How (And Why) We Test USB Power Adapters

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Testing USB Power Adapters

Today, USB power adapters are an essential part of our everyday life, and many enjoy the convenience of having spare adapters at multiple locations for convenient access to a charging source for their mobile devices. Some choose to buy the manufacturer’s official AC adapters, often at great cost, while others take their chances with more affordable aftermarket options.

As evidenced by the occasional news stories about aftermarket adapters catching on fire, people getting electrocuted when picking up their mobile device before unplugging it, and devices getting fried, some of those adapters clearly have severe and in some cases fatal quality control issues.


Over the past two years, I have accumulated a fair sample of AC adapters. It is now time to test them out. We want to determine what proportion of them fall into the excellent, good, passable, bad, and death trap categories. Our selection ranges from Amazon Basic’s $10 adapter all the way down to those frighteningly common $1-2 Apple 1A adapter look-alikes. We'll also compare adapters acquired by other means, such as our SilverStone UC01 review sample. As a stand-in for OEM solutions, I will be using my 2012 Nexus 7 and Samsung Tab A adapters (though I have no intention of breaking those two open unless they fail our tests).

What will we be looking for? From least likely to most likely to be destructive:

  1. Voltage and noise versus load in 500mA increments until hitting the current limit at 115V input (verify how much power each adapter can actually deliver, how clean that power is, and how well-regulated the output voltage is).
  2. ±500mA load transient at half load and 115V input voltage (determine whether there are obvious transient-related issues to worry about).
  3. Efficiency at 1A, half load and maximum usable output at 115V and 230V input (get a general idea of how efficient these units are at the loads they are likely to spend most of their time at).
  4. Standby power draw at 115V and 230V AC (determine the convenience cost of leaving the adapters plugged in when not in use).
  5. Short circuit make and break behavior (see if any of them will self-destruct on short circuit or generate dangerous transients when coming out of short circuit).
  6. Maximum load at both ends of the adapters’ universal input voltage range, where applicable (determine whether the adapters are capable of handling the same load regardless of input AC voltage).
  7. Isolation withstand voltage, also known as hi-pot (detect adapters that may have inadequate isolation from AC mains to their output). This consists of applying a test voltage while monitoring the HV current to detect dielectric breakdown within the device. Different standards have different voltages and procedures ranging from 1500V pulses simulating power line surges to 4000VAC for IEC 60601-1 medical transformer testing. Since I do not have access to the full text for any of them, I decided to aim between the two extremes by using UL’s test for appliance electrical isolation where double the line voltage plus 1000VAC is applied, which makes my target 1500VAC (2100VPk) for universal supplies.

While many adapters will probably under-deliver on their output and be passable on output noise, undershoots, and overshoots, I still predict that most of them will survive the first six tests with minimal fuss. The seventh one, which happens to be the most important for safety, is where most of the adapter fatalities will probably happen. I've seen power supplies with suspiciously narrow separation between their primary and secondary sides, and others have seen highly questionable workmanship in transformer windings (such as primary enamel-coated wire crossing over output enamel-coated wire with nothing in-between, meaning that any chip or crack in the enamel could potentially let a trivial power line surge through). Among units that manage to pass their load tests while maintaining good output quality, this should be the trial that separates wheat from chaff.


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Daniel Sauvageau
Daniel Sauvageau is a Contributing Writer for Tom's Hardware US. He’s known for his feature tear-downs of components and peripherals.
22 Comments Comment from the forums
  • eenoram
    Love the article but the embedded videos arn't working across 4 different up to date browsers running with or without adblocker. Bummer!
    Reply
  • eenoram
    Never mind, working fine now,sorry.
    Reply
  • berezini
    looks like toms doesn't have anything else to write about so they re-write old articles a month or two old to the same conclusion.
    Reply
  • Daniel Sauvageau
    20619893 said:
    looks like toms doesn't have anything else to write about so they re-write old articles a month or two old to the same conclusion.
    The A1265 look-alike story from last month was meant to come out after this one but due to some scheduling weirdness, they got swapped and as I wrote in comments to the "first" story, the set of fake A1265s used for the first tear-down (and also to demonstrate the point of this series which was intended to start with "how we test"), there will be a bunch more to come in the future. The ending was intended to be a teaser (and somewhat of a spoiler) for the first tear-down, which is why my preview here stopped at the first failure. In the actual tear-down, I cleared each successive failure until the adapter was either dead, destroyed or what remained of it passed isolation withstand.
    Reply
  • bit_user
    Thanks again, mate. Looking forward to some good recommendations.

    How can we help fund your efforts? If your recommendations save me even one expensive equipment failure, it'd be worth a lot.
    Reply
  • bit_user
    20619893 said:
    looks like toms doesn't have anything else to write about
    So you read all the CES coverage? I still have some bookmarks to check out, as I couldn't keep up with it.

    IMO, it's understandable if they & the industry are having a bit of a quiet week (meltdown/specter aside), after CES. Most companies with anything to announce did it at CES, so the newswire is definitely going to be quieter this week.
    Reply
  • Daniel Sauvageau
    20621006 said:
    Thanks again, mate. Looking forward to some good recommendations.

    How can we help fund your efforts? If your recommendations save me even one expensive equipment failure, it'd be worth a lot.
    Recommendations-wise, I have one coming 2-3 stories from now which I sent in for editing last week and costs ~$10 and passed all of the tests I threw at it. It did have one potential manufacturing-related safety flaw in it but I'd need multiple samples to determine whether I got unlucky or should flag it as a significant concern.

    As for how to contribute, I'm open to suggestions if there is enough interest in doing so.
    Reply
  • versatileninja
    Love to see recommendations based upon this testing!
    Reply
  • chaz_music
    Something you should add to this: I have noticed many AC adapters being sold on Newegg, Amazon, and eBay that don't have a UL or ETL mark. That means that they did not go through either those agency's safety testing and analysis. Many of the things you found would be caught by UL, especially the shock and fire issues.
    Reply
  • Daniel Sauvageau
    20622599 said:
    Something you should add to this: I have noticed many AC adapters being sold on Newegg, Amazon, and eBay that don't have a UL or ETL mark.
    Marks are only as trustworthy as the supply chain between the buyer and manufacturer is. You can't trust any marks on stuff from generic off-shore sellers who have no domestic liability as the seller has very little incentive to ensure that marks on items are genuine. By the time people realize that they got a fake or fraudulent item (that looked and behaved roughly like the real thing), the feedback period on the sale is often expired and the original listing long gone.

    The first line of defense against no marks and fake marks is buying from a domestic source where liability falls onto the vendor/importer when the manufacturer is out of legal reach.
    Reply