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Temperature compensated current measurement

Hello,

I am currently looking for a way to measure and control the current through a resistive load. I found same standard circuits often used for voltage/current conversion but all of them are temperature sensitive because they simply measure the current through an additional sense resistor. This does not provide the desired accuracy over the entire temperature range. For example if I use a 50ppm resistor and a resistive value of 100R I have about 0.25R over a temperature range of 100K. This is 0,25% which allows maybe 8-9Bits of resolution. Does anybody know a good solution for this problem or can provide some hints on that?

Kind regards, Christian Walter

Reply to
Christian Walter
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"Christian Walter"

** Easy - don't let the resistor get hot.

Use a wire wound type ( typical tempco +/- 30ppm), mount it on a heatsink.

If its temp don't change, neither will its resistance.

Surely you are not subjecting the whole circuit to temps of 120 C ???

........ Phil

Reply to
Phil Allison

What's wrong with temperature compensating the data? Anything is possible with digital data.

Reply to
linnix

The answer is fairly simple - spend more money on your resistor.

Farnell stocks a number of Vishay bulk foil resistors offering temperature coefficients of +/-0.2ppm per degree Celcius. Look at the

100R 0805 part (order code 110-924) the 1206 part (110-9037) or the through hole part 120-3343 - the last only offers a temperature coefficient that is less than 0.6/ppm per degree Celcius form 0 to 60 Celcius. They cost around 15 euro (say $20) each.

Farnell also stocks several 15ppm metal film resistor - the MMA0204 range has a 100R part (order code 308-5703) which is very cheap, while their E96 ranges of axial lead resistors include a couple of 100R parts for about one euro each.

If you are doing seriously precise work you could go directly to Vishay or other specialist suppliers to get four-leaded (Kelvin connection) resistors.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

One time I made a current sensing resistor so small*, I got worried about noise from amplification.

*To reduce self-heating, to reduce R drift.

D from BC British Columbia Canada.

Reply to
D from BC

I care more about the environment. The power dissipated by the resistor is very small. Certainly less than 125mW. Of course I don't want to subject the circuit to 120C. And you are right. If I calibrate the device at 20C the whole spawn is only 0.125R if I go up to 70 and down to -30 using R = R0*(1+alpha*delta_T).

Regards, Christian

Reply to
Christian Walter

"Christian Walter"

** I think you have misread the spec for resistor tempco.

The figures are all " +/- xx ppm " = not just plus or minus.

You would do well to get some sample components and see just how the value varies over your required temp range.

The actual variation may well be no where near as bad as you assume.

....... Phil

Reply to
Phil Allison

If it is really, really critical you could pick the lowest tempco resistor money can buy and then peltier-cool that. But be prepared to shell out close to $100 if you want a TE controller module that is ready to use (meaning no design work there) out of the box.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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