Some time ago, I purchased the FNB58 USB multimeter from FNIRSI with the intention of testing some of my USB cables. Ideally, I wanted to throw away those that are redundant and/or those that pose a safety hazard.
Using ChatGPT and a few Wikipedia articles as my guides, I decided on the following program:
- I would first test the resistance of the cable in ohms.
- I would then compare the difference in voltage, current, and power.
- Finally, I would check whether the protocols supported by the charger carry through the cable.
The first thing I discovered was that there were basically two types of cables - those that came with electronic tools and did not allow the protocols to propagate, and those that came with smart devices that did. The difference in resistance could vary wildly - one cable I tested had a resistance just short of 300 milliohms while another had about 2 ohms. For all of my tests, I used a 5 volt charger with an attached device that consistently drew 0.5 amps - increased resistance in the cables appeared to affect the voltage, but not the current.
Here’s where I need help: At what point do I decide that a USB cable is faulty and poses a hazard? Is there a resistance threshold I should enforce? Also, is there a way to measure data integrity?