Cheap precision sense resistor

Question

I need a precision low value sense resistor to make current measurements e.g. 10R 0.01R. Looking for precise resistors e.g. Vishay, show that these are expensive e.g. $35 for a single 0.001-0.005% tolerance resistor with 10ppm/K thermal coefficient.

Too much for a hobbyist like myself. What cheap workarounds can I use instead?

Currently, I'm thinking of measuring several 1% metal film resistors and using that, but not sure if there is a better option e.g. some other material or whether there is a trick to reduce the temperature variation?

Answer 1

Make your own. I have seen multimeters where it's clear that the shunt resistors have been adjusted after the meter was assembled. They start with a coil of resistance wire of slightly too low resistance, then file away part of the wire to bring it into calibration.

Answer 2

There are a few truths that need to be followed in component selection.

1. The accuracy you need cannot be less than the tolerance of your test and measurement equiment to verify the results.

2. The minimum tolerance values and minimum Ω values have a limited product commercial range of about ±10ppm of 1Ω = ± 10µΩ

   • i.e. ΔR/R*R=ΔR = ± 10 µΩ
   • The laser trimming time increases with added cost as tolerance ratios reduce.
     • for a 1 mΩ shunt , you can find a part with ± 1% or ± 10 µΩ uncertainty (and certainly not ±0.005% )
     • for a 10 mΩ shunt, you can find a part with a 0.1% uncertainty (again not 0.005%)
     • for ±0.001% tolerance it seems the smallest value @ D-K is 1kΩ which has an uncertainty of 10 ppm or ± 10 mΩ which is a big jump in R value.

You can make a non-inductive shunt with folded twited pair magnet wire of desired gauge to yield a shunt or a U shaped stamped copper conductor used for current sensing on PTH technology boards.

• To minimize temperature rise, the rated power must be low, such as a 50mV shunt depending on size and forced air.

• Your measurement system must have very low noise and high resolution to make use of this required accuracy of >16 bits

Making a shunt is not hard. Figuring out how to calibrate it is your 1st goal.

I was once tasked to make a 100kA current sensor on a Diffusion Welder, I just inserted a 10A DC current limited power supply thru the solid copper arms to measure 10 µVdc drop and placed self-tapping screws into those locations for the wire pickup on shielded twisted pairs at right angles.

Answer 3

Consider approaching this problem differently.

Rather than brute forcing your way by trying to source extremely precise and stable (thus expensive) shunt resistors, settle for a middle ground constituting of medium precision and medium stability.

If the projects you are doing involve some kind of digital microprocessor responsible for measuring or producing a digital readout then your in luck. If not consider using one! Software can fix this to a large extent.

Out of the box inaccuracies in the resistor are not going to be a problem. You simply need to measure it to the required precision and apply the corrective action in software.

Now temperature stability is of more concern because it does not involve a simple one point fix and can vary across resistors. However, as you say you are a hobbyist this should also not be a problem.

Start by measuring the drift of your resistor and tabulate a list of Power Dissipation (,temperature,current) vs Drift. Feed that data into a spreadsheet program like Excel, perform regression, extract the coefficients and apply the corrective action in software!

You can get amazing accuracy and stability with this method without spending a fortune!

EDIT:

Here is an slightly unrelated example of a multipoint type calibration i did recently to illustrate the results of calibrating to get your way out of non ideal components. In the example Im using an ADC that exhibits substantial drift at higher voltages measured:

Here is the same ADC after I tabulated the error and performed regression and the software fix: