I have heard horror stories about poor quality USB power adapter. I have a collection of no less than 20 USB power adapters, most are from cell phone manufacturers and some from small projects.

I want to make use of the USB power adapters or give them away to those who needs it. I wonder if there is a way to determine the quality of USB power adapter using a multi-meter (I have a Fluke 87V clone, which can measure frequency and duty cycle)? I also have various USB cables that I can sacrifice.

What I am looking for is if the power adapter can do a proper job of converting AC to 5V DC, a clean voltage without spikes of noise, can supply the current amperage as stated on the adapter. I don't have access to an oscilloscope.

  • \$\begingroup\$ I am using a cellphone widget to show the charging current... \$\endgroup\$ – Eugene Sh. Jan 15 '19 at 19:05
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    \$\begingroup\$ How did you evaluate the quality of the Fluke 87V clone? \$\endgroup\$ – Solar Mike Jan 15 '19 at 19:06

Well, "quality" is a complicated term.

While on a first approximation, one might simply say that "doesn't drop below 4.8V even when drawing ${realistic amount} A current", and that's something that you could measure with a simple constant current sink and a multimeter, things get more complicated the more modern USB power supplies become; phones can communicate their power needs, and a supply that works well for one might not be as great for another.

Then, "quality" is also something that you might be measuring in reliability; since no multimeter in this world can't fast-forward half a year to see whether the supply will break, that's not something you can measure.

A very tangible aspect of supply quality is voltage ripple under different levels of load; this has mainly EM noise effects. However, you'd need an oscilloscope with a couple MHz bandwidth and something to draw controllable current from the USB supply to assess that.

So, while you can certainly assess the main "K.O. criteria" (too little current sourcing capacity), you can't do a more wholistic quality assessment. By the way, a clone is probably not the thing that QA labs would usually employ to assess the quality of a device under test – after all, you need to trust your measurement device more by orders of magnitude to come to a dependable quality assessment.

  • \$\begingroup\$ Thanks for the answer. I was mainly concerned over the quality of the power. e.g. if it performs proper AC to DC conversion, if it can supply the amperage as state. I will update my question. \$\endgroup\$ – some user Jan 15 '19 at 19:51
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    \$\begingroup\$ you can't measure "spikes" (note: you say current spikes, but probably mean voltage spikes; a realistic load for a charger is more of a constant current sink) with a multimeter. These are dynamic properties that the thing simply can't properly visualize – wrong measurement device. \$\endgroup\$ – Marcus Müller Jan 15 '19 at 20:28
  • \$\begingroup\$ My main goal is to weed out the bad USB power adapters. Would it make sense to invest in a sub-$100 USB digital storage oscilloscope? \$\endgroup\$ – some user Jan 16 '19 at 1:07
  • \$\begingroup\$ @someuser is it not likely cheaper to just get a few USB power adapter from reputable sources instead? \$\endgroup\$ – Joren Vaes Jan 16 '19 at 9:08
  • \$\begingroup\$ @someuser if you are to do something with electronics again, then an oscilloscope is a really cool thing to have. Economically, I agree with Joren. \$\endgroup\$ – Marcus Müller Jan 16 '19 at 11:28

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