Some components are relatively unimportant in their effect on overall circuit behavior. Sense resistors are not one of them. It may only be a resistor, but it's a very important resistor at that, with subtleties that can impede design progress.

Sense resistors are usually power resistors in that they dissipate non-negligible amounts of power and are typically rated at 0.5 to 5 W. At the same time, they must hold their resistance value, for it contributes proportionally to the current measurement. Temperature coefficient (TC) of resistance is consequently of high design importance. Using a clipped length of copper wire, for instance, will demonstrate this point. In a motor drive, the torque will fall off as the motor operates due to the increasing resistance of the copper sense resistor, as it heats. Wire made of alloys with low TCs are available to be used instead.

But sometimes a copper sense resistor, whether as a discrete wire resistor or a circuit-board trace, can be used to advantage. The TC of copper is fairly constant over a wide temperature range, and its positive TC of about +0.4 %/°C can be used to compensate for the characteristically negative TC (–2.2 mV/°C) of a semiconductor junction elsewhere in the current control loop.

One simple approach to sense resistors is to make your own out of a low TC conductive material. Manganin wire, an alloy of copper, manganese, and nickel, has a low TC of within  15 ppm/°C from 0 to 80 °C. AWG #18 manganin wire has a resistivity of 0.361 W/m. Smaller diameters are available and the wire can be bought by the roll. Cut the wire to length for the desired resistance, tin the ends, and solder into the board. A manganin resistor is shown below (left). By keeping the half-loop area small, inductance is minimized. The one shown has a resistance of 25 mW.

If you do not want to make your own, the resistors shown (right, front and rear) are commercially available power resistors made of a low-TC metal foil on an anodized aluminum substrate, and in TO-220 or TO-247 packages. Several companies, such as Caddock Electronics, now supply these accurate, low TC, power resistors at an attractive price for power-circuit design.

Another commonly-used low-TC material is nickel-chromium, or nichrome. It's resistivity of 133 μW-cm requires less wire length than mangagin's 43 μW-cm, which can reduce inductance for very low-value resistors. Manganin, however, is superior to nichrome in TC and long-term stability of resistance value.

If future circuit-board fabrication technology allows a wider range of substrate materials (than copper), thin-film power resistors can be integrated onto the board during layout. With clever circuit design, even copper traces can be TC-compensated with BJT junctions.

Series parasitic resistance is also a nuisance for low-value resistors. It is necessary to sense across the actual resistance of specified value and not some of the component lead resistance in addition. To solve this problem, 4-wire, or Kelvin, sensing is made possible with 4-wire resistors, such as those shown below.

The leads are attached internally to the desired resistance and are brought out of the package in pairs.

Kelvin-sensing resistors are also available in foil-on-substrate form, as shown below. (The quarter is for size comparison.) The smaller, inner leads are 4-wire Kelvin sense leads while the wider, outer leads are the drive leads.

Current-sense resistors for power electronics must be of low TC, relatively high power rating, and accurate at low resistance values. Such resistors are commercially available, in multiple packages, and at moderate prices. If prices are too high for a given application, consider making instead of buying sense resistors out of manganin wire, or for the lowest cost, use a copper circuit-board trace compensated by a silicon junction.

Ó Dennis L. Feucht, 2000