HELP: Induction hob power supply fault (incorrect voltage to rectifier)

[DanManTezz]

Hi everyone,
I have a CDA HVN91FR induction hob that partly works.
2 hobs (connected to one PCB board) are not working. The other 3 are.
Can't see any visual damage/problems.
Upon closer inspection, the power supplied to the rectifier in the 'dead' PCB seems too high.
In the working PCB board (for 2 other working hobs), the power supplied to the rectifier is 100V AC.
In the 'dead' one it's showing 230V AC.
It must be a simple voltage divider somewhere in the circuit but I can't figure it out!
HELP!
(Images attached of front and back of board; in same orientation)
Power supply on the right, rectifier on left
What is the yellow part?

 

[Diver300]

The yellow part is an inductor of some kind. So is the green toroid with brown wires.

I would expect 230 V ac on the rectifier, although that may only be there when that part of the hob is running.

The mains power will be rectified. There will be inductors to reduce electrical noise being transmitted back to the supply. There may also be inductors used in power factor correction.

 

[DanManTezz]

Thanks for replying Diver300.

That is what I wondered, but could the yellow component be a transformer? It has 7 pins (see attached)
I believe the green one is a 'choke'.

What is weird is that the unit has an identical PCB for 2 other working hobs, and that shows 100V AC going into the rectifier. I wondered if the 'dead' PCB simply has a switch that has stopped working, perhaps in the relay (white rectangular component), but that seems to work fine, and the voltage supplied to the relay is the same in both the working and 'dead' PCBs. Perhaps there is another switch somewhere?

 

[Nigel Goodwin]

Thanks for replying Diver300.

That is what I wondered, but could the yellow component be a transformer? It has 7 pins (see attached)
I believe the green one is a 'choke'.

Presumably it's a switch-mode transformer?.

 

[DanManTezz]

Presumably it's a switch-mode transformer?.

Good shout, it could be, is there a way to check that it is?

 

[Lightium]

Switch-mode trans. Sometimes use multiple pins for the same coil. Only way to tell is to desolder it and measure with an LCR metre.

 

[Nigel Goodwin]

Good shout, it could be, is there a way to check that it is?

It almost certainly is, and what does it matter? - it won't be faulty. I would imagine an induction hob controller is just a big switch-mode PSU.

 

[Diver300]

To me it looks to small to be a transformer for that much power.

I can't see any reason why an induction hob coil would be isolated with a transformer. The induction hob is a large switch-mode transformer, with the pan as the shorted secondary winding.

I think that the power components are:-
Mains -> rectifier -> switching device -> coil -> pan

It may need an inductor to help with power factor correction. However on our induction cooker I can feel 100 Hz vibration, so I guess that adjusting the switching duty cycle to match the mains voltage will give the power factor correction. There is clearly no problem having the power to the pan varying at 100 Hz.

The coils will be for filtering the noise.

 

[DanManTezz]

UPDATE: looks to be a problem on the low-power side of the board (+ I'm an idiot).

Diver300 was right in saying "I would expect 230 V ac on the rectifier, although that may only be there when that part of the hob is running." - the working hob, when turned on to create current in the hobs themselves, does read 230V (actually 250V).

The problem seems to be that for the 'dead' hob, the relay is holding a few V between the switching contacts, which may be keeping the high power (230V) on. The working hob holds 0V between the switching contacts, until the hob is turned on!

Between GND and the contacts, I've noticed the 'dead' one holds 13V and 16V AC on the relay contacts. The working one holds 16V and 16V AC on the relay contacts (giving an open circuit)

So far, I've found that this 13V is also held on one side of a cap (next to the diode), one side of a a diode (D1) and on the one side of 2 transistors (T1 and T10). The cap and diode (D1) are above the relay (white rectangular block) in the 1st image and the transistors are at the bottom of the 2nd image.

I think I need to find what is causing that 13V!