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So I am dismantling and ripping into a cheap Chinese counterfeit phone charger and revealing all that I see wrong with it. One thing I could do with some insight into though is the purpose of a resistor.

It is basically a 3.3Ω resistor which is in series with the mains (240V) input into the bridge rectifier:

enter image description here

What are they trying to achieve there?

(Mind, given the state of the rest of the circuit maybe even the designers don't know?)

Should it in fact have been a fuse?

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  • \$\begingroup\$ What is the Issm rating of the bridge rectifier? This could be a surge current limiter to "protect" the bridge. Whether it's an adequate one is another matter. 35A * 3.3 ohms plus stray resistance isn't too far off 110V (arguably not 165V, nor 230VAC) \$\endgroup\$ – Brian Drummond Aug 30 '16 at 17:47
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    \$\begingroup\$ Was it marked 3.3 Ω or did you measure it? If you measured it could be a self-resetting fuse. \$\endgroup\$ – Transistor Aug 30 '16 at 17:48
  • \$\begingroup\$ @BrianDrummond \$I_{FSM}\$ is 35A. It's an MB6F \$\endgroup\$ – Majenko Aug 30 '16 at 17:49
  • \$\begingroup\$ @Transistor Both: electronics.stackexchange.com/questions/255091/… \$\endgroup\$ – Majenko Aug 30 '16 at 17:50
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Some resistors are designed so that they will fail open when they overheat. An advantage of such resistors versus fuses is that when failure is a result of an output short-circuit, the resistor will limit how much current will pass in the time it takes to blow. With 120V mains (170V peak) the peak current would be about 50 amps, which is rather a lot, but is still much less than would be able to pass through a "normal" fuse.

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    \$\begingroup\$ You did say "cheap Chinese counterfeit phone charger". It's not like safety is high on their list of priorities. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 30 '16 at 17:48
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    \$\begingroup\$ @IgnacioVazquez-Abrams You should see the PCB layout and soldering... Shocking (potentially literally...) hackingmajenkoblog.wordpress.com/2016/08/30/… \$\endgroup\$ – Majenko Aug 30 '16 at 17:51
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    \$\begingroup\$ I'd not want to specify a resistor here unless there was an "all-fire" specification for it. Something like, "100 Joules delivered in 20ms" guarantees an "open" to occur. \$\endgroup\$ – jonk Aug 30 '16 at 18:11
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    \$\begingroup\$ +1 for super. @Majenko Pretty nasty, but I've seen even worse in the $1 fake iPhone chargers. Usually the touch screen problem is from a missing Y cap, but that appears to have it. Dangerous in any case. \$\endgroup\$ – Spehro Pefhany Aug 30 '16 at 18:13
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    \$\begingroup\$ @Majenko: Given the thermal mass of the resistor, it wouldn't be 45KW for very long; the critical question is what happens when the resistor does overheat as it certainly would. I'm somewhat puzzled by why the resistor has such a low value since a 3300 ohm resistor would dissipate 0.03 watt at 10mA but would limit peak fault currents to a much more tolerable ~1A. That would still be 200W until something failed, but one could be more assured that the fault power would be confined to the resistor. \$\endgroup\$ – supercat Aug 30 '16 at 18:53
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it is for current limit to replace NTC+fuse. It is a common design for 5watts charger. The resistance will effect efficient, the value will be adjusted by different design. The common value could be 3.3/5.1/8.2/10 ohm.

the resistor name Fusible wirewound resistor.

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