Power Supply Failure Found In New Induction Cooker Board
As mentioned by me in yet another article on Induction Cooker Repair, this board failed while trying to replace it on a Butterfly Premium Model, as I thought it was time we went for a replacement of the board after repairing it number of times. But, that decision was a wrong one, proved by the failure of this new board on first switch on. The LED blinked and buzzer sounded repeatedly and one 22 Ohm 2W resistor burst! Well, you can understand my predicament when I had already got approval for replacement with a new board from the customer! After setting right the defects in the old board itself and giving delivery of that, I turned my attention to this board to see what went wrong. In such cases, first and foremost step is to conduct a cold test of all components that could have caused a primary failure. As this resistor comes at the AC in line, after which a rectifier diode IN4007 is used for converting it to DC for primary switching, I concentrated on that section. First of all I removed the IGBT, without which we cannot power this board up for testing. IGBT without the coil load might fail instantly if we switch on. Remember that I had only this board and not the complete set with coil and fan connected like in an Induction Cooker. I checked and found that all components in the primary and secondary were good. I used a socket for the PWM IC Viper12A and tried with a couple of new ICs. But result was same. The only solace was that the resistor did not burst! On further inspection, I noticed that the voltages produced by the secondary were not good enough. The 9V and 18V which should be the secondary output were dropping. I checked the ESRs of all caps.
Found difference in a couple of them and went ahead and replaced all the electrolytic capacitors on this board. Why did I do this on a new board? Well, this board was lying in my stock for more than two years, as I was always successful in reviving the defective Induction Cooker Boards. So, like any other electronic devices, this would have become defective having left unused for such a long time. Checked for any short in 18V Zener, 78L05 regulator IC etc. Found 78L05 showing a difference in resistance reading, when compared to a couple of new 78L05. The resistance between the second and third pin were far lesser than what it showed on the used one. It could be a defect. Anyhow, I replaced it with a new one. But nothing helped restore the voltages to its required level. Naturally the suspicion was on the SMPS transformer, though the ring testing showed ok. It could be a short of some windings on the secondary due to formation of fungus within. Remember that I live in Kerala, India, where monsoon is dominant and we used to have it for more than six months in an year. Moreover, my place is just 9 KMs away from the sea and coastal region will also have more corrosion cases. Anyhow, these are only assumptions based on our experiences.
When I checked with my techie friends, one of the experts who handled Induction Cooker servicing regularly, suggested replacement of the power supply with a readily available board. Pictures of this are below. Kindly use Google Translator for reading Chinese writings, if you want.
I removed the SMPS transformer and its corresponding components of PWM and secondary diodes from the main board, as this board could supply what is needed. The 9V in the board goes directly to 78L05 for supplying 5V to the uController. I could certainly give a 12V instead of that as this Power Supply Board itself is giving out 5V by such an arrangement. I wired the 18V and 12V to the vacant places in the secondary portion of the main board. Checked and ensured that connections were ok and then applied power. The board worked very well, confirming that the SMPS transformer indeed was the culprit, giving way for the satisfaction to jump into the bag. I have to wait for a chance to connect the board in future to see the result fully. Here are close-up pictures of the Power Supply Board along with the pictures for proof of working and also the villain SMPS Transformer:
Special notes: (1) In order to trouble shoot an induction cooker, the easy way is to remove the IGBT and check for proper functions. The power on indication and the panel buttons will work, but with an error. When I switched the Induction on, the indication was E0, because a vessel was not detected. (2) In this condition, we can measure all voltages at the output, including the feedback circuit.
(3) We can also mount the IGBT and induction coil, keep it on the side with a vessel on top to conduct further tests. We can check the wave forms of the pulse given to the Gate. (4) If any of you are interested in knowing more about Induction Cooker repair, kindly use the following link, download the pdf file, and select the category ‘Induction Cooker’ and read several articles written. 300 Repair Articles Published By Parasuraman | Electronics Repair And Technology News (jestineyong.com)
This article was prepared for you by Parasuraman Subramanian from India. He is 72 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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