On Tuesday, 31 May 2016 23:45:16 UTC+1, Rod Speed wrote:
tabbypurr wrote in message
...
On Tuesday, 31 May 2016 14:21:00 UTC+1, The Natural Philosopher wrote:
On 31/05/16 14:00, tabbypurr 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.

Oh dear

Oh cheep in your case.

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.

Lets have a reality check now. Any of the following can result in mains
appearing on the output:

Transformer insulation failure.

Not when there are multiple levels of insulation,
the bobbin, the insulation on the wire of the coils.

In the real world some have the bobbin missing,

Trivial to ensure that those are caught in the quality control.

once in a blue moon one gets wound wrong,

Trivial to ensure that those are caught in the quality control.

or a wire crossing over where it shouldn't etc.

Trivial to ensure that those are caught in the quality control.

Not common with bobbin transformers but it can happen.

And trivial to catch in the quality control.

Optoisolator failure.

Trivial to design those so they fail safe and
dont end up with the mains getting to the USB.

Again uncommon, but a big fat voltage spike, which
isn't too hard to create, can result in failure here.

Trivial to design those so they fail safe and
dont end up with the mains getting to the USB.

Dirt bridging the insulating gaps.

Trivial to ensure that dirt can't get in.

And finally a stray mains wire touching the low v side.

Trivial to design it so that there are no wires long enough
to do that if they do come off what they are attached to.

While good design significantly reduces the risk of
these, all can and have occurred in the real world.

Trivial to ensure that when that does happen
you STILL dont get mains on the USB.

As will some gross manufacturing defects

they do happen IRL.

And trivial to ensure that those get caught by quality control.

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..

I recall watching safety critical components that were
failing tests get a pass. Again IRL it does happen.

Trivial to ensure that when that happens it fails safe.

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

In reality direct mains on output is more
common with wallwart size supplies,

Only because ****wits like you dont use
proper bobbins in the transformer.

and the user appliance connected typically provides zero
insulation between 5v and user. Mains appliances of course do.

Another lie.

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

Should be? Of course. But it's hard to find much that's
completely safe or always as it should be in real life.

Pigs arse it is, just use those produced by
the manufacturers with a clue like Apple.

Welcome back to the retarded morons' corner of the killfile.