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doing a PCB resistor

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Simon:
I'm trying to sense current 20ish amps and have been using a 1 milliohm resistor although I think I can afford to take it to 2 mR. now I also wanted the current limit on the circuit to be a little temperature dependant so that in warm weather the value would tend to be higher than it really is letting less current through and in cold weather it will be low and let a bit more current through. After poking about some ideas I had a thought: resistors usually have a thermal spec as well, they will normally increase resistance slightly when warm, then it hit me: why not make the resistor out of PCB ? in fact copper has a 0.393% increase in R/C so might just work.

Now before I go thinking myself really clever here perhaps somebody can tell me if I'm mad and how I may come a cropper on this one

alm:
I wouldn't expect very good tolerances, the variation in trace width and copper weight is probably a lot more than the 0.4%/degC tempco. Plus .4%/degC is a really lousy tempco, can't imagine that 'real' current sensing resistors are any worse. Make sure to use Kelvin sensing, since resistance between the voltage sensor and the shunt is likely to exceed 1mOhm. For reliable results, I would prefer real resistors. If you don't care about accuracy and can cal variation in resistance out, it seems to me that it would work fine. Dissipating 20mW shouldn't be an issue either, I would try to make the trace fairly long and wide, to make variations less significant. But I've never done it either.

marianoapp:
DN-71 Using Copper PCB Etch for Low Value Resistance

Simon:

--- Quote from: alm on August 12, 2010, 05:17:24 pm ---I wouldn't expect very good tolerances, the variation in trace width and copper weight is probably a lot more than the 0.4%/degC tempco. Plus .4%/degC is a really lousy tempco, can't imagine that 'real' current sensing resistors are any worse. Make sure to use Kelvin sensing, since resistance between the voltage sensor and the shunt is likely to exceed 1mOhm. For reliable results, I would prefer real resistors. If you don't care about accuracy and can cal variation in resistance out, it seems to me that it would work fine. Dissipating 20mW shouldn't be an issue either, I would try to make the trace fairly long and wide, to make variations less significant. But I've never done it either.

--- End quote ---

0.001 R * 20*20A = 400 mW with 2 mR it will be 800 mW

Zero999:
What about making the trace slightly wider than you need and drill a hole in it?

To trim, you increase the size of the hole slightly using a countersink, until the resistance is just right.

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