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USB power supplies: fire risk & testing

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Laz1

New Member
Hi. You know those ubiquitous USB wall power supplies? Well I have a bunch of them plugged in 24/7. (Not just for charging phones, etc, but also
to power homebrew projects.)

I recently was worrying about how often the cheap ones (well all really) flame out? How about under very low load? :)

This brings me to part two of my question: because of my worries I bought what were advertised on ebay as UL listed OEM chargers.
I have the feeling that they are counterfeit. Apart from cracking one open to look, would it be worth comparing the output on a scope to
a known OEM charger from the same company?

Thanks!
 

tcmtech

Banned
Most Helpful Member
There are medications that make these types of concerns far less concerning. :rolleyes:

If you are worried about the cheap stuff then don't buy the cheap stuff. :facepalm:
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
What do you expect the 'scope to show which would enable you to assess whether a supply is hazardous or not?
I suspect a comparison of supply temperature, for a genuine one and a suspect one, under the same load conditions, would be more informative?
 

bountyhunter

Well-Known Member
The really cheap ones are AC transformers with a rectifier and filter cap. To sell in the USA, they have to meet min safety standards. The more expensive offline switchers also have basic safety standards. If you get stuff from China that doesn't meet the approval codes, they could definitely be a risk.
 

ronsimpson

Well-Known Member
Most Helpful Member
The theory is that if they are marked "CSA, UL, VDE" they are tested.
China has both good product and bad product. There is too much counterfeit product. Some times a good product is copied and the UL mark is forged by people that do not understand safety.

DigiKey.com demands copies of the tests. EBay.com does not.
 

AnalogKid

Well-Known Member
Most Helpful Member
To your first question, the world consumes over 10 billion wall warts per year, and the failure rate for any of them, let alone legitimate ones, is very close to the mathematical definition of zero. The components are energy-limiting and the plastic is self-extinguishing.

Second question, of course there are clones, both legit and fraudulent. Using the small white cube Apple chargers as an example, legit clones are perfectly safe, but usually have higher output noise, and don't have an Apple logo. Price is a good indicator - *anything* with an Apple logo that sells for under $10 is bogus.

ak
 
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dr pepper

Well-Known Member
Most Helpful Member
It isnt just the charger that will go up in smoke, its your project too, maybe even yourself, having watched a couple of youtube vids on these thay have been know to put live mains on the o/p, you can imagine what that means.
Since seeing this I only use original ones or reasonably good ones, the only way of knowing that is to pull one apart.

Welcome here.
 

schmitt trigger

Well-Known Member
There was a YouTube video where some guy disassembled and studied several wall warts, some genuine, some cheap Chinese junk.

In addition to the lack of a fuse or other primary safety cutoff, the largest hazard the guy found the clearance between the "live" and output sections of the circuit. They don't meet creepage spacing by a wide margin, so you could get yourself electrocuted or damage the equipment being powered up.

So my advice is the same the other posters have already provided: purchase from reputable vendors only!
 

dr pepper

Well-Known Member
Most Helpful Member
That was big clive and his cheap pink charger.
 
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