Testing 5V Power Adapters

I got a new oscilloscope a few weeks ago and I thought I might take a few moments to investigate just now noisy some 5V power adapaters that I might use on some Raspberry PI projects in the future.  The first unit I tested was an Ikea Nordmärke wireless Qi charger that also includes a USB port.  I have found it does a nice of charging anything I plug into it such as phones and even tablets.  It also seems to perform well with Raspberry PI's and by far is my favorite USB power supply, so I thought I would include it as part of my testing.

Note that all the tested units below had no load on them.

Here is the screenshot of the first test: (Ikea Nordmärke).  It by far has the cleanest output of a pretty nice sawtooth wave my a quick count of waveforms shows something like 266Hz with a 4% ripple.

The second test I did was on an inexpensive dual port Nokoko17.  The first port is labelled "2.1A For iPad" and the second "1.0A Others".  I have used these in the past for Raspberry PI's with varying results and the multiple times I have had problems with the outer cases falling off leaving high voltage parts still plugged into the wall.  I wouldn't recommend purchasing these again, but here is the traces on both ports.  Note that although the amplitude of the ripple is similar, the max voltage is 5.54 (out of USB spec) and both outputs ring in the 3-6 kHz range.

The third unit tested is a Tech&Go model TJD242 dual output unit.  The ports are not labelled but it does have an LED that stays lit for more than 10 seconds after unplugging it from the wall.
Here are its traces of each port.  I apologize that I was unable to get the hardware frequency counter to kick in.  There does seem to be two sources of noise and the ripple is closer to 9% with 440mV of noise.

So in summary, if you have really cheap 5v power converters, they will be noisy and although they might be great for charging phones at 1-2A current rate (where voltage noise doesn't matter as much), on older raspberry PI's where you need a cleaner signal, consider using a cleaner power supply.