No.127 - Kitchen hob PCB non-repair
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- Опубликовано: 30 сен 2024
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A PCB from kitchen induction hob has a blown 'fuse', but I'm not going to fix it!
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To make it more interesting, the 5-part fuse is tapped twice, once for the MOV and once for one of the caps. So replacing it would require at least 3 fuses.
Ah yes of course! Missed that. A multi-tapped fuse!
That's an interesting aspect of the design. Can anyone give reasoned speculation as to why?
I don't know.....can only guess:
Sequential protection, customized protection levels or simply space and cost efficiency..........
The tapped fuse would be able to tell the tech what caused it to blow. If it was only part of it going it could be a lifted neutral wire (MOV triggered) etc.
I head neer heard of a 'kitchen hob'. Had to search to find it was what us yanks call a kitchen stove, or cooktop. Always interesting to learn different terminaology used in different places.
I expect that the large pad area also make the fuse a slow blow, by removing and dissipating the heat in the small tracks allowing for higher inrush currents without lowing the protection for the supply wiring. in the same way they put a blob of metal in the centre of a fuse that slows the heat build up in the wire where it would normally melt! thus slowing down the fuse action allow the inrush current to reduce to the normal conditions and the fuse link / wire to cool.
I've always hated picking the fuse rating and type for equipment, as its got to survive the daily turn on and off's for a good part of the life of the device, but at the same time blow as fast as it can in a fault condition. then given the enormous number and types of fuses out there, it's almost a full time design function. 😞
Prospective Short Circuit Current (PSCC) is likely to be well into the hundreds of amps on a cooker circuit. I suspect the multiple "fuses" is to give the separation to clear the fault current without having a single longer fuse that will create a significant hotspot under load. Those large pads will be to dissipate some heat.
I think PCB fuses are cheaping out on quality (you can see them often on cheap Chinese SMPS) a proper replaceable fuse would be much better. Also I think on a UK cooker circuit you need a HRC fuse.
I agree. Use a real proper HRC fuse with a known rating in a fuse holder. Even a BS1362 fuse would be ok. A PCB fuse is cheap and lazy and has a widely variable rating. You don't know what current it will break at.
@@Alexelectricalengineering Not necessarily an HRC fuse. The fuse type is dictated by the PSCC, but the most straightforward route for compliance is a ceramic fuse.
@@sdgelectronics You are right most ceramic fuses are capable of breaking a very large current (I think something like 1.5 kA at least)
Probably they also want enough open path length to get the arc to extinguish. Mains arc will happily run over multiple mm especially with carbon around.
Getting a bunch of arcs going at the same time is harder. Eventually you'll reach a distance where the spark gap won't fire and it'll stop.
PCB fuses are just ridiculous
It sounds ridiculous, sure, but many designs also use zero-ohm resistors as fuses. It all depends on the conditions under which they are expected to trigger.
In this case the board basically self-destructs which sounds like they are denying "right to repair", but it's also very possible that they are protecting amateurs from trying to fix it and hurting themselves. Ian straight up refuses to fix the board without examining the hob and even then he's hesitant, and that may well be the reason for the design; the hob might now have an invisible failure and powering it up again could cause some serious danger.
If one only has a single short-length fuse protecting a 230V load, then when the fuse melts and an arc strikes, the resulting gap is insufficient to facilitate quenching of the arc. So multiple PCB fuse traces are put in series. They don't all blow at once as one commenter suggested - the weakest "fuse" section blows first, and an arc strikes. The voltage across a short arc for currents
Interesting!
@@IanScottJohnston and @peterh1386 That looks like a job for The Slow Mo Guys.
@@Brian_Of_Melbourne Great idea Brian - that would be cool to see!
I was thinking they put 4 in series for voltage requirements.
Induction hobs are common to blow boards. Often it’s caused by people dragging pans over the induction plates and it puts massive load on the board. The general layout is 2 board one for a pair of rings.
Too cheap to even put in a proper fuse to allow for repair, so that is a brand to avoid, what brand was it ? If it were mine (not someone else's) I would drill holes through the PCB and install a flying lead barrel type fuse or mount a fuse holder somewhere else for one.
Flying lead barrel fuse capable of up to 30A??
If it were my hob then maybe I would try something……but it’s not mine and would be irresponsible to hack a fix for a friend IMHO.
They bought a new Pcb for £250.
I think it's probably not do with cheap, but rather ensuring the PCB doesn't get repaired after a catastrophic failure. Even if it goes back to the manufacturer under warranty, it's likely more cost effective and more straightforward from a risk/quality perspective to swap out the PCB
@@sdgelectronicsrisk mitigation.
Shameful design, the manufacturer needs to be exposed. I've repaired a single hob induction cooker, and again MOSfet issues, I replaced it with the latest version of the part from a reputable supplier not had a problem since.
Looks like they went to the Apple school of design, ie, instead of a simple replaceable fuse lets make sure it totally destroys the PCB so you have to buy expensive replacements.
I would fix/re-engineer it regardless, because it would either pop straight away - not a big deal if you stand back -, or it will work fine.
What a waste to replace the whole thing, I would name and shame the brand of stove.
They use this method to share the voltage drop equaly on fuse element, so when overcurrent occur they fall at a same time and the small distance is enough to disconnect low voltage high-amp current, without mutch arcing.
The wide traces are cooling the thin traces and set the Amp square sec. Ratio of fuse.
I think the start and the continius input current can mesaureable after repair. The trace resistance to mosfets is mesaureable, so the max current is can calculateable when they go short, so olso get the max. Disconnect current for the correct and safe fuseing.
Than the simulator with known pcb material says the thin trace widht to get the normal working currents and get the fail current at same time.
Then can made a new fuse on a small PCB and it may bond onto the old fuse to get the same thermal as the original had.
The main thing: the small fuses opening at the same time, so any kind of bridgeing as repair is not an option.
Good to see a video that comes to the conclusion 'not repairable' AND lays out the reasons. This is as helpful to the repair community as a successful repair IMHO.
Inline fuse on the cable coming in to the unit and its done.
You could even try to fit in a SMD part though that would require grinding of the PCB ( SF-3812F3500T-2 )
If there was enough space, maybe a turret (like in the hardwired guitar amps) through a drilled hole in the pcb could be something. But meh, still doesn't feel safe. I completely understand why you don't repair the board. Good call.
Left me wondering who the manufacturer is - would like to make sure they're on my "do not buy" list...
Looks like its WKK according to the label.
@@simontay4851I looked at the video to try and figure it out, but then assumed it must be a European brand I didn't recognize.
I think WKK is affiliated with BSH appliances (Bosch/Siemens)
I would just scrape de soldermask and put some SMD fuses.
I had a single induction hob that kept blowing the igbt and bridge rectifier, it only didit when turned up full, ended up binning it
Stupid design!
I believe the transistors are IGBT's, not MOSFET's.
Yes you are correct......I was on a roll making the video...🙂
There’s a lot to be said for a gas hob. Just saying…
Not wrong!……virtually no electronics in our Rangemaster.
Shhh! - You'll have the green lobby camping outside your house holding candlelit vigils for Gaia whilst throwing orange paint around.
What in the world is a hob?
It’s a little known medical condition.
I wonder if the four fuses in series is to reduce the voltage across each fuse section to avoid flashover.
eror root cause and non repair very well explained. Thank you, Ian
Definitely a disposable PCB!
Even if you did replace the PCB fuse with a proper HRC fuse in a fuse holder mounted off the board and replaced the shorted MOSFETs, high voltage will have back fed through the gate pins and fried the control circuit. If high voltage has fried the PIC microcontroller, there is no point replacing that. You would need the firmware and good luck finding that.
Yep, can of worms no doubt.
Had similar issue with my dryer. Blew the pads off where the relay was soldered onto the board. I patched it up with some wick and she's good as new. It never occurred to me to check for faults in the board. Lol I just assumed it was due to a bad solder joints going more and more resistive over time.
Never assume, always verify!
Looks to me like the pcb fuse did it’s job, shouldn’t it be fused further upstream anyway, maybe that’s the thought of the designer and this is a last resort protection they get for free without increasing the bom ?
Upstream fusing is there to protect the fixed wiring rather than the appliance. It would be normal to have this on a 32A to 50A circuit, depending on requirements
@@sdgelectronics so if you were designing this would you put an actual physical fuse on the product or would you think the pcb fuse is adequate
@@XORMeUk Personally would use a physical fuse, but for the reason that this would reduce the burden of proving compliance.
amazing that 5 fuse tracks blew at the same time?
Huge amount of current…….they didn’t stand a chance
@@IanScottJohnston Poor fuses just a massacre of the protectors 🙂
No my friend ... It is repairable circuit board and this fusing design is the worst stupid idea of the designer EE ... Just use a thin 1 hair diameter copper wire and solder all these pads at once in series and go on finish repairing ...
That may well be, but the PCB is already damaged, and it will look rough around the edges with such a fix. The only fix I’d dare to do would be to mill out the old fuse footprint and make a shallow seat in the board to fix a custom replacement fuse PCB. It would look at least somewhat professional that way and I wouldn’t have trouble sleeping at night. Also no wire to curl off the short to the nearby case.
@@absurdengineeringHit the nail on the head…….it’s about how to sleep at night.
@@absurdengineering My friend , you made so complex solution , it is only soldering a thin hair diameter of copper wire on the same damaged PCB area which is available for soldering , why would you add more PCB to replicate the same stupid idea of that designer ??.. Are you also trying to save some money of using real fuse component too ???... Come on ! , be realistic , PCB fuses are always much trouble with smoking and firing other components when blowing unlike glass/ceramic fuses ...
@@IanScottJohnstonNo you won't sleep safe at night with that solution at all .... The core problem is ; Using PCB printed fuses is a very stupid money saving idea rather real glass/ceramic fuses ... PERIOD ! ...
@@ahmedalshalchiTrue, but sort of beside the point: the thing is broken enough that even if you fix the fuse and replace the mosfets it fails again (if I understood the video correctly). They may have known something about their design, perhaps :)
Knowing when something is beyond repair is just as important as knowing how to repair.