AMC7135 or the revival of a notorious flashlight.

Not long ago, I posted a review of a small flashlight.
In that review, I described a problem with the flashlight: its driver was poorly designed and simply no good.
Read on to find out how I got around the problem. It won't be a long read, but I think it'll be useful.

So I decided to order some fairly popular LED current-regulator ICs (you could call them LED drivers), AMC7135. The IC comes in two versions: 300–340 mA and 340–380 mA.
The seller had the first version.

The seller shipped the order the next day, but it took more than 40 days to arrive :(
There won't be a photo of the envelope, since the order contained more than just this IC (there'll be more reviews).
There were several identical little bags inside the envelope, and the AMC7135s I ordered were in one of them.


There were 10 ICs in the order, and all 10 arrived.


There's not much to say about these ICs.
I'll just mention that the IC requires very few external components. It's a good idea to add a couple of ceramic capacitors to prevent the IC from oscillating, especially if there are long wires to the LED. The datasheet has all the information you need about this; I can explain if necessary.

And this is the flashlight we're going to modify. It had been sitting on my desk for ages, waiting for the ICs. Well, now they're here :)


Here's the notorious driver. It was probably fine originally when powered by a single 1.2 V rechargeable battery or an AA cell, but it's absolutely unsuitable for a lithium battery.


And here's what we'll need for the modification (well, a soldering iron, solder, and a steady hand too).


Next, I used a hot-air gun to remove everything from the driver board. I didn't throw the parts away; the old driver still had a small inductor and a Schottky diode, which might come in handy somewhere.


Then, with a soldering iron, some solder, and a couple of short wires, I put the whole thing together using the board's contact pads.
I don't think it's worth spelling out what goes where, since boards vary and the work needs to be done differently for each one.




Next, I used the handle of a screwdriver to press the board back into its original aluminum sleeve.


Hooray, it works :)
By the look of it, it's only a little dimmer. It doesn't get hot and works perfectly.


The regulated current is 330 mA, well within the specified range.


At this point, I could have said, “Bingo! I did it. Everything’s great.”
But no, things were not great at all.
The photo below shows how many chips I ruined while modifying it.
Yes, I ended up using the fourth chip.


As I found out while working on it, these chips (I mean chips from this particular seller) are VERY sensitive to overheating during soldering.
First, I soldered in the first chip, but thought it was shorting the board (the board wasn’t designed for this chip), so I removed it with a hot-air gun and soldered it back in. When I powered it up, the LED barely flashed for a fraction of a second, then went out.
I thought I’d just got a bad chip. I soldered in the second one and checked it—it worked, but didn’t look properly soldered, so I warmed it up a little with the soldering iron. Turned it on, and it was dead.
I don’t remember what happened with the third one, but it joined the first two.
I soldered the fourth one like a bomb disposal expert. The result is in the photos above. Everything works.

I took photos of the underside of the chip. You can see that the thermal pad is slightly crooked, with bare copper showing.


From the top, though, it looks like a perfectly normal chip.


Conclusion.
Well, what can I say? I did achieve the result I wanted—the flashlight works—but it cost me three burned-out chips. I think the chips were probably rejects, although I had no such problems with the other items in the order.
As you can imagine, I have plenty of soldering experience, and electronic components usually handle being removed with a hot-air gun and soldered back onto a board without any trouble. Well, most components do; we’ll leave out the unusual ones that are heat-sensitive or, conversely, designed to operate at 300 degrees.
But the flashlight shines really well now—I’d even say brilliantly. My family really likes it. Now I just need to get it a good battery.

All in all, I wouldn’t buy from this seller again. Better try someone else.