Samsung C21RJAN ceramic hob

[starLED]

I have a 15-year old ceramic hob Samsung C21RJAN

Bottom circle (I don't know what you call it ) is not turning on when I press the sensor button.
Buttons are working and I can select heating level with +/- and it shows numbers for the circle.
I took it to the local repair shop, and they only said it was electronics and not the heater element.
I am not sure that they have done proper job and tested it correctly, so I would like to try see for myself if I can repair it.
I have found images of the PCB on the web:

I can see that there are not many components, only chip and on other side few dozen components.

Is this worth a try to repair?
Does anyone have a clue what a problem can be, what to search for?

Thank you.

 

[Diver300]

That is not an induction hob. It's a ceramic hob so the heating elements are powered from directly from the mains at 220 V 50 Hz.

The control board consists of a low-power switch mode power supply on the left, a microprocessor on the bottom and the three relays on the right (although there is space for a forth). Under each relay there is a SOT23 transistor that powers the relay coil and is controlled by the microcontroller, presumably. The level of heating is set by turning the heating on and off every few seconds.

The first thing to check is if the relays are working. I've repaired appliances before now by just replacing relays like that.

 

[starLED]

Yes, it's a ceramic hob, I didn't know the right translation for it, sorry, my english bad .

OK, I will check the relays.
Will post images next days.

 

[Diver300]

My only other advice would be to buy an induction hob, as they are so much nicer to use than ceramic hobs.

I replaced a ceramic hob with an induction hob and it was really easy to swap them over. That was a 4-ring hob, but there are plenty of 2-ring hobs available.

 

[starLED]

I actually have bought exactly the same ceramic hob, and I am very satisfied with it.
But I would be happy if I fix this one, if possible.

 

[Diver300]

It's certainly worth a look at the relays. I think that they are something like this:-
https://uk.farnell.com/schrack-te-connectivity/9-1393243-7/relay-dpst-no-250vac-8a/dp/2060869
The 2-pole NO (normally open) ones have 6 pins in total. Farnell show the same picture for any in the range, and some have 8 pins.
I don't know what coil voltage you need, but it will be written on the existing relays.

 

[starLED]

I will disassemble the device these days, so I will definitely check that one.

My main concern is the chip, but there are no indications whatsoever that it is faulty, as far as I can tell by using the buttons.
The rest of the components seem easy to replace.

 

[starLED]

I have disassembled the hob, and I have taken out 2 PCB boards.
The first one is the touch board and second one is the main board.

Here is what I've checked so far:
- heater elements are good
- ribbon cable is good
- relays have coil resistance about 345 ohms (360 is in Datasheet) which is ok
- relay terminals when switched OFF are in open state

What I plan next to do is to connect 9V battery to coil terminals to check if relays are switching ON.

Here are the images:

 

[starLED]

I've tested all relays with 9V battery (datasheet states that coil needs min. 8.4V).
The middle (RED) relay doesn't switch ON, while two outer ones (BRN and ORG) switch ON.
I can hear two outer ones click and terminals have continuity, while the middle one clicks very faintly and it doesn't have continuity (as you can see in the image).
So, the middle one (RED) is faulty.
Red wire goes from middle relay to bottom heater element, so this is the reason why is not turning on.

 

[Diver300]

The middle (RED) relay doesn't switch ON, while two outer ones (BRN and ORG) switch ON.
I can hear two outer ones click and terminals have continuity, while the middle one clicks very faintly and it doesn't have continuity
So, the middle one (RED) is faulty.
Red wire goes from middle relay to bottom heater element, so this is the reason why is not turning on.
View attachment 148517

That is almost certainly the problem.

Relays like those can't be repaired but it might be interesting to open it up to see what has gone wrong. In my experience there is often a lot of damage from heat when they have gone wrong. The normal amount of heat generated when the relay is working is minimal. It is only the coil current times the coils voltage plus the load current times the voltage drop. A bad connection can make the voltage drop increase a lot, maybe from a few millivolts to a few volts, and the heat will still work well enough that no problem is noticed, but now there is a lot of heat generated in the relay, and parts will melt quite quickly.

 

[starLED]

That is almost certainly the problem.

I will replace the relay to see if it fixes the issue.

I've found the relays in a local store that match, I think.
I've looked for SPST-NO, 16A, 12VDC, 250VAC relay and found two Schrack relays that could do.
Can you confirm they are a match, and what is the difference between them?
The price is the same.

RT334-12
RZ031A4D012

Relays like those can't be repaired but it might be interesting to open it up to see what has gone wrong.

I'll try to open it, if possible.


Thank you for your help, couldn't solve it without you.

 

[starLED]

Just removed faulty relay, until I buy replacement.
Couldn't open it to see why it braked, it's sealed from all sides.

 

[Diver300]

I can't see any significant difference between those two relays and I think either would work.

The relays is not sealed if you are prepared to use a saw. I expected that it would not be possible to repair, but you can still open it. I would suggest sawing all four sides near the base.

 

[starLED]

I am looking at dimensions for Schrack relay.
It seems that Schrack is slightly bigger than the original.
Can this be an issue?

Left is original relay from PCB board, right is Schrack.
Seems that holes for Schrack are 1,3 in diameter while the original is 1.

 

[Diver300]

I don't think that there will be a problem. I think that there is only one size of pins. One of the reasons for the slightly larger hole size of 1.3 mm is to allow the relay to be used with a metric spacing of 5 mm or with an imperial spacing of 0.2" (5.08 mm).

 

[starLED]

The relays is not sealed if you are prepared to use a saw. I expected that it would not be possible to repair, but you can still open it. I would suggest sawing all four sides near the base.

As requested, I opened it.

Contacts in the relay have degraded, the coil does work and the leaver joins contacts but there is no continuity.
Probably the heat destroyed it.

 

[Diver300]

Relays often have one moving contact and one fixed contact. I guess that the heat caused the plastic holding the fixed contact to start to melt, allowing the fixed contact to be pushed away from the moving contact. The lack of a click is because the contacts aren't hitting each other any more.

Anyhow, it's always nice to confirm the real cause of a fault.