Measuring USB power adapter quality using multimeter

Question

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.

Answer 1

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.