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I've installed new car audio into my car. It works fine for a few months now. There is a USB port on the car audio for charging. Before actually using it, I wanted to test if it is safe enough and outputs about 5V.

So without giving a second thought, I grab my multimeter and set settings for direct voltage measurement. As I touched USB port ground and +5V pins, there was a pop in speakers and car audio shutted down, which pretty much indicated a short circuit.

After the incident I checked youtube video on how to measure USB port with multimeter. In the video, man used small metal pin between usb port pin and multimeter positive probe. (https://www.youtube.com/watch?v=xvSg1RiYDKQ)

So, as I understand, small metalic pin plays a role to reduce current? But how come measuring car battery or wall socket is fine by using multimeter probes, but not with USB port? Why short happened?

I have little knowledge about electronics and probably shouldn't stick multimeter probes in things, but curiosity took the best of me. Would appreciate any help or links to understand the issue.

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  • \$\begingroup\$ Did your multimeter probe accidentally touch the metal cover of the USB port? \$\endgroup\$
    – crasic
    Commented May 16, 2021 at 4:56
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    \$\begingroup\$ I cut the bad end off of a USB cable and added 2 banana plugs, I plug this into my meter and the other USB end into the USB port. It adds another test cable but surprisingly I use it more than I thought. It also works for powering small 5V gadgets for testing. \$\endgroup\$
    – Gil
    Commented May 17, 2021 at 22:38

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As mentioned in the video, the outer metallic, rectangular casing is connected to the ground. So inserting the thick lead of the multi meter to touch the positive supply will cause the lead to short the positive supply and the outer casing which is ground.

The thin metal pin is used only to ensure a gap between the pin and the outer casing while it is touching the positive supply. It is is not a mistake proof method. If that pin leans too much, it can still short the supply and the ground. The pin may not limit the current.

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  • \$\begingroup\$ Thank you very much! This seems to be an issue. I've probably skipped the explanation part in the video. \$\endgroup\$
    – sverek
    Commented May 16, 2021 at 5:03
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To test tiny things like that, I will often (as in almost always) make a break-out cable.

Get an old usb cable and cut the end of with about 10cm or 4 inches of cable. Strip the outer covering off and then separate and bare the wires.

Often I will put them into a terminal block with labels if the colors don't indicate what they are (usually red is +, black - and the other two data but do check).

That way I reduce the chances of shorting things, but I did my apprenticeship as a vehicle electrician, so making and using things like that is a habit.

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  • \$\begingroup\$ I've tried your method and it worked perfectly. Exactly what I needed. Thank you for the tip. \$\endgroup\$
    – sverek
    Commented May 30, 2021 at 13:18
  • \$\begingroup\$ @sverek if it worked then give it a vote :) \$\endgroup\$
    – Solar Mike
    Commented May 30, 2021 at 13:20

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