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[Phys-L] Fwd: AC power calculations

The 65W apple power supply is a high current device;
as a general rule capacitive droppers are not, though there
are some pretty skeevy aftermarket consumer power supps out there.

There’s a bit of a history of aftermarket phone chargers in
particular killing folk charging their phones while talking in the tub.
Just google “phone charger kill” for stories. US code requires isolation
yadda yada but there are some cheap devices from China that
counterfeit the markings or simply sell direct. Second and third
world devices are simply dangerous; most of the resulting
deaths occur in Russia, India, China etc.

If these are plugged in backwards (or someone reversed polarity
at the outlet), some of these devices put out stupidly high voltage
and (stupidly high voltage - 5V) on the USB output pins.

These aftermarkets that kill folk are usually capacitive droppers.
A fun guy who talks about CD in everyday devices whilst
“taking them to bits” and analyzing the circuits is Big Clive:

The 65W apple charger is a-going to be loaded with inductors and
is almost certainly a switching PS. I’ve recently discovered to my dismay
that I can hear faint 60Hz buzzing on my microphone line teaching
online classes from an apple laptop ONLY when I use aftermarket
USB C power supplies (I’m assuming NOT a capacitive dropper).
It definitely pays to buy better equipment if you press it hard.

Dan M

On Feb 23, 2021, at 02:12, bernard cleyet <<>> wrote:

On 2021/Feb/22, at 13:06, Daniel MacIsaac via Phys-l <<>> wrote:

Actually Bernie, there are a lot of cheapie power supplies out there using capacitive droppers to avoid the transformers in wall warts. Many LED lamps use these.

If you buy a super cheap (eg quite lethal when wet) $2 cell phone charger that’s for sure a capacitive dropper. The Apple charger costs 10X as much, but is much safer (real HV isolation).

Dan M


I obviously miscommunicated.

I meant as a load, i.e. the cap. across the AC domestic power. I may try that again to verify.

One may test if a cap is the connection to the wall wart or a better high freq W/toroid v. low standby consumption P/S (< one watt) if it sparks on connection. The 65 W apple book P/Ss (curiously) does this! The usual, now ancient, transformer types don’t, and standby is often about 4 W.

bc uses his kill a watt v. often. He's taken apart many wall warts, but not yet a “modern” one. (Too valuable and usually potted. He does not like potted electronic.)

BTW, I previously used a d’ Arsenal type watt meter, but now in the to be repaired box. They’re very interesting: The W/Q [1] is the armature, the current is the magnet. Smart, nicht wahr?

[1] I suppose nearly universally voltage, which makes as much sense as using feeterage or meterage for length. But then using voltage indicates not abvolt or statvolt. OTOH, coutures use yardage. Oh, I can use pd, except w/q is specific potential. The English maybe use tension especially in EHT The French tension, and the Germans a very long word, typical! (according to google translate)