I'd like to test the current output of the USB port in my car. It's not labeled, and I can't find a spec.

My testing setup is VERY simple. I dissected a USB cable, cut the red wire (+5V), put a multimeter in the middle of the cut wire, and plugged the USB cable into the car and a (not fully charged) mobile charging battery.

In V mode, the multimeter shows 5.09V. In A mode, the multimeter shows 0A.

In addition to the first mobile charging battery, I also tried another battery and two old phones. Same result for all four devices on the multimeter, and none of the devices themselves indicated charging.

I grabbed another USB cable, plugged it in, and all the devices indicated charging immediately.

I guess my test cable could be bad, but... If the multimeter is showing 5V, that means a complete circuit. And if the circuit is complete, why aren't the batteries and devices charging? What did I mess up when I dissected the cable?

Thanks!

 

This is a common mistake made by the uninitiated to electrical circuits.

You cannot measure current like that, if I can imagine what you are doing.
You most likely blew the fuse in your multimeter.

A multimeter set to measure current, mA or A, is like a short circuit with low resistance.

When you switch the meter to measure current while the meter is still connected in parallel with a voltage source, it appears like a short circuit across the voltage source. The voltage souce will attempt to supply as much current as it can, limited only by the total resistance in the circuit. A typical DMM has a 350mA fuse (yours could be different - check the fuse or the user manual). Most likely the fuse is blown.

Open the DMM and check the fuse.

Come back and ask how to check the fuse.
Then come back again and ask how to measure current properly without blowing the fuse.

 

Mini, micro, A, B? Did you connect D+, D-

The micro-usb has 5 pins instead of 4. Did you use the 5th pin?

Interesting info here: https://en.wikipedia.org/wiki/USB_On-The-Go

 

Actually, contrary to what MrChips said, it sounds to me like you are trying to measure current (but not voltage) correctly.

If I understand you correctly, you cut the + wire between the USB port and the device and then connected one probe of the multimeter to one side of the cut wire, and the other probe to the other cut wire, which puts the multimeter in series with the charging circuit. Is this correct?

If so, you should be able to measure current that way, with the multimeter on the 10A range, not on the mA range, which would probably blow the fuse, which you might have done. On the 10A range, most multimeters use a different jack for the + lead, so the question is:

Did you use the 10A range? and
Did you change the probes to the 10A input?

Bob

 

Thanks for the replies. Turns out the 10A fuse is blown. I did not use the mA side, as I was expecting something like 0.5A - 2A, and my mA fuse is 440 mA. I also tested the mA fuse this morning, and it's fine.

The meter is in series with the charging circuit. I get what MrChips is saying, but in this setup, the current is "regulated" on either or both ends (car and device).

For KeepItSimpleStupid, this is a micro-B cable, and the only wire I cut was the red one. Per above, the meter is in series on the red wire. No other wires cut.

Will try to find a replacement fuse today and give this another go.

Thanks!