Testing USB phone chargers

[HenrysCat]

Hi all, I have just purchased a Rigol DS1102Z-E and am looking in to measuring the noise from various cheap Chinese USB phone chargers just to see how bad they really are, I am going to a genuine Apple charger as a benchmark.
I have been watching videos from youtube including eevblog ones, all very informative some very very technical, but none actually show to connect up the test gear.
Thus far I have gathered the scope needs to be in between the charger and load (my load is a UD18 from Aliexpress, yes cheap Chinese  )
So how do I connect the scope? is literally a case of chopping up a USB extension lead and making it thay way? or is there a better way/device/lead etc?

And any other tips greatly appreciated also.

Thanks all

[tooki]

Take a close look at this, one of the first in depth USB charger analyses: http://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html

[HenrysCat]

Thanks that's interesting but according to the reddit post also about 8 years old, I have a box of cheapo chargers here, also nothing about exactly how to connect the kit.

[tunk]

Test them at different loads - if you have a 2A charger, load it with
e.g. 0.5, 1 and 2A. And I guess you've watched diodegonewild's
youtube videos, and this: https://www.youtu.be/xaELqAo4kkQ

There's a lot of tests here, many of them really bad:
https://lygte-info.dk/info/indexUSB%20UK.html

[antimatter503]

I assume you mean USB chargers that go from mains to 5v, etc., like a phone charger?

Since you are new to oscilloscopes I would recommend you also get an isolation transformer or a differential probe so you don't accidentally blow up your scope by probing the wrong thing!! Plug the USB charger in and tap off the two wires that go to the load you can look there for noise from the output. Notice the difference between no load and loaded output. After you have some fun with that open up the case (this is when you need the isolation from ground loops and to be careful because there is mains voltage) and you can probe the switching transistor and before it pretty much anywhere and you can see neat things.

Just be careful if you don't have an isolation transformer or diff. probe because you can easily blow up your scope . I have done it before by testing mains power supplies and slipping off my test point.

[PeteH]

You could also build a small usb type A plug "PCB" which has a BNC on it... Can create a universal test fixture so that you can easily migrate the setup from unit to unit with a load tap for a generic usb load (adjustable linear load).

[Ian.M]

A significant proportion of cheap USB chargers are defective by design with minimal isolation between the mains supply and their low voltage output.  e.g:

If you measure the output ripple and noise, without using an isolation transformer for its mains supply, sooner or later you will encounter one that's got defective insulation, shorted between its primary and secondary sides which will blow up your scope.   Its possible to use a CAT II or better rated high voltage differential probe or isolated differential probe, to avoid the need for an isolation transformer, but such probes are expensive, and have significantly lower bandwidth than even an ordinary cheap x10 probe, so unless you intend to take measurements on the primary side of mains PSUs or other circuits that have no mains isolation, (for which differential probes are vastly preferable^*), I would recommend an isolation transformer.  Note that the isolation transformer must be wired for test bench use with no ground connection on the secondary side.

It would also be desirable to do a 1KV HiPot test (or PAT test) on all the chargers to eliminate those that have already failed (or are about to fail) dangerously.

With that out the way, you'll need to be able to tap USB Vbus and Gnd right at the charger output.  Others have suggested a few options for doing so.

Also note that cheap SMPSUs can interact badly with active loads (e.g. the control loops of a SMPSU and OPAMP+ MOSFET current sink can be unstable when connected together resulting in oscillation), so even if you use an active load for most measurements, its highly desirable to have suitable power resistors for a purely resistive load, (and preferably a power rheostat to vary the load resistance) so you can be certain the noise/ripple is from the charger and not its interaction with an active load.

* Novice and less experienced technicians should *NOT* attempt to make oscilloscope measurements on any sort of non-isolated mains circuit (e.g. the primary side of a SMPSU) without appropriate training and a formal risk assessment, even with a differential probe.

[tooki]

Thanks that's interesting but according to the reddit post also about 8 years old, I have a box of cheapo chargers here, also nothing about exactly how to connect the kit.

If you don’t know how to connect the test gear, then you’re in over your head and should defer this project until you’re more experienced, since — as others have said — there are some serious hazards involved.

As for age: the physics of electricity are still the same today as they were in 2012. The methodology is still perfectly current.