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In general, USB recommends that the maximum capacitance placed on the end of the device is 10uF, in order to reduce the "inrush current" when the device is plugged in. While it is possible to use special circuitry to reduce the current drawn, some products somehow manage to use large caps with a simple fuse or ferrite.

For example:

PupDAC (C22 is a 1200uF cap!)

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Sparkfun USB supply (100uF)

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How can these devices, and many others (which should work as they are sold commercially) manage to place a large cap on the USB input without any problems?

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For the same reason that many devices ignore the requirement to enumerate and ask for more than 100mA of current.. because they can get away with it (most ports will work okay even with a big capacitor on there) and since there are lots of devices that ignore the 10uF limitation, then the computer designers have to make their ports work or face a lot of complaints from users about their crappy USB toys not working.

Your assumption that these work "without any problems" may not be correct. They could have problems with a minority of available host ports and not be overly concerned about that.

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  • \$\begingroup\$ Well this probably explains why my ability to achieve reliable USB functionality when using many devices is limited at best. +1 \$\endgroup\$
    – horta
    Commented Jul 14, 2014 at 18:03

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