How To Repair 12VDC Output Power Adapter
I’d got a call from my friend with the complaint of his Set top box was not working. During the conversation I came to know that he was watching some TV shows and suddenly he heard cracking noise from the TV side and the TV was showing “no signal”. This means the set top was not working (no power LED lights on) causing the TV to show “no signal” symptom. So I asked him to get the Set Top box adapter to my workshop for further testing.
Next day he visited my workshop with his power adapter. As I am sure about the culprit I have directly opened the adapter and started visual inspection. Visually checked found ok from the component & solder side of the PCB there was no dry solder as well no burning marks.
From component side I have tried to locate Fuse but there was no fuse, so I have traced the circuit from main line wire; there I found one resistor connected in series on main line this resistor showing 10 Ohm as per color band on it (Brown Black Black Gold) . So I have checked with multimeter on the circuit it shows no reading that means resistor was open.
After removing the resistor I found damage mark on resistor-see the photo below:
Again checked with multimeter and confirmed the resistor was open circuit. The function of this resistor is to act like a fuse and this is why you can’t find any fuse in this power adapter.
The corresponding components could have fault also thus connected a 60w bulb in place of that resistor which I have removed. After plugged in the AC source to the adapter, the 60w bulb did not glow and the power LED was working. Next I checked the output voltage and it measured 12.39V DC which mean it was good-see the photo below:
Once I got the correct output voltage, this means there was no other shorted components and now I can solder the replacement resistor to the circuit.
Now the power adapter is back to the life again. If you would like to learn how to repair switch mode power supply, I suggest that you check out Jestine’s ebook in power supply repair.
This article was prepared for you by Yogesh Panchal who works as a Computer Hardware Engineer in Mumbai India.
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Note: You can check his previous repair articles in the link below:
https://jestineyong.com/no-power-in-seagate-external-usb-hdd-repaired/
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Parasuraman S
March 28, 2017 at 11:01 pm
Good job!
Yogesh Panchal
April 3, 2017 at 3:30 pm
Thank you Sir.
Albert van Bemmelen
March 29, 2017 at 12:42 am
Bringing back to life a dead Power Supply always gives a repair engineer one of the most satifying feelings. And as a reward a very happy customer who is able to use his old device again without having to buy a new one. Thank you for the article Yogesh.
In order to limit overcurrent using resistors are okay. But when we want to limit high in-rush currents they are not. Then we need NTC resistors that after warming up lower their resistance. And after the short in-rush overcurrent is gone these NTC resistors are completely bridged over by a relay. I need a in-rush current limiter for my 709A+ spot welder just to spare the fuses in my home when switching on the device (it creates overcurrent switch-on currents upto 63A!).
Although my spotwelder machine works fine after a L16A automatic fuse, I hate to see my main fuse blown! Just finding the right in-rush limiter that can handle the 63A x 230V power peak will be the question.
Henrique Jorge Guimarães Ulbrich
March 29, 2017 at 7:54 pm
Albert, unfortunately I do not know inrush limiter suitable for handling 63 a x 230 V. I understand that this problem would be approached just in the design of the spotwelder, in order to not bring about problems to the users. Is your house protected just by melting fuses? Or alternativelly by circuit breakers?
Albert van Bemmelen
March 30, 2017 at 5:27 pm
Hi Henrique. I know that this is related to the transformer's primary coil
that at switching on has not yet build up a magnetic field and why the in-rush current is so high. And at that point the primary coil is just a large low ohmic resistor.
I do have circuit breakers (L16A changed to C16A upstairs in my living room, and changed another L25A into a C25A downstairs) or probably also called resetable automatic fuses. But my main fuse (25A) is just a sealed boxed in melting fuse. And when that one breaks open, the bill for replacing it will be quite if not very high! And the safety rule is to have at least 2 fuse steps inbetween 2 fuses. Because now my C25A is slower but also the same amperage as my melting 25A main fuse is and previously only my L25A automatic fuse broke open, I do not dare to use my Spot welder upstairs. Using my L16 automatic fuse downstairs of my small 'garage' is safer and still worked 2 days ago. Because from L16A to 20A upto the 25A mainfuse are 2 steps. (also they often use the 1.6 fuse value safetyfactor). Anyway it is disappointing that theoretically a coil tries to prevent the current from rising which is a good thing here, but just not fast enough. Whereas a capacitor resists to any change in voltage.
Albert van Bemmelen
March 31, 2017 at 8:37 pm
Hereby a very good link that explains how to calculate an inrush limiter circuit I just found:
https://www.ametherm.com/inrush-current/transformer-inrush-current.html
Henrique Jorge Guimarães Ulbrich
April 7, 2017 at 2:53 am
Thanks, Albert. A very good article.
Yogesh Panchal
April 3, 2017 at 3:40 pm
Albert, Thanks for your comment.
for your problem 63A MCB trippers are available in the market.
Just for your reference pls. visit the site.
http://www.hager.co.in/product-catalogue/energy-distribution/main-protection-device/miniature-circuit-breaker/miniature-circuit-breaker-10ka/4526.htm
Albert van Bemmelen
April 4, 2017 at 3:43 pm
Thank you Yogesh. The Hager trippers/breakers you opted are not the same as Inrush current limiters. Because a current limiter doesn't break open like a fuse.
To explain this I added yesterday this information to my "How to repair Laptop Batteries" article":
I bought a Schwabe 60533 MIXO Einschaltstrombegrenzer , IP44 Gewerbe, Baustelle 16A 230V (German poduct, about 92 euro) to prevent blowing up my 25A mainfuse after switching on my 709A+ 2 in 1 Spotwelder.
It should perfectly well work because the inrush limiter can handle upto about 3800 Watt of Power!
Albert van Bemmelen
April 4, 2017 at 9:58 pm
Correction: Not 3800 Watt but 3500 Watt max which is about equal to 16A x 220VAC. And my Spotwelder is only 1900 Watt at 230VAC.
Albert van Bemmelen
April 7, 2017 at 7:22 pm
Just tested the yesterday recieved Schwab 16A 230VAC Inrush current Limiter.
It works great !!
Worth the money for sure!! (So Gerald, it will certainly work at your home too if there also is a USA current Limiter version).
I only had to change the USA AC plug with travel adapter plug to a decent EU plug on my 709A Spotwelder because the crappy travel adapter (USA to EU) did not stick/fit in the Schwab inrush current limiter!
MUSTAFA HAKAN ÜNLÜ
March 29, 2017 at 2:25 am
Hi,I admire you like the other repairists about your success at this forum;couldn't we use 100 watt light bulb instead of 60 watt?
Yogesh Panchal
April 3, 2017 at 3:44 pm
Mustafa,
Thanks for the comment, Of course you can use the 100W Bulb but advisable to use either 40W or 60W bulb for this type of low power adapters.
reza
March 29, 2017 at 1:08 pm
Hey Yogesh Panchal
Good Job!
Yogesh Panchal
April 3, 2017 at 3:45 pm
Thanks Reza.
Abhinav Chaitanya
March 29, 2017 at 1:12 pm
Hi,
From the photograph of the power adapter, it seem it was easy to open the case, probably it was encased with screws or it was a locking type, (I do not see the messy gum on the edges or the cutting mark on the case nor the holes for the screws).
Most of the adapters are firmly glued and the only method I am aware is to cut open at the joint. But then after repair again we have to glue it.
In case you have opened a glued adapter conveniently, could you share the procedure you have used to open such adapters.
Thanks.
Yogesh Panchal
April 3, 2017 at 4:01 pm
Abhinav,
This adapter is just locking type by using flat screw driver you can open it.
for glued adapter just apply petrol with the help of cotton on joining edge of the adapter leave it for five minutes and try to open.here is the link for your reference https://www.youtube.com/watch?v=a00esZ3B-aQ
zaheedi farjan
March 31, 2017 at 12:05 am
This was a very easy task changing just a simple resistor who act like a fuse but if instead after changing the resistor it not only blow but blow other component this would be very interesting to learn these kind of fault findings
Repect n regards
Yogesh Panchal
April 3, 2017 at 4:05 pm
Zaheedi,
Yes you can find many tricks and tips on this place thanks to "jestine sir" for providing this platform to share ideas.
suraga Electronics
April 4, 2017 at 7:59 pm
Hi. Mr-Yogesh.
Good Repair Article. !
Thanks .
Yogesh Panchal
April 5, 2017 at 4:21 pm
Thanks Suranga,
Humberto
April 13, 2017 at 1:45 pm
Congrats. for a successful rapair
Yogesh Panchal
April 13, 2017 at 9:05 pm
Thanks! Humberto.