How (And Why) We Test 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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