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The Natural Philosopher[_2_]

On 31/05/16 14:00, wrote:
On Monday, 30 May 2016 10:15:32 UTC+1, Rod Speed wrote:

Perfectly possible to design the charger properly so that it is
always completely safe even when something has failed.

Good chargers aren't quite that good. But designing a charger to stay
safe no matter what fails is an impossibly tall order. As anyone with
skills in the relevant area knows. Rodney's ignorant child-like
assumptions are frankly not very constructive.

Lord. I am going to - if not leap - reluctantly step to to the defense
of wodders. Random monkeys and all that.

The way SMPSUs work, is ultimately by having a high voltage bit and a
low voltage bit and separating them physically. They are bridged by
typically a high frequency ferrite cored transformer, and its possible
to do things that isolate mains from LV almost completely. with several
mm of plastic in between.

The only other linkage needed is some kind of negative feedback to
achieve regulation.

You can, in uncritical applications take that off a third winding on the
transformer, or for more precision use things like optical isolators.

What this means is that there is no chance of mains getting onto the LV
side *due to electronic component failure*. You need to somehow breach
the integrity of the physical insulators inherent in air gaps, plastic
transformer bobbins, or optical isolators.

Dropping the thing into a bathtub will do just that of course.

As will some gross manufacturing defects - I once sawed the top off a
power transistors to find out why it had developed a base emitter dead
short. It it was the only time I ever had such a fault, and on opening
the can it became clear. The base wire had been spot welded within a
micron of the emitter pad, and when it got hot, had expanded and arced
across, and welded itself there.

That was a one in a million chance. To be so close and yet pass cold
testing, to fail in a hot amp in a hot climate..

In the context of USB sockets in bathrooms, they would however certainly
be no worse than an electric shaver socket.

Ex of gross and deliberate assault with a deadly soldering iron a USB
in wall charge point should be mains isolated and completely safe.

NT

--
Future generations will wonder in bemused amazement that the early
twenty-first centurys developed world went into hysterical panic over a
globally average temperature increase of a few tenths of a degree, and,
on the basis of gross exaggerations of highly uncertain computer
projections combined into implausible chains of inference, proceeded to
contemplate a rollback of the industrial age.

Richard Lindzen