Blown bulb renders microwave oven unrepairable - Page 1
Forum section: electronics / repair

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 23, 2017, 11:20:19 am
content: Be careful when you use PCB tracks as fuses!

author: bktemp
author_rank: Super Contributor
timestamp: May 23, 2017, 11:39:59 am
content: Interesting failure mode of a microwave oven. That shows why good fuses are really important.In the video you said, there is a vacuum in the bulb. But can there be an arc inside a lightbulb with a vacuum? I thought this only happens with gas filled bulbs because the gas is necessary for forming a conductive plasma. Or is metal vapour in a vacuum enough to form a conductive path?Since there is also copper deposit on the inside of the bulb, could it be one of the wires holding the actual filament wire failed and shorted the input?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 23, 2017, 11:48:40 am
content: This makes me think: How do you replace a blown pcb fuse? Of course you can just solder a wire over it but yeah...

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: May 23, 2017, 11:52:28 am
content: Of all the things I've taken apart in the home I have the most admiration for microwave oven pcb design. If you can't figure out how to repair one of these providing it has a good chip and not damaged in any way you should give up. On the other hand trying to fix a washing machine mcu malfunction is a real challange.

author: rsjsouza
author_rank: Super Contributor
timestamp: May 23, 2017, 11:54:36 am
content: Wow, that's shameful on Panasonic - at least around here, they are not the "el cheapo" of the brands...

author: Cliff Matthews
author_rank: Unknown
timestamp: May 23, 2017, 11:57:44 am
content: Was the owner asked, at what moment it occurred? I've seen door interlock switch configurations instigate this level of destruction too (especially with careless door slammers).

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: May 23, 2017, 12:51:17 pm
content: Thank you Mike Harri^w HolmesIsn't it amazing the extent some naughty pixies are willing to go in order to help "the creator" cash in some more $?(No, no, George, that can't be it!)

author: MK14
author_rank: Super Contributor
timestamp: May 23, 2017, 01:02:09 pm
content: Surely the bulb should have a fuse built into it.Such as these:https://www.mygreenlighting.co.uk/replacement_bulbs/pygmy_and_other_small_bulbs/e17_ies/519.htmlQuoteHence the "fuse" gets changed when the user (or service people), change the bulb.Then the user, does NOT need to disassemble the Microwave to replace internal fuses, etc. Which would be a very bad idea, anyway, because of the possible dangers of people who don't know what they are doing, messing inside microwave ovens.I wonder if it was the original bulb and if the original was fused ?Maybe a cheap (unfused) replacement bulb was fitted ?

author: AndyC_772
author_rank: Super Contributor
timestamp: May 23, 2017, 01:11:49 pm
content: Perhaps one of the microwave experts here can help...I have a microwave which works fine apart from the LCD display, which hasn't worked at all for years.I'm sure it's an easy fix; it probably just needs a good clean of the zebra strips. But I'm nervous of taking the covers off in case it's not properly RF tight when back together.Is that generally a concern? Other than getting an eye full of microwaves, how would I know if it's started leaking?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 23, 2017, 01:20:39 pm
content: You just take the cover off to get to the LCD and electronics, there's no need to touch the cavity and the door so you will not change anything RF-wise.To be safe leave it unplugged for a day before opening it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2017, 01:26:54 pm
content: And you thought ESD was bad...Try mains arc flash into your micro!Arc flash is crazy. It's not so big a deal around these parts -- USA 120V just kind of goes pop, thump, dead. 240V+ is where things get interesting. It's not "interesting" enough to jump out at you (that's where "medium voltage" begins, technically 600V+), but once it's jumped, it does like to keep on going!Some years ago, I'd designed an industrial 10kW inverter module. 480VAC input, so about 650VDC total supply. This used SOT-227 "minibloc" modules, four in an H-bridge, screwed to the bottom side of the PCB. I was testing with IGBTs, to see if they would be suitable at lower switching frequencies (the module was designed for MOSFETs). I of course discovered they weren't suitable, and the aftermath was interesting to behold.What happens in a situation like this, is:1. Starting from the time a fault happens (in this case, a device failing shorted from excessive heat dissipation), in about 20 microseconds, it's past the point of no return. It can't turn off, no matter how hard you try. It becomes an ever-lower value resistor.2. After about 100 microseconds, the die and bondwire melt, then vaporize. (Effective resistance rises, because it's now an arc.) Gas production begins. Soon, the case fractures.3. After a millisecond, the case explodes, and the gas expands. The gas is hot with assorted metals and organics from the connections and packaging material, and rich with ionization from the arc. It's very hot, and very conductive.4. The expanding gas touches the PCB underside. The PCB is cold, so vapors begin to condense on it, leaving a powdery black (and also maybe gummy, from organics) residue.5. Where the PCB is conductive (uncoated areas: pins, pads, untented vias), and any of those sites have an opposite polarity from the expanding arc, the arc glomps onto them as well. We now have inadvertent arc electrodes...6. The arc continues to draw fault current from the supply (capacitors are discharged by now, but mains is still flowing). This superheats the air around the arc, vaporizing more material, and melting the "electrodes".7. After 1-100ms, the fuse finally blows, clearing the fault. (In this case, it would've been a couple ms, because semiconductor fuses were used. In the worst case, the mains wiring breaker might open in 10-100ms.) The arc's ionization dissipates, the gas cools, smoke condenses onto nearby surfaces, and whatever remains in the air, bellows out from the area with that characteristic smell.Tearing it apart, I observed a nicely cratered SOT-227, plenty of black smudge, and erosion and cratering of nearby traces on the PCB. One copper pour was actually burned away several mm, probably beginning at a soldermask pinhole. A blobby melted copper edge was left. There was erosion around a capacitor pin, as well, though it didn't melt (it was too thick for that, I guess).All in all: expect everything in the vicinity of an arc flash to become conductive. Expect any exposed metal to become a target of violence. Arc flash indiscriminately fucks shit up. And, yes, needless to say, that includes the technician, so don't forget your PPE, and guards and all those precautions, when working on industrial equipment!Domestic appliances aren't quite so violent, but this microwave oven case demonstrates how things can still turn nasty, on smaller scales.Tim

author: rsjsouza
author_rank: Super Contributor
timestamp: May 23, 2017, 02:25:06 pm
content: T3sl4co1l, pretty interesting sequence of events. I just have one question:QuoteWouldn't it be the opposite? Given that:Quote

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2017, 03:00:30 pm
content: At university we had a light bulb (230 V 15 W) blowing with an arc. As the outlet was rather close to fuses and a high power distribution and the distribution transformer also not that far, the current was really high and the first 16 A fuse failed - so it blew additional 32 A and 100 A fuses. So it made quite a mess - the cables from the 16 A fuse turned brown from the outside and needed to be replaced. Lucky nobody got heart from the glass part flowing through the room.Even though it was at a normal outlet, one might want a meter with true CAT III rating at such an outlet close to a breaker box. I am not sure the typical HRC fuse would have worked here: the 16 A fuse that failed was likely rated to break at least 6 kA.At 230 V it is quite normal for a light bulb to fail with with blowing the fuse - but usually it is just the fuse.

author: james_s
author_rank: Super Contributor
timestamp: May 23, 2017, 05:09:03 pm
content: When I work on a microwave, I normally connect a clip lead across the HV capacitor while I have it open. Other than that, there's really very little risk working on one, just don't power it up with the cover off unless you really know what you're doing. The RF stuff is not a big deal, it's pretty difficult to get one to leak significantly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2017, 05:13:17 pm
content: Quote from: rsjsouza on May 23, 2017, 02:25:06 pmWell, simplified. "Stages" would be a better way to put it, than "events". But yeah -- gas would be wheezing through the fractures during that time, pushing things apart and eroding stuff somewhat. A lot of noise is also produced in this stage, because the gas is under high pressure and temperature, creating shockwaves as it slows into normal air. That's why transistors make such a resounding CRACK when they die in this way.(And yes, mechanical stuff really does move this slowly. Bullets leave their gun at a leisurely pace, leaving enough time that you could, say, get a quite reasonable amount of computation done on an average microcontroller, in the time it takes a bullet to travel a few 10s of cm. And that includes the response time from, say, a photointerrupter, assuming you've arranged it so muzzle flash doesn't overwhelm it, that is. Even high explosives -- which ignite via shock waves in the 2-10 km/s range -- don't do very much (a few mm worth) over the course of microseconds.)Tim