Cheap USB cell charger, how's this work?

One of the power bricks on my network switches burned out. I figure I might as well replace the whole switch considering how old it is. I might reuse it tho for something else, and that got me thinking about building a supply for it out of spare parts. The old supply was 5V, 800 mA and from what I can tell there’s no way to crack it open without destroying it. They probably glued the thing shut at the factory.

Stuff I have on hand? A couple of 7805’s mainly. Assorted caps, diodes, usual parts. And… I have a couple of those cheap chinese made USB chargers you get at the dollar store.

Those are 5V/500 mA. No surprise when I tested them for load, couldn’t get more than 350 mA out stable.

So I opened one up, to see if there’s a way to make it more reliable. But as luck would have it I can’t get my head around what they’re doing here. Wondering if anyone can clue me in on it. See pics.

Looks sort of like your typical transformer-based supply. That little black square is probably a no-name rectifier (doesn’t have any part number). Smoothing caps. Indicator LED. And a transistor which I guess somehow regulates voltage.

But if I follow the traces, that’s where I get lost. AC is going through the black diode, past the cap, into the transformer. Then it goes to several places, among them the transistor (it says 13001/6B331). Then another output goes into the rectifier and to the USB connector, past another cap.

Why all the components on the AC side, and almost nothing on the DC side?

It's a cheap switch mode power supply. The black square is a opto coupler. Now you know how it's called you can easyly look it up :smiley: But it probably looks a lot like this Teardown, schematic and analysis of a single transistor PSU. - YouTube

And to get the switch running again, just buy a better 1A USB charger and wire it up :slight_smile:

Well, if it confused bigclive, doesn’t seem like a good idea to open that can of worms :o

Yeah, I thought of a cell charger, except the noname ones I find that say they’re 1 amp are pretty wonky most of the time and start to give out way before 1 amp.

That means I’ll probably have to find a decent brand name charger, and pay a small fortune for it :grin:

For the time being I found myself a 9V/2amp supply and put it into a 7805, spliced into the salvaged cable and connector. The 7805 isn’t extremely happy dissipating that voltage drop (at about 60C last measurement I took), but it’s holding up nicely under load. Should do the job until I can get a half decent cell phone charger.

Yeay , the are a bit overrated most of the time. So just go for a 2A version :stuck_out_tongue: Or instead of the 7805 go for a car USB charger, a 2A+ is easy to get and no/little heat.

Or go to a thrift store and look over there. But don't buy a heavy one, that probably has a transformer. You want a switch mode one.

Or go to a thrift store and look over there. But don't buy a heavy one, that probably has a transformer. You want a switch mode one.

Thrift store/ garage sales - excellent but somewhat slow and patchy supply of 5 V supplies.

For once a cheap chinese mains supply that doesn't look ludicrously dangerous. But
then again I can't see if that transformer is split-bobbin or not.

ludicrously dangerous?

Marginally safe maybe. I would not push that supply... it's pretty weak.