me learn, so I have learned something about MOVs. Now, if I use them, I'll buy whichever, and make sure to buy the ones with internal thermal protection.After all, it doesn't take much to add a few components on a PCB, so I thought making a fancy power supply would be nice.

author: helius
author_rank: Super Contributor
timestamp: January 11, 2021, 03:44:08 am
content: The "10m from the service entrance" bit is also how the IEC61010-1 multimeter CAT ratings are specified. A CAT II rating pertains for measurements on receptacles and hard-wired equipment more than 10m from the panel (or 20m from the service entrance). Closer than that, and CAT III applies.

author: bostonman
author_rank: Super Contributor
timestamp: January 11, 2021, 04:21:16 am
content: Littelfuse has some nice products. They have MOVs with a third lead so a LED can be connected to show a fault condition.Seems like a waste of power having to drop nearly 120V AC across a current limiting resistor for the LED.