Unlikely Cascade of Failures Leads to Microwave’s Demise

Surely a blown light bulb can’t kill a microwave oven, right? You might not expect it to, but that was indeed the root cause of a problem that [mikeselecticstuff] recently investigated; the cascade of failures is instructive to say the least.

While the microwave that made its way to [mike]’s bench wasn’t exactly engineered to fail, it surely was not designed to succeed. We won’t spoil the surprise, but suffice it to say that his hopes for a quick repair after the owner reported a bang before it died were dashed by an arc across the interior light bulb that put a pulse of mains voltage in places it didn’t belong. That the cascade of failures killed the appliance is a testament to how designing to a price point limits how thoroughly devices can be tested before production runs in the millions are stuffed into containers for trips to overseas markets.

Even though [mike] made his best effort to adhere to the Repair Manifesto, the end result was a scrapped microwave. It wasn’t a total loss given the interesting parts inside, but a disappointment nonetheless unless it forces us to keep in mind edge-case failure modes in our designs.

[via r/electronics]

37 thoughts on “Unlikely Cascade of Failures Leads to Microwave’s Demise

          1. I LIKE THE CUT OF YOUR JIB!

            No! That is ebin AF!

            I can’t find a pic like that with monetary coins. OMG, I feel a hackaday travel box swap forming!

            Beer bottle glass for sand fuse internals? Hmm!

  1. There’s no feedback path between a gadget’s fragility and the revenues of those steering the design decisions.

    As an engineer (no, I’m not living in Oregon :) I think the result is obvious. Open loop designs tend to move to some extreme position.

  2. Assuming a micowave is used for 3 minutes at a time and a pygmy lamp has a life expectancy of 1000 hours this gives you 20,000 uses of the microwave, I feel this explains all the design choices, I’m surprised the lamp is replaceable, or have I become cynical?

  3. Microwaves are even boobytrapped: A friend unloaded an expensive Samsung model at my place because it had stopped working after the corner of his shirt was caught by the lower latch in the door. It had blown his house fuses, and it wouldn’t power on with new wall fuses ??? Taking it apart it was apparent from the schematics glued on the inside, that there were 2 microswitches detecting door closure and a 3’rd microswitch at the bottom of the lower “latching hole” that detected the hook inside the oven being depressed further than it normally would. This switch simply shorted the A/C after a slow fuse. So that’s why it had blown his wall fuse !! So after remediating this design flaw, by emulating correct switch position for operation with some crocodile clips, I thought it would work but sadly, the short had killed the driver gates that closes the relays for the grill, hot air heater, and magnetron, so albeit the microwave believed it was working now, nothing besides the light coming on and the plate rotating, worked. So this is possible a tamper failsafe so if some teenagers try to jam the door switch to fry each other with death rays, they will destroy the oven! This is deliberate design by Samsung, so I put it back together and suggested he should have it replaced under warrenty, but he is a subtle guy so his felt it was his own fault.

      1. Recently had that exact thing happen with a ‘new’ < 1 yo GE. Loud bang, dead. Pulled it out of the wall (wall mounted), wall outlet was dead. Took awhile to figure out which breaker (none appeared to be tripped). Microwave still worked, but no light inside, and the stupid bulb is buried deep inside, requiring disassembling the unit to replace the bulb. Ack!

          1. It makes sense, if it blows the internal fuse. It means that the owner has to take it to a repair shop, who’ll notice Little Billy has been dicking with the safety interlocks. Or else Big Billy will eventually learn not to be so stupid, if it’s him who bought the thing. Either way, it stops your messing about in a very definite way. It’s not possible to just wait for it to reset (say) then carry on trying to build your death-ray.

            Of course the internal fuse SHOULD blow, but then a microwave uses a lot of current, so it has to have a high value fuse with enough leeway not to blow in ordinary use. I suppose if your kitchen outlet only just supplies the needed power, then that might cause a problem.

            Solution for that, though, is to have a sensible electrical distribution system in the first place, with a decent voltage and current rating.

    1. This is a very important safety feature, to ensure that NO MATTER WHAT microwaves are not being produced when the door is opened. Virtually all ovens going back to the beginning have this design, though it’s unfortunate that in this case it blew more than just a fuse.

        1. All microwave ovens have an internal fuse. If the internal fuse didn’t blow it’s because the outlet breaker was insufficiently rated (15A in place os a 20A) or the circuit was already loaded with something else.

  4. I also suffered from a blown PCB trace when the range light on a over the range microwave burned out and arced smoking the trace off the board. I thought the fuse had simply blown as the microwave had no power to the control panel but the fuse was fine. A rather large bodge wire for main power trace that blew off and its still running years later.

  5. I am not going to get into the design of the microwave lead to this issue, but I do have a question.

    The narrator of the video stated that a light bulb burning out will blow fuses/trip breakers: Is that something that is common in countries with 240V mains power? I ask as I live in the US I have never once seen or heard of a light bulb burn out that tripped a breaker or caused a fuse to blow.

    1. I’m also in the US with our nice tingly 120VAC branch circuits, and I’ve seen a bulb blow and take out a breaker before. “Candelabra” bulbs are the worst, I think. I had one blow in a ceiling fan.. it tripped the breaker, and destroyed the “wattage limiter” circuit inside the fan. The built-in fuse on the wattage limiter was still fine, but traces were blown off the board.

    2. I don’t think it’s anything directly to do with 240V.
      I’ve never seen a device dying trip the current breaker, but it’s not uncommon for the RCD to trip when a bulb or device goes; I think it’s down the the type of RCD; happened almost every time in my parents house, but very rarely in mine.
      RCDs are mandatory in the UK, probably not in the US?

      1. RCDs (called “GFI” or “GFCI” here) are mandatory in areas with exposure to moisture, such as kitchens and bathrooms. They’re also mandatory for outdoor outlets.

  6. I don’t know about the situation with 120V, but I have observed quite a few bulbs shorting out completely and tripping the circuit breaker. I think it is plausible that this happens ore often with 230V.

  7. That video makes me wonder if it perhaps it makes sense to look into your devices before they die to see if you can spot and repair the built in redundancy factors.

  8. This reminds me of a time when I had a VW Rabbit and the alternator stopped working most of the time, but not always. It turned out to be a burnt out idiot light! Hard to imagine. Porsches and BMWs (cars and bikes) also used this charging circuit from Bosch. I put a resistor in parallel with the light after I figured out what had happened. More on the circuit here: http://www.buchanan1.net/charge.html

  9. Hmmm. Mine keeps killing bulbs but works fine. One.fine.point. If I happen to run it 10min or 20min at the end of power cycle it will perform an auto shut off. What bugs me is why is it overriding heat after 11min… Okay, might be my fault a bit, I summoned a plasma wisp from a grape.

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