USB Power Supply/charger Test and tear down - Page 1
Forum section: electronics / general

author: HKJ
author_rank: Super Contributor
timestamp: February 13, 2013, 09:20:49 pm
content: I have been testing and tearing down a couple of USB power supplies/chargers, the result can be seen here:http://lygte-info.dk/info/usbPowerSupplyTest%20UK.htmlIt is mostly cheap crappy units, but I have also included a few good ones. The worst ones has about 0.5mm between mains voltage and low voltage, the best is about 5mm.

author: mariush
author_rank: Super Contributor
timestamp: February 13, 2013, 10:08:23 pm
content: Very nice and detailed pictures.I've bought a charger that looks on the outsides as your second charger... totally different inside compared to the 2nd and the repeat further below.But just the same, couldn't do more than 0.3a and stay above 5v with about 400mV ripple. I upped the input and output capacitors and put a bridge rectifier and managed to get about 0.45A out of it with around 1-200mV ripple but that's pretty much it.Mine was from dinodirect.com "Our future" charger.

author: biot
author_rank: Regular Contributor
timestamp: February 14, 2013, 01:21:06 am
content: Great work, very educational. Thanks very much!

author: amyk
author_rank: Super Contributor
timestamp: February 14, 2013, 08:19:20 pm
content: Looks like they ran out of optoisolators for #2...So the ones you'll be using also happen to be the most expensive... since you bought them already I'd agree, but out of the $1.60 ones #3 would be my pick.#6 is odd in that it has far less ripple than the other cheap ones, but also the lowest efficiency.The clearances look OK for most of them too.Keep in mind that for dangerous situation to occur there must be the possibility of completing a circuit through the mains via a person, and in practice it is actually very rare when using one of these chargers that you'll be both grounded and touching something live on them or the device it's connected to; even if via some fault the shield on the USB cable became live, people usually will grab the plastic, they are usually not grounded, and a phone or whatever else plugged into the charger almost always has a nonconductive case. While the individual risk maybe quite high, when taking everything else into consideration it decreases.

author: HKJ
author_rank: Super Contributor
timestamp: February 14, 2013, 08:33:19 pm
content: It looks like $2 chargers are to cheap, maybe I could have found some better chargers for $5.The clearances are not up to the required distance on many of them and as you can see in the table, all the cheap ones failed at 5kV between primary and secondary.Standing on a floor can be enough grounding and my mobil phone has a metal cover, I do also uses the USB chargers to power some battery chargers.Safety was only one aspect of the problem with the cheap charger, current output and noise is also a problem. I have tried to use a phone connected to a charger with to much noise, the phone did strange things with the touch input.

author: lapm
author_rank: Frequent Contributor
timestamp: February 15, 2013, 10:13:44 pm
content: Nice testing... Didint expect that much noise in them, but i quess you get what you pay for...Have couple of cheap ones myself.. im almost temptet to test them myself, but that would require me building dummy load first...

author: amyk
author_rank: Super Contributor
timestamp: February 16, 2013, 06:23:27 am
content: Quote from: HKJ on February 14, 2013, 08:33:19 pmI wouldn't expect them to survive 5kV; in your test methodology you mention "Safety standard says 3000 V AC or 4243 V DC" so they'd be perfectly fine if they fail at 5kV but not at 2.5kV.You're right that the current is below spec on most of them but that just tends to be how they rate things at 2x or 3x the real limit. I think they were all 500mA designs remarked for higher. Noise is a problem if you're using these for more than charging phones, but that's why: to cut costs, the manufacturers design them so they're just barely adequate for their intended use. You need to spend more for a stable filtered 5V PSU. The typical mobile phone also doesn't use the input 5V directly for much if anything; most of the logic is 3V3 or lower so it gets regulated down from the nominal 5V input, and the ICs receive a stable supply anyway. It's the same reasoning as why many AC adapters are unregulated, because the end-device has its own regulator.

author: HKJ
author_rank: Super Contributor
timestamp: February 16, 2013, 08:05:48 am
content: QuoteThat is the reason I only marked failed 2.5kV in red, but if you look in the table, you will see that all the more expensive chargers did survive the 5kV.QuoteGenerally I do expect equipment will follow the specifications and again the more expensive chargers does.QuoteAs I wrote above, my mobile phone is critical. For proprietary adapters it is no problem, they are only supposed to be used with a specific piece of equipment, for usb it is different. There are specifications for usb and the adapters are supposed to be used with 1000's different pieces of equipment.

author: Fezder
author_rank: Regular Contributor
timestamp: March 09, 2013, 07:17:51 pm
content: Hey, nice article, newer knew about issues about mains voltage. i heard some talk about some cheap chinese chargers set on fire, but that close mains voltage to low voltage....phew. you've seen really effort in purchasing and even testing those things, great!