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I'm trying to diagnose a faulty car usb power supply. I can't identify one component however. I tried to take a picture

enter image description here

The part on the bottom left. Its light blue and the only thing on it is a big O. I couldn't get a better picture, unfortunately, as it is very small. It sits right between positive power and the resistor above it. I thought it might be a fuse or a diode. Seems not right though, when I use my multimeter in diode mode, it lets current through for a second or two, then stops. Switching the probes shows the same result, letting current flow for a while, then stops. So it might be a capacitor? It shouldn't be, though, as the only other connection from positive input to the rest of the circuit is through another 330uF capacitor, and the input is DC.

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    \$\begingroup\$ probably a jumper ... zero ohm resistor ... and it is open \$\endgroup\$
    – jsotola
    Nov 23, 2020 at 22:19

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The blue component is a zero-ohm link resistor. It seems to be used as a fuse. Since you measure something else than 0V over it in diode mode, the resistor fuse has blown open.

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  • \$\begingroup\$ Interesting. I would've expected to read nothing in diode mode for a blown fuse, similar to how I read nothing when the probes are not connected to anything. \$\endgroup\$
    – tkausl
    Nov 23, 2020 at 22:24
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    \$\begingroup\$ But the rest of the circuit migh be "something" instead of "nothing". That's why you can't measure resistances, capacitances, diodes etc in circuit, while they have other components connected to them. \$\endgroup\$
    – Justme
    Nov 23, 2020 at 22:27
  • \$\begingroup\$ Oh right, I forgot about that part... Thanks! \$\endgroup\$
    – tkausl
    Nov 23, 2020 at 22:28
  • \$\begingroup\$ Assuming its being used as a fuse makes sense because there's no other practical reason to otherwise separate the two sides of that trace. Nothing is going under it on one side and its unlikely something sensitive is going under it on the other side of the board. \$\endgroup\$
    – Passerby
    Nov 24, 2020 at 5:19

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