Strange design of Induction Hob power input, advice please

jaytee · Apr 21, 2019

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?

[ElectriciansForums.net] Strange design of Induction Hob power input, advice please

marconi · Apr 23, 2019

Re #15 - interestingly at para 7.3 of the reference - second bullet reads:

The fuse preventing overcurrents is done so using a restriction on the board pad (Fuse pad). Actually, there are two series-mounted fuses. Only the IX1 generation is protected by a cartridge fuse, it is a so-called ‘Very rapid’ fuse.

Two series mounted fuses is what I think is seen on OP's image of the perfect EMI board. I still reckon the second one is a 'very rapid fuse' as I suggested.

OP: I cannot be tell by looking but is there a wire link at FP1 on the component side across where the first unblown pad fuse sits underneath?

freddo · Apr 23, 2019

Shoei said:
he attached file is a service manual for similar induction hobs to yours,

That's an informative manual! Rarely got anything that detailed when I was repairing white goods.

jaytee · Apr 23, 2019

Perfect, many thanks for that! The filter PCB is shown on page 40 complete with the fuse pads.

Its the 5A rating that I am struggling with, I would have thought the zones should be able to run a full non boosted power. Assuming that's only 3kW it should still draw 13A unless there are power factor things going on...

I can try to measure the track as well but it does not look any more meaty than a normal PCB. The fuse section of track is 0.5mm wide and 1mm long.

jaytee · Apr 23, 2019

marconi, no there are no wire links in either position.

Shoei · Apr 23, 2019

jaytee said:
Perfect, many thanks for that! The filter PCB is shown on page 40 complete with the fuse pads.

Its the 5A rating that I am struggling with, I would have thought the zones should be able to run a full non boosted power. Assuming that's only 3kW it should still draw 13A unless there are power factor things going on...

I can try to measure the track as well but it does not look any more meaty than a normal PCB. The fuse section of track is 0.5mm wide and 1mm long.

You're welcome

. I doubt the fuse is too critical, so long as you replace it with something that will blow before the board catches fire you should be OK, fuse tracks on PCBs are not fast or precise. Does the relay have a current rating on it? That could give a clue of the maximum rated current for the board.

jaytee · Apr 23, 2019

That's good thinking, the relay is an OMRON G2R-1A-E-DC12 but its a general purpose 16A. A couple of PCB track calculators are giving values around 1.5 A for a 0.035mm track 0.5mm wide so I don't have any confidence in that. I guess a 7A fuse will be a good starting place and I can increase from there!

Shoei · Apr 23, 2019

jaytee said:
That's good thinking, the relay is an OMRON G2R-1A-E-DC12 but its a general purpose 16A. A couple of PCB track calculators are giving values around 1.5 A for a 0.035mm track 0.5mm wide so I don't have any confidence in that. I guess a 7A fuse will be a good starting place and I can increase from there!

That's fair enough, you wouldn't think the relay would be maxed out so 10A protection may be a good starting point, as has been suggested. For confidence it's not a bad idea to solder in a fuse anyway, maybe 7A-10A 20mm ceramic, or whatever you have to hand.

Track calculators will give you the current limit for a modest temperature rise, typically 10 degrees, for a fuse you need to exceed the melting point of copper so the target temperature is a bit more extreme.

a-z electrics · Apr 27, 2019

Shoei said:
Chances are the original problem was a mains spike that made the capacitor fail short.

Is this a case of best fitting a SPD on the installation then do you think, as the latest regs update are advising we consider?

jaytee · Jun 1, 2019

So, 1st June. How did that happen!

The hob has been temporailiy istalled to give it a test drive and so far so good! The 10A normal mains fuse is working, I've had both hobs running at maximum, the highest output is Boost on the bigger one and power 8 on the smaller one and it was quite happy boiling water until the cows cam home - so its fixed!

Its noticeably faster than the resistance type, on boost it sounds like a kettle and boils very quickly. The heat immediately disappears so it been a good experience.

Here's a picture of the temporary installation, to amuse, baffle or anger at the viewers choice ;-)

Cheers,