[chbot] Why galvanic isolation is important
m.beckett at amuri.net
Mon Jun 30 06:22:17 BST 2014
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.
This is a comment made here
(/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
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
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
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.
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