USB socket charger, voltage too high! - Page 1
Forum section: electronics / beginners

author: step_s
author_rank: Regular Contributor
timestamp: April 12, 2016, 08:13:08 pm
content: Hi EEVbloggers.I've bought a few chargers off eBay, and they perfom very well, but my phone doesn't like them.It seems they give out around 5.6V with no load, and around 5.4V when under load, so the phone shuts charging on and off, to declare the problem.I'm not too familiar with how these fly-back transformers work, so I was wondering if there was a way to make it lower the voltage a bit, without having to stick on a linear regulator?I found this crude drawing, and it basicly sums up the entire design.https://www.tablix.org/~avian/blog/images2/2014/05/schematic_of_a_cheap_usb_charger.pngSo how does this regulate it's voltage, and can it be lowered?Thanks in advance!

author: michaeliv
author_rank: Frequent Contributor
timestamp: April 12, 2016, 08:17:49 pm
content: From the schematic, ZD1 + the 510 resistor above seems to be the ones responsible for regulation via the octocoupler.Try increasing the value of the 510 resistor and see what happens.

author: Delta
author_rank: Super Contributor
timestamp: April 12, 2016, 08:43:37 pm
content: Get a longer / shittier USB cable, that'll drop a bit of voltage for you...!

author: botcrusher
author_rank: Regular Contributor
timestamp: April 12, 2016, 08:44:44 pm
content: As thin a guage as you dare... Thin enough and it could double as a heater!

author: hydrogen maser
author_rank: Regular Contributor
timestamp: April 12, 2016, 10:36:04 pm
content: just put a beefy diode in series with the output, a lot of them drop about .6V - perfect.

author: step_s
author_rank: Regular Contributor
timestamp: April 13, 2016, 12:11:04 am
content: @MichaelivThe schematics seem to be different. The resistor is in series with the optocoupler, not in parallel. Putting a resistor across the optocoupler increases the voltage I'm afraid. .Doesnt seem like there's much of a choice then :/@Delta, they will, when under load. I'm afraid these chargers are made for shitty cables, but when there is no load, the voltage will spike regardless.@Hydrogen maserJust tried that, and it seemed to work with a schottky diode in series, and a 100ohm resistor across to give a small bit of load.But, nvm all of this, i tried soldering it together, and something went wrong, so it blew up :/Atleast I did find out that all of my sockets and lights are connected to the same 10A fuse. . . That can't be right?

author: michaeliv
author_rank: Frequent Contributor
timestamp: April 13, 2016, 12:18:45 am
content: Quote from: step_s on April 13, 2016, 12:11:04 amOK, if you have more chargers & 10A fuses, than decreasing the value of the in-series resistor would have decreased the voltage.

author: wraper
author_rank: Unknown
timestamp: April 13, 2016, 12:24:00 am
content: Quote from: step_s on April 12, 2016, 08:13:08 pmClueless, you just don't understand that have been screwed upQuoteImpossible, even under 500 mA they drop below 4V (after the cable), therefore charging cycles when the phone tries to draw more current. Though you likely measured before the cable which have some hair thin strands, voltage drop on it is like 1-1.5V at 500mA, 1A just is not possible. Or try some 700mA load, it will stop to work at all. Throw that garbage into the trash bin, they are dangerous anyway. No sufficient insulation from the mains voltage.The voltage "under load" you were measuring most likely was between those short shutdowns, phone thinks a bit before starting to draw the current.

author: step_s
author_rank: Regular Contributor
timestamp: April 13, 2016, 01:37:19 am
content: @MichaelivTried to do that, and it did lower the voltage, but directly soldering it across only brought it down 0.05V I'm afraid.Tried messing around with the resistors for the transistors on the high voltage side, but that didn't make a difference either.Honestly think that the only option is getting a better charger, or adding a high current linear regulator to cut off that last 0.5V.@WraperEhm, I'm afraid I don't really understand what you mean?Ye, they are cheap chargers, but they had very good insulation from mains, and internal fuse.The cable is another story entirely. I have some pretty short and beefy charger cables that have very low loss, but that has nothing to do with the charger.The issue here wasn't the drop in voltage, but the high voltage when no load. The phone does have short periods where it stops charging to check the voltage, and if it then spikes to 5.6V the phone will simply shut off the charging. Even though the voltage might be in the "safe range" which seems to be 5.4V max.

author: sleemanj
author_rank: Super Contributor
timestamp: April 13, 2016, 06:17:26 am
content: Replace Zener ZD1 with a lower voltage one so that the optocoupler conducts sooner. How low exactly :shrug: try a 3v3 or 3v they are common enough.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 13, 2016, 07:23:19 am
content: Cheap chargers output a higher voltage since they are often used with cheap cables which will drop a significant voltage.You should get the matching cheap (cca) cable.LPT: Don't buy chargers of ebay. The risk of fire or electrocution will not justify the 5 euro you saved.

author: wraper
author_rank: Unknown
timestamp: April 13, 2016, 08:08:11 am
content: Quote from: step_s on April 13, 2016, 01:37:19 amHave you measured this with oscilloscope. Phone should not stop charging because of higher voltage. The fact is, Chinese usually lie about the current rating. Can you please post a photo of the PCB?

author: step_s
author_rank: Regular Contributor
timestamp: April 13, 2016, 03:36:46 pm
content: @SleemanjWill look into it! Maybe the tolerance of the zener is what is causing to high voltage on some of the units?@Jeroen3As stated before, the voltage drop will be non-existing when no load, so this wont work :/Isn't the threat of electrocution and and fire what makes it more interesting?@WraperYou were thinking about the ripple on the output?Android phones will refuse to charge if the USB voltage is too high, for safety reasons, and 5.4V is the absolute max rating of USB 5V.I have charged USB batterybanks with the chargers, and they work very well, and seem to be able to push 5V at 1.5A.The PCB is burned after I messed it up, so don't know how much you will get from seeing it

author: wraper
author_rank: Unknown
timestamp: April 13, 2016, 04:32:53 pm
content: Quote from: step_s on April 13, 2016, 03:36:46 pmIf the phone was refusing the high voltage, it shouldn't start to charge at all. But it's cycling in your case.QuoteNo, I'm talking about if there are short voltage dips which you unlikely to see on multimeter.

author: tooki
author_rank: Super Contributor
timestamp: April 14, 2016, 09:37:11 am
content: Quote from: step_s on April 12, 2016, 08:13:08 pmIf your phone doesn't like them, then by definition they do not "perform very well".That said, use the poor regulation as an excuse to get full refunds, then throw them out and go buy real chargers that a) won't damage your phone, and b) won't kill you.http://www.lygte-info.dk/info/usbPowerSupplyTest%20UK.html