how can i fix this power adapter? - Page 1
Forum section: electronics / beginners

author: vicolodo
author_rank: Newbie
timestamp: September 04, 2015, 01:22:48 am
content: Hello,recently i bought on ebay a cheap chinese power adapter 220V to 12V 6A .It does not work well, it is defectiveIts behaviour seems equal to the behaviour of a similar 5V 2A power adapter i bought some months ago:Measuring the output with a tester, i read correct 12V voltage, but the current is low and it varies in time continuosly from 0 to 1.5 Ampere.To explain better to you the behaviour of the power adapter, i connected it to a 55W bulb and made a video and uploaded it in youtube.Here is the video:https://youtu.be/JsC_0k1DCt8I made some snapshots of the PCB:top:bottom:Hete is also another video that shows the behaviour when i connect the adapter to a fan:https://youtu.be/IVyrdqqT0f8Some months ago i was able to fix the other power adapter simply substituting an electrolitic capacitor.But that time was easy to find the faulty capacitor, because it was swollen.This time instead seems not easy to find the fault,do you think that can be a capacitor again?can you someone help me?do you have any suggestion to help me to find the faulty component?thanks!

author: Chris C
author_rank: Frequent Contributor
timestamp: September 04, 2015, 01:50:32 am
content: I won't be much help if the power supply really is faulty, but I suspect you have an issue with your test setup.A 12V 55W tungsten bulb draws 55/12 = 4.58A. That's within the specs of your power supply.However, the resistance of a tungsten filament when cold can be as little as 10% of the resistance when hot. Which means it's going to be drawing more than 4.58A, and likely more than 6A, until it reaches full brightness. Which would trip the overcurrent protection feature of the switching power supply, shutting it down, until it resets a moment later. That's exactly what I'm seeing in your video.If you haven't tried this power supply with a more predictable load, do so now.

author: jitter
author_rank: Frequent Contributor
timestamp: September 04, 2015, 05:53:54 am
content: I agree with Chris about tungsten filament resistance. But I also wonder about the adapter's quality because with just the fan connected to it, it should work fine, yet it still seems to go in and out of protection. A brand new adapter should not have faulty caps.At first sight, the board layout looks OK with decent filtering and isolation slots in the pcb. But those heatsinks look rather small for a 72 W adapter, or do they sit inside the housing against some more aluminium?

author: kripton2035
author_rank: Super Contributor
timestamp: September 04, 2015, 06:13:47 am
content: check the output capacitor (the brown one) for a bad esr, or just try to change it if you dont have an esr meter.

author: mij59
author_rank: Frequent Contributor
timestamp: September 04, 2015, 06:21:40 am
content: The build quality is not that great, the distance between the primary and secondary side is not enough at R13.You could try , bad solder joint of R9, replace the caps on the secondary side, if no success throw it away.

author: wraper
author_rank: Unknown
timestamp: September 04, 2015, 06:49:38 am
content: There is Delta written on the PCB, so it should be high quality. Though I doubt this is delta PSU (except PCB), rather seems they put PCB salvaged from the old PSU into new enclosure. Did you change the high voltage electrolytic capacitor? If not, this is dead giveaway, as it should be mounted horizontally, there is even glue left where it was mounted originally. Also it is obviously hand soldered.EDIT:One solder joint of this capacitor is bad, this is likely the issue.

author: wraper
author_rank: Unknown
timestamp: September 04, 2015, 06:56:35 am
content: Quote from: mij59 on September 04, 2015, 06:21:40 amThere is more than enough distance. Don't you see the value of resistors, they intentionally connect the primary and secondary through 5x of 22 MOhm resistors to bleed the charge between the output and input. R13 IS connected to the primary side, why should there be any distance.

author: mij59
author_rank: Frequent Contributor
timestamp: September 04, 2015, 12:44:19 pm
content: Quote from: wraper on September 04, 2015, 06:56:35 amThanks, you are right, its connected to the primary side via the heat sink.

author: Cliff Matthews
author_rank: Unknown
timestamp: September 04, 2015, 01:37:36 pm
content: That's not a Delta. They'd have the class to install a MOV.

author: wraper
author_rank: Unknown
timestamp: September 04, 2015, 07:10:10 pm
content: Quote from: Cliff Matthews on September 04, 2015, 01:37:36 pmAre you blind? There was a MOV... in the past, before was cut off.

author: jitter
author_rank: Frequent Contributor
timestamp: September 04, 2015, 07:56:21 pm
content: Quote from: wraper on September 04, 2015, 06:49:38 amI agree that this could be the case.@vicolodo:There seems to be a datecode on the pcb, but it's obscured by a big white blob. Could you tell us what it is? That might also shed some light...

author: Cliff Matthews
author_rank: Unknown
timestamp: September 05, 2015, 12:07:28 am
content: Quote from: wraper on September 04, 2015, 07:10:10 pmYeah sure, small screen I used and didn't see the cut leads.. Still, I never trust labels from China.In 30 years, I've mostly found Delta PSU's rather pleasant to look at.A bit ugly that response though..

author: vicolodo
author_rank: Newbie
timestamp: September 05, 2015, 04:37:34 pm
content: hellothank you to alli will answer tonighti will add all additional information you needalso if is needed some measurement with my tester, please ask and i will do itthank you!

author: wraper
author_rank: Unknown
timestamp: September 05, 2015, 05:16:45 pm
content: Quote from: Cliff Matthews on September 05, 2015, 12:07:28 amThat label is on the PCB, not enclosure. Many PSUs, chargers from China have salvaged PCBs from disposed equipment inside. To the extent, you can buy two exactly same PSUs from one seller and there will be different PCBs inside.https://www.eevblog.com/forum/reviews/chinese-ebay-power-supply-lottery/

author: Cliff Matthews
author_rank: Unknown
timestamp: September 05, 2015, 09:00:49 pm
content: Quote from: wraper on September 05, 2015, 05:16:45 pmYes, I saw it. So it is - and it isn't. If I read you right, it's something like:Over-run PCB's are sold to overstock warehouse. They get sent to a sub-par assembler who drops-in a dogs mix of components and semi-dry hot snot, while agreeing not to put a Delta label outside.I guess the Chinese over produce and the JIT supply chain is a Japanese concept - sad.

author: wraper
author_rank: Unknown
timestamp: September 05, 2015, 11:16:27 pm
content: Quote from: Cliff Matthews on September 05, 2015, 09:00:49 pmYou've got it completely wrong. They just take E-waste dumped from the Western world on China, salvage old boards from the junk and put them into the new enclosures, profit. The same with salvaged parts being sold as new.