[Solved] Touch sensor circuit board (lamp repair)

[Toblag]

Dear Electro-tech-online,

I am repairing a LED floor lamp in which the touch sensor (for on/off and dimmer) has broken. The lamp itself is an 8W LED. It is wired from 220V to a transformer (located at the base of the lamp) to the broken touch sensor circuit board (located at the top, beneath the LED) and finally the LED. I have attached pictures of the floor lamp, the transformer and the touch circuit board.

I am now looking for a replacement.
Does anyone know where I can find a replacement? All circuit boards in the lamp are made by GYOTEK but I can´t seem to find any info about them.

Best regards,
Tobias

The whole lamp and the head of the lamp (the brass "button" is the touch sensor)
Transformer
Broken circuit board

 

[Pommie]

That's not a transformer! How do you know the touch part is getting power? Note, that circuit is possibly dangerous when powered. What voltage is your main supply?

Mike.

 

[dougy83]

That's not a transformer!

It looks pretty transformery to me. It'd be a flyback converter. Presumably the orange wires are the secondary winding.

 

[Toblag]

Thank you for the reply. Not sure if transformer is the correct word (I'm not a native to English), but isn't the larger circuit the one that converts the voltage from 220V AC (main supply) to what the LED uses (DC and probably 12-15V or so)?

The separate circuit board, which the touch sensor is connected to, does not transform the main outlet, just on/off and dim of the supply from the transformer circuit. The touch circuit/part is getting power since the lamp turns on when connected to an outlet, but it won't dim or turn off by the sensor. Unfortunately, I don't have my multimeter here right now so I can't measure the voltage directly at the sensor circuit board, but since the lamp gets current, I'm quite sure there is a problem with the sensor board.

I've done quite a lot of works on ordinary 220V lamps and 12V DC boat electronics, so I am aware of the danger and I am very cautious when it's powered.

Tobias

 

[Nigel Goodwin]

Thank you for the reply. Not sure if transformer is the correct word (I'm not a native to English), but isn't the larger circuit the one that converts the voltage from 220V AC (main supply) to what the LED uses (DC and probably 12-15V or so)?

It's a switch-mode power supply, so includes a high frequency transformer (the yellow square shape) - but not specifically a 'transformer' as such.

 

[dougy83]

The separate circuit board, which the touch sensor is connected to, does not transform the main outlet, just on/off and dim of the supply from the transformer circuit. The touch circuit/part is getting power since the lamp turns on when connected to an outlet, but it won't dim or turn off by the sensor. Unfortunately, I don't have my multimeter here right now so I can't measure the voltage directly at the sensor circuit board, but since the lamp gets current, I'm quite sure there is a problem with the sensor board.

The photo doesn't show enough for me to see the exact circuit, but I'd guess that the small 3-pin device is a mosfet, that switches the GND wire of the LED. It's possible that it has failed in the ON state. You can try replacing it with a suitable SOT23 mosfet, e.g. if 12V supply, you could use a SL2302 (2A/20V).

The other option is to remove the broken board and add a dangling touch switch, e.g. **broken link removed**

 

[Toblag]

The photo doesn't show enough for me to see the exact circuit, but I'd guess that the small 3-pin device is a mosfet, that switches the GND wire of the LED. It's possible that it has failed in the ON state. You can try replacing it with a suitable SOT23 mosfet, e.g. if 12V supply, you could use a SL2302 (2A/20V).

The other option is to remove the broken board and add a dangling touch switch, e.g. **broken link removed**

Dear Dougy,

Thanks for the reply. I have attached more detailed pictures of the circuit board, does that help you to help me?
I prefer to change the circuit board or necessary parts on it to keep the lamp in it original design.

Best regards,
Tobias

 

[dougy83]

Yep, very good pictures, thanks. The schematic for your circuit board is this:

Datasheet for touch IC: http://www.coldtears-electronics.com/images/SGL8022W.pdf
Datasheet for MOSFET: http://aosmd.com/pdfs/datasheet/AO3402.pdf

Anyway, I'm still of the opinion that your MOSFET is failed as a short-circuit. You can replace it, and you may find that the lamp is working again.

If you want to check that this is the problem in the interrim, you can measure the voltage across R2, that is, the voltage between the negative supply line and SO (the SGL8022 pin 7). The voltage should change each time you touch the touch pin.

 

[Toblag]

If you want to check that this is the problem in the interrim, you can measure the voltage across R2, that is, the voltage between the negative supply line and SO (the SGL8022 pin 7). The voltage should change each time you touch the touch pin.

Dear Dougy,

Thank you very much for taking the time to help me and for informative answers, you´re fantastic! If you want to, you can pick a charity of your choice and I will pay your help forward with a donation to that charity.

I have got a hold on my multimeter and have done some measuring. The input voltage to the touch sensor circuit board is DC 22.9-23.0V. The output to the lamp is 21.7V and after touching the sensor (the LED flickers a bit) it is 22.2V.
The voltage across R2 (measured between the SO and the negative cable to the LED) started at 3.3V and varied between 3.9V to -1.2V when “dimming” on the sensor. The lamp did not change more than the small flickering previously described. Does this confirm your theory about the broken MOSFET?

Best regards,
Tobias

 

[dougy83]

Happy to help; no donation required.

The voltage across R2 (measured between the SO and the negative cable to the LED) started at 3.3V and varied between 3.9V to -1.2V when “dimming” on the sensor. The lamp did not change more than the small flickering previously described. Does this confirm your theory about the broken MOSFET?

Sounds like you measured the voltage across R2 as well as the MOSFET, which is why you got a negative value. So the values would be 4.5V at the start, and between 0V and 5V each time you touch the sensor (i.e. just add 1.2V to each measurement). This means the IC is working. I'd replace the MOSFET.

I don't know what is causing the flickering, unless there's a mechanical/connection issue somewhere. I'd ignore it for the moment.

 

[Toblag]

Happy to help; no donation required.
Sounds like you measured the voltage across R2 as well as the MOSFET, which is why you got a negative value. So the values would be 4.5V at the start, and between 0V and 5V each time you touch the sensor (i.e. just add 1.2V to each measurement). This means the IC is working. I'd replace the MOSFET.

I don't know what is causing the flickering, unless there's a mechanical/connection issue somewhere. I'd ignore it for the moment.

That is indeed what I did... Just to be sure I will measure it again when my girlfriend is back home (need an extra hand to touch the sensor while measuring). If those measurements are in agreement with the previous (+1.2V) I will go ahead and order a new MOSFET. When it is replaced I will get back to you with the results!

Once again, thank you!

 

[Toblag]

I'd replace the MOSFET.

Hi again,

It took me some time to get a hold of the right MOSFET as well as to find someone who could change it for me (Denmark is pretty much in lock-down). Yesterday it was replaced and now IT IS WORKING PERFECTLY!

Thank you so much for sharing your knowledge. It wouldn't be working again without this forum and you!

Best regards,
Tobias

 

[Pommie]

Great to hear of a positive outcome.

Mike.

 

[JohanT]

I have exactly the same problem with the same Muuto Leaf lamp. Toblag, do you have a link to the exact MOSFET you ordered and from which store you ordered it? I live in Sweden so can hopefully order from the same store. I have some basic soldering skills so hope to replace it myself.

Thanks for a lot for creating the thread and huge thanks to Pommie for the detailed instructions.

Cheers, Johan

 

[Toblag]

Hi Johan,

I ordered this MOSFET (https://www.ebay.com/itm/171027560618). They are sold in packs of 10, so I have 9 spare ones. If you want to I could send a couple to you (for free). I too have some basic soldering skills, but it is a very small solder, so I got help from a friend of mine who works with computer repairs.

Nevertheless, the first thing I would to is to contact Muuto or the retailer where you bought it. Muuto is aware of this problem and sent me a new lamp for free (I got to keep the broken one and thus I wanted to fix it).

Best regards,
Tobias