# How to calculate the effect of a resistor's TCR?

• posted

I'm trying to determine the power rating I need for a current sense resistor and am having trouble calculating the effect on the resistance due to the resistor's temperature coefficient. I want to minimize the temperature's effect on the resistance as much as possible (without having to go to a huge resistor, that is) to decrease any drift in my current reading as the resistor heats up.

If a resistor has a TCR of +/-20PPM/degree-C., is the effect calculated as follows?

- Assume worst-case 100C temp rise.

- 1PPM for a .010-ohm resistor = .010/1,000,000 = .01 ohm

- 20PPM would then = .2 ohm

- (100C rise) x (.2 ohm/C) = .00002 ohm rise in resistance

- Resistance at 125C (assuming 25C ambient) = .010 + .00002 = .01002 ohms

At 10A, that would only change the sense resistor's voltage from an "ideal" of 0.1000V to 0.1002, a change of .2mV.

Is the proper way to calculate this?

• posted

Micro-ohms

Ditto

Yes. But it's a lot easier to just say: 20 PPM per degree C times 100 is 2000 PPM, which is a 0.2 per cent error.

Current shunts are usually made of manganin. Manganin has an inverted-parabolic resistance versus temperature curve. "Resistance wire" manganin peaks near 25C, and "shunt manganin" peaks around 60C. Either is intended to have a flat, near zero tc, near its nominal operating temperature. So don't assume that the tc is linear; total error is usually less than 2000 PPM. Within +-15 deg c of the sweet spot, tc will typically be under 5.

I have some graphs and stuff if you're really interested.

John

• posted

LOL! Thanks John, I missed that. I mistakenly said "change" instead of "ignore" when the spell-checker flagged "uohm" as being incorrect...ended up deleting the "u". :-)

Definitely easier to remember it that way!

I'd be very interested, thanks! John

• posted

The resistor is the first thing to worry about; you also have mechanical stresses at attachment points to worry about (and even thermocouple effects) that won't necessarily be negligible. The mechanical mounting of your resistor should be symmetric (both ends at nearly same temperature) and some flexibility should be arranged for the connecting wiring (because the tempco doesn't include any thermal-stress allowance, and strain does raise resistance).

Finally, note that the sense resistor also has some series inductance, and your low resistance means even a low inductance can be significant. Calculate it, and think about the implications.

• posted

Here's some badly-photographed specs and test data on the two kinds of manganin.

ftp://66.117.156.8/Manganin.zip

Shunts in the milli-ohm range are usually manganin, although some low-ohm wirewound resistors may be nichrome sort of stuff, with different behavior.

The really good stuff is zeranin, but it's hard to come by.

John

• posted

Thanks John! At first glance, I'm able to read all of them. Time to do some learnin'. :-)

John

• posted

Thanks for the tips! I'll definitely pay attention to the mounting and wiring.

The resistors are being used in a very low-frequency application (current measurement for the output of a DC power supply) so I believe their inductance won't be a problem. Though, I guess at turn-off any inductance that is present will cause a voltage spike, though probably very low.

John

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