Learning is fun

[Rick Law]

Here is a bad experience turned good.  I benefited from it (learning with help from fellow EEvBlog member - thanks, c4757p), and an honest eBay seller "worldwindow2010" who did what he said he would do.

My work area is a non-smoking area, but I give dying components some leeway.  And... a PCB has no business pretending to be Siamese twins.

I ordered a CC/CV power board to replace my dead cell phone charger.  It was shipped fast, and arrived in good shape.  I checked it quickly and looked like it worked.  So I “wrap up” by giving the seller an excellent feedback.

Weekend came and I had time to really test and use it.  After about 10 minutes, I saw smoke - I thought my battery was about to explode!  Following the smoke, I found the smoker: a regulator (78L05) on the PCB was literally going up in smoke.  I've equipment smoke before, but this is the first time I saw a component de-soldered itself and moved around as the board bubbles like an egg being boiled in water!  I emailed the seller.  He apologized and said he will send me a new one.  I received the news with some skepticism, wondering if I would ever really see a replacement.

Since some of you vent about bad sellers, so let me sidetrack here - here is one that is honest and wants to make the customer happy.  “worldwindow2010” - he said he will send me a replacement and he did.  It arrived quicker than I expected, it is working and this one is a non-smoker.  I think the first board just had a bad component. 

Before I got the replacement, good thing about a dead board is I don’t have to handle it with care.  I realized that when the 78L05 was smoking, the board was still putting out the right voltage and charge current was stable.  Not only did the 78L05 smoked, the solder actually melted and the PCB was bubbling.  Now that the board became disposable, and not knowing if I would ever see a replacement, I want to at least learn something from it - I traced the entire PCB and I measured what I can.  With some help from fellow EEVBlog member (thanks, c4757p), I figured out how it actually did “constant current”.  (It was bugging me when I didn’t see any feedback mechanism, but c4757p’s comment points me to the op-amp wiring).

https://www.eevblog.com/forum/beginners/it-works-but-how-there-is-something-i-am-missing/

Realizing that the 78L05 is only responsible for the current-control part of the circuit, I experimented, and got the board working to the point of having the CV part without CC.  After more experimentation, I got partial control of CC without the 78L05:  I connected the LM2596’s input (27Volt) via a resistor to two white-LED in series then to ground.  Two white-LED give me 5.2volt-ish.  I use that to power the op-amp controlling current and the rest of the CC circuit.  It even kind-of work as long as the real load on the LM2596’s output doesn’t draw so much current that the “LED voltage regulator” has no power!  The two LED looks like a couple of heads, and the board looks like a Siamese twins’ cranial separation surgery went horribly wrong, but it worked within a range of current.  It looks hilarious!  I like this board.

I posted a picture - the soldering is horrible.  Not only are my hands shaky, my soldering iron split open due to age, so I can only use it in one angle till I get a replacement.

Had the board not died so quickly, I would probably never dig so deep.   So, I had the opportunity to learn and understand what each component on this board do, and I really like the board because I understand it well.

I would have been happy even if I never seen a replacement since I learned so much; but this eBay seller is an honest one.  My skepticism was misplaced.  The seller did what he said he would, I received the replacement board, (thanks “worldwindow2010”) and I learn a lot by understanding how this board got the job done.