avr current measure
Sinisa said:
No, you didn't do 4 wire method because you attach your electronics with only two connections on J1.
Ahh, I see what you mean. I've ignored the cables from J1 to the wire as the resistance is negligable. (~ 0.1 ohms versus 15 ohms for the hot wire).
I would suggest using wire that has as high tempco as possible, because it makes it so much easier to measure. Ni would be good material I presume.
I'm constained to using something fairly strong to survive the tension on the wire. I'm currently using 0.5mm stainless steel wire, which is a bit over 0.003 ohms per degree.
You should also work backwards, set your desired tolerances, than from that you can calculate what is your error budget and see if you fall within those parameters.
I've shown my calculations at
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The key number is that the voltage over the current sensing resistor will vary by about 40uV per degree.
I've never had to do design around these sort of numbers and I'm worried about the noise levels. As you can see, I've put 20Khz low pass filters on both the voltage and current measurements and I'm now using precision op-amps (~ 0.8uV offset, ~ 130dB CMRR) but I suspect there's a bunch of things I should be worried about that I'm missing.
The critical path is obviously around I2C which comparing the output of the low-pass filtered PWM from the CPU with the current sense voltage and then amplifying by 200x.
I'm worried that the 3 stage low pass filter won't have low enough ripple, and I'm worried that there'll be noise ingress that that I haven't acounted for.
My gut tells me that you will quickly run into problems. Running PWM and trying to do measurement at such low signal will quickly turn into NASA project. You did not specify what is ppm/K of that wire? Work in steps, don't try to come up with solution right away, because you will likely run into corner otherwise.
More details of my working out are on the wiki above. I'm planning to PWM fairly slowly (the thermal inertia of the wire is relatively high, so a 50Hz or even 25Hz PWM should be ok).
I was intending to run a fairly simple control algorithm: Turn on current, and sample at 5Khz until the wire resistance reaches a given value, then turn off the current for a fixed duration. This is actually variable frequency modulation rather than pulse width of course.
Current circuit is:
**broken link removed**