One of the burners stopped working in my ceramic cooktop

Thread Starter

rambomhtri

Joined Nov 9, 2015
397
Hi, I have this ceramic cooktop with 4 burners. I bought it like... 13 years ago?

Anyway, about 5 years ago one of the burners stopped working, so I disassembled it and found out the coil (resistor that heats up) was split, damaged, open. I replaced it and it is working fantastic so far. Today another one failed, but this time the whole burner is OK, it has its 24 ohm and everything gives me correct readings. So, I think it must be something else. The other 3 burners, including the one I replaced, are working just fine, it's just one of them that you set to 9 (from 1 to 9 it goes) and it stays turned off instead of red and heating. Here's a pic of the broken burner (the one down), although the burner itself seems to be totally fine, so I guess it's not correct to say the burner is broken:
1.jpg

After some investigation and tinkering, I noticed this:
2.jpg

Middle orange box seems to have a burnt hole, but I don't know what is that and how it could affect just one burner. There are 4 orange boxes and 4 burners, so it looks like one box for each burner, but then there are only 3 white boxes, so...

I ran out of ideas bout how to diagnose this or fix it...
 
Last edited:

AlbertHall

Joined Jun 4, 2014
11,505
It is a 12V coil relay which can handle 250V at 16A. It would seem you have found the problem. Look for Schrack RTH34012.
The white are also relays, rated at 10A but I don't know what they are switching.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
397
I expected that, but the problem was/is... how can I diagnose that component?

I indeed already did, checked the ohms between the 6 pins (random selection) and I got the same readings in the burnt box and the one on the right, but again, I don't know what they have inside and reading ohms randomly over pins seems to be a crazy "just because" action with no reason.

It's soldered to the PCB, I would totally love it if I could check it without desoldering it.

As far as I can tell, the main cable comes from the wall, and it has the hot wire, the neutral and earth. Earth is connected to the chasis, all fine. The hot wire is separated into the white and blue (first image, top left), and neutral into the black and brown. Then the rest of the cables from each burner plus the neutral and hot all go to the picture number 2 PCB, which has the touch thing connected, and those boxes. That's pretty much it. On the left there's also a big black box that I don't know what is it neither.
 

wayneh

Joined Sep 9, 2010
17,153
It's soldered to the PCB, I would totally love it if I could check it
That can be done by several ways, but doing it safely requires care and attention. Having the pinout of the relay would help a lot. If you know which contacts are connected when the relay gets energized, you could jumper across them. The burner should then turn on whether the relay is energized or not.

Given that there’s hole burnt in top of it, that’s good enough me to just replace it without additional testing.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
397
Thank you so much for the ideas. I have checked that component in the original page, tried to learn what those 6 pins mean, but I think you have to be some kind of dealer or something to see the schematics and that...

What's the infamous "magic smoke"?
I'm not familiar with it, but again, I'm not really into these kind of electronics.

I know a relay prevents over currents by using an electromagnet that creates a force under the desired maximum amps (or higher) that opens the circuit, or by deforming a piece because of the heat by the amps that opens it. How can something burn there?
I'm a little rusty here hahaha.
 

LesJones

Joined Jan 8, 2017
3,497
Here is a link to the datasheet on the relay.
Farnell have them in stock at £3.74.
If there was no visible damage to the relay you would follow the red wire from the faulty ring back to the relay board and then check which relay it is connected to.
It also looks like a connection on the blue wires to the right of the ring that is not working has been overheating.

Les.
 
Last edited:

schmitt trigger

Joined Jul 12, 2010
463
The “magic smoke” is both a joke and an euphemism to indicate that a component has been severely overloaded and damaged.

The phrase was coined because most of the time an overload causes a device to burn up. Releasing smoke in the process.

Thus engineers like to joke: “Did you capture all the smoke that the transistor let out? Make sure to Inject it all back and it will work again.”
 
Whether it's grease buildup or food crumbs, one of the most common causes for a gas burner that has trouble igniting is debris blocking gas flow to the igniter. The fix: Begin by removing the grate covering the troubled burner. Next, remove the burner cap, which should lift off with ease.

www.krogerfeedback.com
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
397
This is no gas burner, this is obviously a ceramic cooktop (title says it), which works by heating up a resistor.

Thank you all for the help!
Today I will desolder the apparently bad relay and check if it is effectively broken. It has 8 pins, sorry, 6 pins:
1615893333924.png
Which pins should I check to make sure it is broken?
To my understanding, there must be an almost 0 ohm resistance in one path through the relay, that has broken and become and infinite resistance path, right?
Looks like number 12 and 11 should be shorted and are now open, right?
I don't see it clear what each pin has attached to it and what reading should I get...

A relay is a switch which is operated by an electric current rather than a lever. It will have a coil of wire which forms an electromagnet and the magnetism operates the switch contacts.

https://en.wikipedia.org/wiki/Magic_smoke
Yeah, so basically I guess there was some kind of over current and that created a bigger electromagnetic field that pulled or pushed some kind of switch (permanent switch I guess?) and opened the power intake of the burner.

I don't remember to have repaired the blue wires... that right there that you see is an insulating electrical tape (I would have used a tube) that covers what appears to be a damaged cable. I might look into it as well, I indeed thought about it when I opened it the other day and came here, but left it.
 
Last edited:

LesJones

Joined Jan 8, 2017
3,497
From the part number (RTH34012) that I think I read correctly from your picture and the data sheet I would expect the relay to have 6 pins (Not 8 as you state in post #11) The resistance between terminals A1 and A2 should be about 360 Ω. With no power to the coil (A1 A2) there should be no continuity between 11 and 14. If it does have 8 pins there should be close to zero ohms between 11 and 12. Apply 12 volts DC to pins A1 and A2. (The polarity should not matter.) The resistance then between 11 and 14 should be almost zero ohms. (If it has 8 pins then there should be no continuity between 11 and 12.) With the power removed from the coil there should be no continuity between A1 or A2 and 11, or 12 or 14.

Les.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
397
Quick note: I counted physically yesterday 6 pins, but then the product page shows 8 pins, that's why I said it has 8 pins...
 

AlbertHall

Joined Jun 4, 2014
11,505
Quick note: I counted physically yesterday 6 pins, but then the product page shows 8 pins, that's why I said it has 8 pins...
The writing on the relay indicates it has one normally open contact, so six pins.
'Normally open' means the contact will be open with no voltage on the coil but will close when 12V is applied to the coil.

This was not necessarily caused by an overload. When relay contacts get old and worn they don't make good low resistance contact. When there is a heavy load on the relay contact, as in this case, the load current in that contact resistance makes heat. The heat further damages the contacts -> more resistance -> more heat -> magic smoke.
 

Alec_t

Joined Sep 17, 2013
12,204
Apply 12 volts DC to pins A1 and A2.
Before you do that, check with the meter probes both ways round that there is about 360 Ohms between A1 and A2. Some relays (I don't know about yours) have a built-in diode between those terminals, so are polarity-sensitive and connecting 12V the wrong way would destroy the diode.
 

drc_567

Joined Dec 29, 2008
1,154
... Sometimes it is possible to carefully pry off the plastic case, which may allow a better view of the relay parts and their connections.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
397
More pictures!
I have poked a little bit with a screwdriver the black zone, and it crumbles itself pretty much. By the way, the fact that it's black inside, carbon coal like texture, means it's gone bad?
What's supposed to look like if not damaged?
IMG_20210316_192725.jpg
IMG_20210316_192751.jpg

There's no way I can remove the orange cover... That goo down there looked like it couldn't handle a sharp cutter... Wrong, it's solid and very hard... Any ideas to view the insides and get a better idea of what happened?
I fear that the relay was a consequence of another fail, not the cause itself!
 

LesJones

Joined Jan 8, 2017
3,497
I suspect the relay was the cause of the problem. I believe the contacts became high resistance and generated a lot of heat causing the burn to the casing.

Les.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
397
I finally opened the relay!
With pliers I managed... Here are some pictures:
IMG_20210316_201805.jpgIMG_20210316_201841.jpgIMG_20210316_201913.jpg

I am having trouble understanding it. There are basically 4 real pins: 2 for one contact (left plate, just one pin), 2 for the middle plate (another pin), and the coil has 2 pins of course.
I understand the coil must push the middle plate to touch the left plate, because it must be touching the right plate (open) if unpowered. The thing is, I have not seen any coil in the middle plate, it looks like a solid copper rectangle. May be the material of the middle plate is ferromagnetic?
 
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