steinie44:
DO NOT USE SWITCHING power supplies. There is no transformer in them. That is very dangerous.
That is of course, complete nonsense. If anything, the opposite as the transformer in a switchmode supply being smaller, has a smaller area of insulation and could be argued to be on that account, less likely to fail.
If of course, it is properly designed and constructed. The strict requirement of these two-pin appliances is "double insulated", so there must be two separate and complete layers of insulation. One fascinating problem is whether the feedback opto-isolator is technically compliant with this.
michinyon:
Looks like a carefully covered up murder to me.
Hmm, I suspect you have been watching too much television!
Anyway the article was on TV last evening, my wife as a matter of course said to me "Do you have any of those?" to which I replied along the lines of "only about half a dozen". From eBay of course, costing little over a dollar each. At $4.95, those local sellers are making a good profit.
Not sure what to do with them. Use an isolation transformer I suppose!
cjdelphi:
Many of the cheap power supplies rely on a capacitor to limit current from mains voltage, yesterday someone on died after the phone charger failing and sending 120/240vac into her phone...
You seem not to understand what a SMPS is. Perhaps you should study up on it. It does not "rely on a capacitor to limit current". What is however interesting, is the popular type of LED light globe which I had (innocently) presumed to contain a swithchmode device like the CF globes. I have now realised that they must use a capacitor as a "ballast", with a bridge rectifier and as many LEDs as possible all in a series chain to use the minimum current at the maximum voltage.
cjdelphi:
What's the easiest/best way of stopping this? I want to sit a circuit between a cheap power supply (5v out) and your phone/ device, if it goes over 5v kill it.
That is of no use, since the problem is not the differential voltage between the output wires, but the possibility of the mains voltage being imposed as a common mode on both due to insulation failure.
cjdelphi:
But will a dose of 120/240v also kill the protection circuit.
It's not a protection circuit. What is required is insulation.
cjdelphi:
TVS diodes?
MOV?
cjdelphi:
I'm not sure... if it's possible without some kind of isolation? (Transformer) but if so that defeats the purpose of cheap power supplies!
And indeed, that is the only answer to a faulty manufactured device - in this case.
rogerClark:
In which case the exact problem has not been determined i.e the article reads
It is believed a dodgy $4.95 phone charger sent a high-voltage electrical pulse into her phone, which transferred to the earphones she had connected to a laptop.
Note the word believed
I don't think they know for sure.
If indeed she had burns on her ears and on her chest, that would certainly suggest something.
rogerClark:
And I'm not personally convinced that the diagram in the article is correct, as it implies that somehow the laptop is the return path to earth.
If the laptop has a three pin plug (as most seem to) you will find that the earth pin is common to the negative output. Why? Because certification on the power supply is easier if it does not have to be "double insulated". This arrangement is safer, albeit a complete menace if you are doing any sort of audio work.
Added:
BillHo:
Only some of the cheap power supplies rely on a capacitor to limit current from mains voltage, if you buy one of these cheap power supplies then it will be dangerous.
That does not make sense. Any such power supply which was not isolated with a transformer would electrocute everyone using it - at least 50% of the time.
Reminiscent of the old "transformerless" radios in the days of valves.
BillHo:
You can always open up these cheap power supplies to find out yourself, how it wire up.
Looking into that ...
