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| Test Equipment Recommendations for Measuring USB Cable Resistance |
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| reborn1212:
--- Quote from: Henrik_V on July 21, 2022, 12:25:42 pm ---While getting a 4W (milli) Ohmmeter is easy , you want to measure the cable , not the connector ;) While applying a 4W measurement on a USB C connector with it's build in Kelvin contacts , connecting a USB A/B (mini/micro/...) pin with two contacts will need some DIY connector mechanics. Not impossible, but assuming some 100 cycles not an easy part. Another testing method: Apply more current and apply a thermometer (FLIR) >:D --- End quote --- There are USB C connectors with built-in Kelvin contacts?? |
| alm:
I don't have a datasheet handy, but I'd think that a general purpose bench meter with its low test current won't be very stable or accurate in the dozens of milliohms range. With the low voltage across the DUT noise and thermal EMFs might become an issue. Dedicated milliohm meters, or solutions with a current source and sensitive voltmeter (SMU, separate current source), can mitigate this with a higher test current and by either switching polarity of the current source or pulsing it on and off. --- Quote from: reborn1212 on July 20, 2022, 01:18:14 pm ---On a side note: Using USB breakout boards inevitably adds significant error when measuring. Any advice on how to mitigate this? --- End quote --- I think the best you could do is design your own breakout board with Kelvin connections right from the connector pins. This doesn't eliminate the resistance of the connector but does eliminate the resistance of the breakout board and connections. You don't need 4W probes. Any way that separates current carrying conductors from voltage sensing conductors will work equally well. |
| Someone:
--- Quote from: alm on July 22, 2022, 05:40:49 am ---I don't have a datasheet handy, but I'd think that a general purpose bench meter with its low test current won't be very stable or accurate in the dozens of milliohms range. --- End quote --- Just read the data sheets and pick one that does.... 34461, 100Ohm range, 1mA, +/- 4mOhm error DMM6500, 1Ohm range, 10mA, +/- 0.2mOhm error Its getting to the corners of "normal" but still pretty comfortable. |
| alm:
--- Quote from: Someone on July 22, 2022, 05:57:01 am ---34461, 100Ohm range, 1mA, +/- 4mOhm error DMM6500, 1Ohm range, 10mA, +/- 0.2mOhm error --- End quote --- So for 10 mOhm, the uncertainty would be +/- 40% for the 34461A. And the DMM6500 2%. So the 34461A is clearly not sufficient for dozens of mOhms with 2% accuracy, and the Keithley barely. The Keithley has offset compensation and a higher test current, so could probably barely do this measurement. But a dedicated milliOhm meter will likely be better. I have a Ballantine 3205B (up to 1A test current) and Keithley 580 (up to 100mA test current, thermal offset calculation). The Keithley would measure 10 mOhm with 0.24% uncertainty. I'm not sure about what is currently on the market. I think Keithley wants to sell you a current source + nanovolt meter. |
| Someone:
--- Quote from: alm on July 22, 2022, 10:22:11 am ---So for 10 mOhm, the uncertainty would be +/- 40% for the 34461A. And the DMM6500 2%. So the 34461A is clearly not sufficient for dozens of mOhms with 2% accuracy, and the Keithley barely. The Keithley has offset compensation and a higher test current, so could probably barely do this measurement. --- End quote --- The OP asked for 2% in "few dozens" milliohms, and there is at least one bench meter that gets there. That's not barely, that's guaranteed within worst case. All that was needed was a quick perusal of data sheets. So this: --- Quote from: alm on July 22, 2022, 05:40:49 am ---I don't have a datasheet handy, but I'd think that a general purpose bench meter with its low test current won't be very stable or accurate in the dozens of milliohms range. --- End quote --- Pure guessing, and shown to be wrong (also with an example of one bench meter that is poor in the same range despite having 4 wire mode). All it takes is picking a suitable bench meter. The bigger challenge is probing/fixturing to isolate the actual characteristic wanting to be measured. |
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