Cheap USB charger - Is This Good or Bad? - Page 1
Forum section: electronics / general

author: amspire
author_rank: Super Contributor
timestamp: October 24, 2016, 02:16:17 am
content: Just bought a Buddee 3.1A dual USB output wall charger from BigW in Australia for $12, and I was wondering if it was OK or dangerous rubbish.Initial tests were impressive - it supplies the rated 2.1A and 1A (3.1A total) without a problem and it runs really cool. I tried it on a Android TV dongle that dislikes most of my other wall adapters and it worked perfectly.The thing that stuck me as odd was that the initial output voltage was 5.01V. When the load increased, the output voltage increased up to 5.25V. I hadn't seen this before so I was wondering if this was by design (5.25V is the maximum USB voltage allowed) or was this a really cheap regulator?Turns out it is intentional. I opened it up with the Dremel (the case was pretty strong) and the build is good. Excellent clearance. A fairly big, well-made EF-20 transformer that seems to handle the power easily. It uses a PN8386 switch/regulator on the primary, and on the secondary a PN8305 trenchfet switching/monitoring IC to rectify the output. The resistance of the switch in the output IC is 6 to 14 milliohms. It is designed to increase the output voltage as the load increase. With 2A continuous out, the supply does not get warm. Idling, the charger draws under 50mW so that is not bad.Has anyone seen wall USB chargers that deliberately increase voltage like this that I guess compensates for the resistance in cheap cables? Is this a good idea?The tolerances have to be spot on for this to work. In the device I have, it was accurate on to much less then 1%. Since both the outputs are in parallel, when the loaded output goes high to allow for voltage drop in the cable, the other output goes to the maximum as well. It seems that it is a bit risky to actually use both the outputs unless you know that the connected devices will handle 5.25V without any risk of failure. Unless you check your own device for accuracy, then with tolerances, it could be slightly over the maximum USB voltage.$12 for what seems to be a safe, well built 3.1A USB supply from a major retailer is a pretty good deal here. Electrolytics of course are lower grade - VENT brand (CapXon). Not the worst, but definitely not the best.I found Test info page with details on similar supplies but with really bad layouts. The Buddee has the proper clearances:http://blog.chinaaet.com/duozuoshi/p/5100017211http://www.eet-china.com/ART_8800721642_628868_NP_75169bb5.HTMGoogle Translation:https://translate.google.com/translate?sl=zh-CN&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fblog.chinaaet.com%2Fduozuoshi%2Fp%2F5100017211&edit-text=&act=urlThe layout in these pages is totally different to the Buddee. In the Buddee, the transformer is bigger, and the output cap is a single 1500uF solid aluminium next to the PN8305 IC in parallel with a ceramic capacitor near the connectors. Minimum clearance is 7.25mm. I can post images of the teardown a little later if anyone is interested.Richard.Edit: in bagging the layout on the Chinese site, it may be the case has a plastic moulding that fits in the slot between primary and secondary sides. That would probably make it safe. If not, there is only about 0.05" between the USB connector pads and the Mains circuitry.

author: retiredcaps
author_rank: Super Contributor
timestamp: October 24, 2016, 02:26:32 am
content: Quote from: amspire on October 24, 2016, 02:16:17 amYes, teardown pictures please. Coincidentally, I'm going through some learning about USB charger devices right now when my Samsung tablet wouldn't charge overnight after I ran the battery down to 0%.

author: Monkeh
author_rank: Super Contributor
timestamp: October 24, 2016, 02:30:38 am
content: I've seen this before. The charger Samsung provided with my Galaxy S4 does the same, and almost perfectly matches the drop of the supplied cable.No USB device should care about 5.25V, that's well within tolerance. The current spec is for 5.5V. Nothing should care below 6V, certainly.

author: HKJ
author_rank: Super Contributor
timestamp: October 24, 2016, 04:12:49 am
content: Some chargers do that, this is usual types without optocouplers.You can see test data and tear downs for many usb chargers here:http://lygte-info.dk/info/ChargerIndex%20UK.html

author: DBecker
author_rank: Frequent Contributor
timestamp: October 24, 2016, 11:03:24 am
content: This is a typical design feature for high-rate USB chargers and devices.The device monitors the voltage as it ramps up the current drawn. If it detects that the voltage rises, it knows that the charger is capable of providing high power and will ramp up to a maximum of 1.5A over about three seconds. If it detects the voltage sagging, it slows the ramp. If the voltage continues to sag, it stops ramping the current and monitors the voltage. If the voltage continues to drop, the device quickly reduces current draw to slightly under 500mA to avoid triggering a polyfuse.

author: HKJ
author_rank: Super Contributor
timestamp: October 24, 2016, 11:53:41 am
content: Quote from: DBecker on October 24, 2016, 11:03:24 amNot really, as I wrote above it is often chargers without an opto coupler.You can get some special output curves from some chargers:On a multi port high power charger there is no compensation:

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 24, 2016, 08:53:55 pm
content: It would be greatly appreciated if you could post a link to and or picture of the charger, decent chargers are hard to come by.

author: amspire
author_rank: Super Contributor
timestamp: October 24, 2016, 10:29:36 pm
content: Quote from: peter.mitchell on October 24, 2016, 08:53:55 pmI will post pictures sometime today. Just a matter of finding the time.