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Author Topic: Resistivity benchmark ONLY for talented Precision benchtop multimeter  (Read 754 times)

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Online Kiriakos GR

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Originally I thought to perform testing of contact resistance of new (four pin) Molex type connectors, the ones that every computer power supplies they come with.

A new Molex plug this includes four male or four female pins among plastic cover.
Price of single plug with pins this is very inexpensive.

Primarily I thought to measure resistivity (contacts resistance) from side to side which the cable this gets installed and this is two milliohm in a new and unused set of pins (male connected to female).

Then I thought to measure resistivity starting from cable installation point, and up to the center that is half distance and this now measured at one milliohm.
I do consider now these inexpensive Molex pins as one material that it can be used by anyone as Resistivity benchmark.

Benchtop multimeter that it can be benchmarked this is the one including 10 Ohms measuring range and also come with high resolution regarding digits.
( 6 ½, 7 ½, 8 ½ ) .
Four wires measurement, this is another requirement among with the use of Kelvin clips.

 
Bellow are test results of Fluke 8846A when this tested against HIOKI dedicated resistance meter this worth three times more than the multimeter.

HIOKI meter with on-board Comparator function this is able to make a reliable judgment down to 0.005% of resistor tolerance, RM3542A this does auto-calibration every few minutes, test leads resistance this is automatically zeroed, and this is something that a regular multimeter can not do.
 
RM3542A this translates to hassle free repeatability and measuring precision, because of it several internal circuits (Helper Functions) them operating as active guardians always awake and always present.

Now that all details they are explained, your may proceed to discover how your own benchtop multimeter this measure up.


Fluke 8846A this has precision specification for Ohms at 0.05% (12 months).
Specific DMM this is not calibrated for six years, even so it delivered measurement result which I do consider it as very respectable regarding calibration stability.
And it Resistivity sensitivity this is also amazing. 
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Online Kiriakos GR

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Few days ago I did take apart few laptop battery packages, so to salvage any still working Lithium cell.
And I discovered one special component soldered over the BMS board, this is a Current Sense Vishay Dale 10 mOhms R01F resistor.

1W (6.35mm x 3.18mm)  ±75ppm/°C  Operating Temperature -65°C ~ 170°C 

First thought which I did was to rescue it by transferring it at another PCB, then I did discover it tight tolerance and build quality characteristics, this is a high quality 1W Current Sense!
Then I did concluded that best use for such a resistor this is to convert it to another calibration inspection gadget of ITTSB Blog. 
It is known that surface mount components cannot be inspected with any sort of Kelvin clips.
Therefore I did solder it over copper contact points (spacers) and I did recover compatibility with  Kelvin clips. :)

This is now my first (free of charge - Quality Sample) for further more upcoming Resistivity benchmarks.   8)
   
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Online Kiriakos GR

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Here I am investigating the portion of damage at square pin Molex, this be widely used at CPU, PCI-E and PC motherboard harness.

The Male CPU connector this was extracted from a motherboard with damaged CPU pins in the socket, in other words this is a healthy part which we may baptize as Good REFERENCE.
Now the electrocuted female square pin this has received unknown portion of arching (in seconds), the rising temperature it did succeed damaging the specific Nylon sleeve at the 4Pin plug.

By inserting the burned female pin at the healthy male plug for a first time I got a value of 120 miliohm ...
At the second insertion the resistance it did stabilize at 10 milliohms.

The post above this is a valuable guide about expectations from a new unused round Molex, regarding contact resistance.

The burned pin it must be replaced because there is not anymore any positive expectation that this connection will be secure and arching will not be repeated again under load.
   
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Online Kiriakos GR

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Re: Resistivity benchmark MOLEX Micro-Fit 3.0 Crimp Terminal for 18AWG
« Reply #3 on: July 23, 2020, 10:04:27 PM »
Now it is time to explore Molex actual specifications of  contact resistance.

Unfortunately their testing procedures about measurements of contact resistance they do not make any sense to me.
They mention test under 20mV (as voltage limit) and 100mA as testing current ?
It makes you wonder of what testing equipment they use.

https://www.molex.com/pdm_docs/ps/PS-43045-001.pdf

https://www.molex.com/molex/products/part-detail/crimp_terminals/0430300038
   
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Online Kiriakos GR

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Re: Resistivity benchmark MOLEX Micro-Fit 3.0 Crimp Terminal for 18AWG
« Reply #4 on: July 23, 2020, 10:49:16 PM »
This is the ITTSB Blog aha moment, this time under the microscope there is the nearby healthy pin.
To be honest I would not expect the contact resistance to be equal with the round pin of MOLEX, simply because at the larger in diameter round pin there is much more contact surface than the narrow Micro-Fit 3.0 Crimp Terminal.

Either way I did manage to confirm 2.5  milliohms :)   
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