Those cheap Apple knockoffs are just as good, right?

Those cheap Apple knockoffs are just as good, right?  Wrong!  Ken Shirriff does a stellar side-by-side comparison between two products: a genuine Apple iPad charger ($19), and a cheap knockoff ($3). 

I once bought one of the cheap ones myself, and did a much less rigorous test: I connected the output to a 2.5 ohm load (to get 2 amps/10 watts of output) and looked at the output with a 'scope. 

First problem: the output was just over 3 volts, not the desired 5 volts.  That's 40% under voltage; the USB spec says 5% at most.

Second problem: there was almost 1 volt of 25 KHz noise on the output.  Apparently the switcher operated at that frequency.  One would expect to find no more than 100 millivolts of noise, and ideally less than 10 millivolts.  This thing was producing 10 to 100 times that!

Third problem: after about 2 minutes of under-load testing, a wisp of smoke started curling up from the charger block.  A few seconds later there was a loud pop, the case cracked, and clouds of black smoke shot out.  Simultaneously the house's circuit breaker blew.  I opened the windows and let the stupid thing cool down, then threw it away.

I ran the same tests on the Apple charger, and it passed with flying colors.  The block got slightly warm, otherwise I couldn't see any change at all.  I've happily paid my “Apple tax” ever since.  I prefer my electronics to work, and without noxious smoke, please...