[chbot] Why galvanic isolation is important

[Charles Manning]

These articles in the news are written by morons who don't understand
physics who then dumb things down even further for their readership to
understand.

Many of the devices are first designed to work at 110V with no margin.
After being dropped a few times and a few ESDs punching through
insulation they don't stand up to 240V + maybe a simultaneous extra
discharge.

SSRs are a completely different kettle of fish. They can be very
quirky. The AC models behave differently from the DC models. The AC
models only turn off at zero crossing.  No matter what wonkapedia
says, they are not at all a solid state equivalents for their
electromechanical cousins.



On Mon, Jun 30, 2014 at 5:22 PM, Mark Beckett <m.beckett at amuri.net> wrote:
> Volker
>
> I'm sure someone as intelligent as you can see the difference between a USB
> phone charger and an optically isolated SSR.
>
>
> There is a picture here showing the outlet which would seem to explain why
> it occurred.
> http://rt.com/news/168772-faulty-usb-electrocutes-woman/
>
>
> This is a comment made here
> http://www.austech.info/electronics/92054-cheap-usb-devise-kills-woman.html
> (/including the  incorrect spelling/)
>
>    The switched mode transformer in these things is tiny. The only
>    thing inside them that is separating mains from you touching it is a
>    couple of layers of insulating tape and not even caplan tape. It
>    melts away easy with a soldering iron. There are no separate
>    chambers like there was with the good old 50Hz transformers.
>
>    The way these cheap chargers flood the market there is no
>    Fairtrading or any other department that can keep up with this.
>
>    Cheap or expensive, every switched mode PSU is a fire hazard.
>    A voltage surge (indirect lightning) can short a diode and the large
>    electro cap explodes on the AC mains voltage or they explode because
>    they fail as electro caps commonly do.
>    This happens instantaneously unlike a faulty old fashioned 50Hz
>    transformer that would just get hot and activate the thermal circuit
>    breaker or
>    the hair thin copper wire on the mains side would melt at a weak
>    spot and break the circuit. No fire.
>
> We had some switchmode psu's used on good brand monitors that had some parts
> get hot enough to melt the case, and in most others the case mountings
> became fragile and the whole thing fell apart exposing the internals.
>
>
>
>
>
> Yes there is a very remote chance that one day, the SSR may fail, but
> equally you may step onto the road and into a sink hole, be run over by the
> bus that had a COF hours before and then suddenly had no brakes, no steering
> and the driver had an unknown heart condition.
>
> Do you check every appliance in your house daily, weekly, monthly or yearly
> as you are running the same risk as you have cited with every electrical
> appliance you touch.
> Don't think that the fancy label on the outside is anything to rely on,
> since many of the 'known' brands contain the same parts used in cheaper
> brands.
>
>
>
>
> Of course for the truly paranoid, you could add a fuse and varistors across
> the low voltage side and from each leg to earth, which would limit the
> voltage to  12/24/48 volts whichever you want.
> This would result in the fuse blowing and rendering it safe.
>
>
> Mark
>
>
>
>
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