Cheap USB charger

A small wall wart USB charger I was counting on failed to charge my phone, looping in a charge-for-half-a-second/won’t-charge cycle.  When I got home and started to investigate, I was stymied by lack of a way to get my hands on the wires in the USB cable.  That had happened before, so I bit the bullet, cut up some cables and terminated the ends with nice friendly 0.1″ header pins.  Fine – back to the problem at hand.

Patching the USB cable halves together with a little breadboard and plugging the phone in to charge, I watched the voltage as the will-charge/won’t-charge loop played.  The voltage dropped to 4V or so while it was trying to charge.  OK, it’s pretty clear – the charger can’t supply what’s being asked of it.  So replace it.  I scrounged up a higher capacity wall wart USB charger, and my charging needs were met.

But that charger was to live in the car, and that left me with no highish capacity wall wart in the box/as a spare.  Ebay served me well (I thought!) and I ordered a 300mA charger (from China, of course) – for $0.99 with free shipping.  (How do they do that?)

Oops – what arrived was a duplicate of the one that didn’t work.  A quick load test (facilitated by my new breakout cables!) confirmed the problem: though rated at 300mA (+/-30mA) it would barely do 200.  Since it was spec’d at 110-240VAC, I tried it at 136V – the most my bench variac would put out.  It did moderately better.  At 240V, it might even have hit the 270 mA spec.

OK – two next steps.  One, write back to the vendor, asking that he supply a charger that actually at least met the spec he advertised, or I’d have to provide negative Ebay feedback.  That’s apparently a pretty big club.  Yeah, for 99 cents it’s a great little 200mA charger.  But that’s not what it said, and that’s not what I needed.

Second, can I goose it up to better output on 120VAC?  What I see:  There are 2 LEDs – red for power and a green apparently for “charging”, starting to light around 100mA and getting brighter with more current.  At higher currents, the red one dims.   There is a zener, no visible 7805, 2 transistors, some diodes, a full wave bridge, and a transformer with 6 leads.  Is it really a little switcher?  The ripple on the output under enough load to drop the output voltage is 60Hz – not 120, and not the 50 or 100KHz I’d expect with a switcher.  There’s a diode in series with one of the AC leads – probably related to the 60 Hz ripple.

Guess I’ll need to reverse engineer a schematic out of it (more challenging with that 6-lead transformer, but at least it’s only a single sided board).  Ugh.  Is it worth it?  There’s a medium chance I can get the other 100mA out of it for swapping a couple of parts – though I don’t actually know if that’s enough for my phone.  If I just leave it open and postpone the decision, it’s yet another bit of crap on my bench.  Or I can call it a great cheap 200mA charger and put it back together.  Decisions, decisions.  I guess that’s how my bench gets so full of  crap.  Ugh.

2/7/21:  Interesting.  I happened to be casually looking at this old post, and while I haven’t reverse engineered a schematic, a new possibility occurred to me.  There’s what could easily be at least an attempt at an AC input/DC output physical distance separation across the PCB.  Two hypothetical ‘output’ terminals of the transformer are on the DC side, where the Zener, a potential filter cap, LEDs and the USB port live.  Looking back at original, higher res pics, that light-colored 4 pin device, which spans the HV/LV divide is marked ET1109, which is indeed an opto isolator.

So one possibility is that it is in fact a switcher, with the two transistors as the oscillator, and the opto providing feedback to the HV side.  Possibly the oscillator failed, and the poor transformer – designed for 100KHz or whatever – had crude half wave rectified AC rammed thru it, and sure enough, the DC side took what it got and put out what 5V current it could.

That’s all pure hot air, of course, but it’s interesting to look back later and see a possibility that I missed 9 years ago.