So we've heard a lot of sensational stories about people dying from cheap mains->5V USB converters. What sort of corners do these manufacturers cut and do they even save much money than if they made a robust circuit?

I'm interested in circuit diagrams and their flaws. I just figured the standard switch mode supply was simple enough to copy so I don't know how they can get it so wrong. Do they just leave out fuses?


There is lots on the web on this, but the personal safety aspects can be largely summed up by - AC in becomes AC out, sometimes.

All countries have regulatory bodies which set standards or (more usually) adopt internationally recognised standards for the requirements for AC line powered consumer equipment. A significant proportion of the relevant regulations elate to the appropriate means of keeping AC mains where it belongs. Doing so costs money in design, manufacturing, testing and ongoing compliance. Cheap-junk makers do not care.

Some major areas where corners are cut are:

  • Bad insulation on transformers. Primary to secondary direct breakdown,arc over between layers on same windings causing overheating or fire and transformer insulation breakdown.

  • Underdesign in various areas causing thermal break-down leading to mains escaping its cage.

  • Bad clearances - PCB track to track, inferior board materials, lack of absolute mechanical clearances allowing flexure or creep or general aging changes to allow contact.

  • Poor isolation prim/sec side generally - this is a subset of bad clearances but is often chronically bad. Designing PCBs with regulation track to track clearances, slots etc takes an amazing amount of PCB space when things are tight (ask me how I know :-) ) and even designers who intend their equipment to meet standards will still push the design to the acceptable edge of the regulations. (The regs are invariably enough over engineered that this is safe - the main hazard is having the test lab reject your design on the last -.1mm lack of clearance.

  • Opto isolators, pulse transformers etc that do not meet real world isolation requirements. (It is common to not see AC mains "Y" capacitors equipped for filtering where they are required - but also to see capacitors used that do not have formal Y ratings. Y caps may be connected from phase and neutral to output ground. Failure of a Y cap will place mains on the output ground. If the ground is not grounded "it's all on".

  • Fusing - what that ? :-). Where fuses are fitted they may be OVER dimensioned - which is worse than being to small. A mains fuse not rated for adequate rupture capacity will in some cases allow an arc to form and maintain when the fuse wire is long gone. People have died due to this.

  • Mains leads - plugs that fall apart, cables that are too thin and/or not well terminated. Heating happens or a wire disconnects and moves ... .

  • Case insulation: Cases may crack, come apart without tools, or occasionally just for fun. AC mains does not care which failure let it out.

  • Accessible terminals: Terminals are meant to be proof from poking fingers. An uninsulated screwdriver will ideally (ast least) not be able to come near a live terminal on an unopened device.

    I have some LED power mains to 12VDC power supplies which I bought in a Shenzhen market which have helpful holes in the outer case that allows one to unscrew the low voltage terminals and insert or remove low voltage wiring and then to refasten the screws.
    The mains connectors on the supply have the same arrangement !!!!!!!!!!!!!!!!!

And more ...

In a market where reorder business is not expected and where the maker "does not give a damn" anything that seems to work initially may be offered, alas.


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