Strange design of Induction Hob power input, advice please

[jaytee]

Hi, hope everyone is enjoying Easter!

I have boon looking for an induction hob recently and found one on ebay for £25 and close by so I bought it, yay!. It was purchased as defective, its a 5 sone hob and the two left zones did not power up. A bit of research before purchase shows there were separate circuitry for the left, middle and right zones.

On disassembly I found a blown WYO4N7 safety capacitor in the initial power handling PCB. On removal I also found burnt out tracking on the underside of the module. As the 3 sets have the same compoennts I swapped the PCB assembly from the right side to the left and it is working fine so the cause of the failure was the capacitor.

On looking at the good PCB there is no fuse fitted but there is a thinned section of PCB which is the weak point and appears to be designed to act as a fusable section. This is not as expected because I have seen catastrophic failures of PCB's where carbon has continued to conduct the mains.

The hob is rated at 10,800W total and the two zones involved are rated at 4kW. This infers that up to 17 amps is required. The thinned PCB track is approx .5mm wide and 1mm long, there are two in series.

There is no other electrical path and the full current flows through the two sections. I don't understand how this can work but clearly it did before the cap failed.

Could there be something funny going on with volts and current phase so the effective power is much lower? The zones are resistive, I don't see how they would be any different to a halogen load unless the presence of the pan makes the load inductive?

Does anyone have any thought on how 4kW can go through the tiny tracks, and how can this be repaired? some 13A fuse wire?

 

[marconi]

Happy Easter.

Yes, some type of fuse but not any type of fuse such as 13A fuse wire.

Before I advise what are your competencies with electricity and electronics including repairing pcbs?

And when you reply is the previously defunct set of hob rings now working fine on the swapped pcbs?

 

[telectrix]

lokks more like either dry joint on pcb or arcing between those w close tracks. clean and repair.

 

[marconi]

#3:

Dry joint - maybe yes. But I don't think so.

Arcing - no. Think about the absolute potentials of these undamaged copper tracks and pds between them and adjacent tracks. Arcing happens when you have high pds/field strengths - these do not exist in the vicinity of the damage.

I reckon it is a pcb fuse (I think fast acting semiconductor fuse) which bridges the right hand gap.

All the damage( from excessive overcurrent) was kicked off by the failing/failed mains input shunt capacitor.

 

[Lucien Nunes]

Yes that little section of track carries the full load current, which it can do because of the heatsinking provided by the adjacent large areas. That is why its aspect ratio is as it is, which makes it unsuitable to protect against overload, but it will act as a backstop in the unlikely event of a short (these caps are designed to fail open, yours is a less common failure mode). In this case it is likely to blow the copper clear very quickly, before the resin in the board has time to heat, char and become conductive. Basic, cheap, not very nice in theory but it did the job.

You could bridge it with a piece of tinned copper wire as you say, however you may be able to purchase the PCB for a modest sum as there's not much on it, which will then give you a fully compliant repair in accordance with the MI's.

 

[wheeto]

Is it a Whirlpool / Hotpoint unit?

 

[marconi]

Ok I hesitated - but since it has been suggested you bridge the gap with copper - and you might just do that - I would recommend you buy a 10A pcb fuse and use that to bridge the right hand gap - having cleaned up the region first.

It is too late for me to explain why 10A may be good enough - but it is a safe value to start with which one can then uprate.

See:

Schurter 3405.0176.24 UMF SMD Fuse 3x10.1mm Quick Acting 10A - https://www.rapidonline.com/schurter-3405-0176-24-umf-smd-fuse-3x10-1mm-quick-acting-10a-26-3046

 

[freddo]

If it was mine, and I couldn't easily get a replacement PCB. I'd just remove the blown filter capacitor, clean all the blast marks off of the PCB. New filter cap, bridge/bypass the blown tracks and leave it at that. The next fault will just be a little louder... If you fit a replacement fuse (good idea) I'd probably go for a time delay type rather than a quick acting one. All that filtering on the PCB suggests the hob is not just resistive.

Edit I see it is an induction hob.

I remember checking an electric oven that the customer had called in saying it was dead. The 45A cartridge fuse was blown. With the power off I turned it all on, insulation test all ok, and one at a time L-N resistances seemed normal as each element was switched in and out. So I thought it safe to put a new fuse in and switched each control on. Got to the 3rd hob ring and there was a tremendous bang. Bright flash from the control, ears ringing, soot all over my hand and up my arm, as well as over the white control panel. I jumped about 3' off of the floor. Good times.

 

[marconi]

With no knowledge of the original pcb fuse I stand by my first recommendation in order to protect better the downstream power semiconductors on the even more expensive hob power controller boards. If the quick acting fuses don't hold up then perhaps the OP can let us know.

(The board you provided images of is the EMI board with some circuitry for a fan control)

 

[jaytee]

Hi All, many thanks for your replies.

I'm HNC Electronic Engineering and many years as component level repair tech for Wavetek & Fluke with AC/DC and HV calibrators, DMM and waveform generators. But my intelligence has been dimmed with years of being a Sales man now but still competent to wave a hot iron around!

The replacement PCB is £168 +shipping and VAT and the circuit is in good enough shape to try and repair.

There was no fuse fitted, there is an empty position marked as "FP1". The designed weak point was just the 2 small PCB tracks in series which failed O/C when the filter cap failed.

The unit is made by DeDietrich, seems to be an established French Company so it may even be designed and built in France instead of China for a change, perhaps..

Marconi, if you have time to explain why the 10A could be suitable it would be appreciated and interesting, this is the theory part that I cant recall. I expect it may be to do with phase angle if the loads are inductive.

The failed side has one zone of 2800W and one of 3200W when the boost function is active, and the manual says there is power sharing in this case.

 

[jaytee]

Just regarding the PCB's, in the initial pictures the unburnt PCB is the one from the other side of the hob and its the same. There are no components just the very thin tracks which vaporise when too much current flows.

 

[marconi]

I reckon the hobs are cycled on/off/on , the actual duty cycle being varied by the knob for that hob. Thus the average power is somewhat less than the peak or rated power of the induction hob. For a duty cycle of 50% a 2.8kW would produce an average 1.4kW output. I believe that the hob rings are never operated at 100% duty cycle because of thermal limits of the hob ring itself and the power electronics which drive it plus the pan.

The power sharing feature manages the average power consumed by two rings which are on at the same time so that it does not exceed a pre-defined limit, something of the order 10.8kW/3 = 3.6kW - whatever the hobs' knobs setting ie: Full on both would be regulated so that combined power is no more than 3.6kW.

Induction pan heating is efficient so the pan and contents reach the desired temperature rapidly. Once a pan is at desired temperature the average hob power drops markedly to cover pan heat losses to the ambient air.

3.6kW is of the order of 15A.

Take a look now at the pre-arcing times for the 10A fuse I suggested:

https://static.rapidonline.com/pdf/560622_v1.pdf

( A bit of trial and see with a low safe fuse value because I did not want to recommend 15A for the first repair attempt. A 10 A fuse at 20A has a pre-arcing time of 120seconds).

 

[jaytee]

Thanks marconi. The 10A fuse pre arc values look viable.

I may pop a clamp on and see what current it is drawing. The plan is to get it running again and trial it for a couple of weeks to see if we get on with an induction hob so this will show up the need for a larger fuse. Its actually really easy to get to the PCB's which makes a change!

Cheers,

 

[Rpa07]

With the modern technological advancements of hobs and ovens these days, I am now thinking that the diversity calculations could or should be changed.
We are all going around installing 10mm and 6 mm cables that end up terminating a 1.5 - 2.5mm flex into the ovens and hobs.
I know it’s future proofing BUT old ovens would have had no cycling control over the appliance, it would have been all on or all off depending on Sunday lunch or a midweek snack.
Fascinating thread though.

 

[Shoei]

The attached file is a service manual for similar induction hobs to yours, there's a circuit on page 23 which shows the general schematic of the filter board, including the 'fuse' on the mains in.

It looks as though the popped capacitor is one of the two connected to earth shown top-centre of the schematic, the WYO4N7 you mentioned above.

Safest bet is to replace the board, they're a bit scarce but there are some about, this one at £100 for example.

If you want to do a repair, clean up the board with a drop of IPA (not beer). Bridge the fuse, remove the ceramic capacitors and give it a try. I'd take all four of the capacitors off (C4,5,6,7) but at least remove the two that are connected line-earth. For a test it should work fine without the capacitors, they're just there to stop RF getting back into the mains.

If you want to replicate the PCB fuse you can estimate it from the track width on the board that hasn't popped. If it's 35um copper (most likely), 1mm width would be roughly equal to 5A fuse wire. You could check the copper thickness with a vernier to confirm, it may be thicker (70 or 105um) in which case the rating would be higher.

If it works with no capacitors and the fuse link, it's easy to source replacement capacitors with suitable X1 class parts. Chances are the original problem was a mains spike that made the capacitor fail short.