5 mohm resistor

fredag den 13. maj 2022 kl. 01.50.45 UTC+2 skrev John Larkin:

Why bother if there are ready-made four-terminal devices available at Mouser? I am happily using its 500uOhm version.

Best regards, Piotr

We do want giant pads in big solid copper pours, without thermals, to heat sink these resistors.

And we can adjust our scalings for any measured systematic error.

5% accuracy would be OK in this application. We could calibrate each resistor if we wanted to, but it's not worth the hassle in this case.

The resistors each drive an INA281, on the +48 bus. We measure the ca

20 amp current from the power supply, and the current into each of the 8 plugin boards.

We have that part in stock. At about 10 amps per resistor, we need a part with big pads, for good electrical and thermal behavior.

Pseudo-Kelvin contacts to 2-terminal resistors, or arrays of parallel resistors, seem to work very well.

This ain't bad:

It should be even better on a proper PCB with pseudo-Kelvin between-the-pads connections.

lørdag den 14. maj 2022 kl. 01.44.41 UTC+2 skrev John Larkin:

how did you measure it?

Lascar power supply, short-circuit current 1.27 amps, and my Fluke benchtop DVM probing the two red dots.

It's a 1% resistor, and a pseudo-Kelvin connection should get me close to the resistor tolerance.

I wonder how they connect to them while they are trimming/machining the 5m value.

lørdag den 14. maj 2022 kl. 16.37.13 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

on the second one you could directly get a measurement of the difference by comparing the voltage on the red dots and on the two "main" connections

You mean to measure where the end wires are soldered?

The "1 oz" copper is about 580 uohms per square, so that would add 2 mohms or so.

I could dremel/xacto a bit better and get more Kelvin-ish.

søndag den 15. maj 2022 kl. 02.09.41 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

no, same place as the dots on the first one

Both measurements were on the dots.

søndag den 15. maj 2022 kl. 03.49.17 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

compare measurements at red and blue dots

I could do that Monday. The blues will obviously be more voltage drop.

søndag den 15. maj 2022 kl. 07.10.14 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

sure, the idea is that it'll directly show you how much it actually matters

My first test did that. 5.41 mohms.

Here are some better measurements.

When I get the actual PCB, with the connections between the pads, it might be better. At any rate, I can use a 2-terminal 2512 5m resistor in my gadget, no problems.

mandag den 16. maj 2022 kl. 19.22.37 UTC+2 skrev John Larkin:

yeh, 2% off with a 1% resistor and just a bit of thought to the layout isn't bad

I calculated that the resistance of the solder, from both 2512 pads to the PCB copper, is around 15 uohms. Tiny.