It can be very important to the proper functioning of an electronic device to know how much current is flowing into or out of a node in a circuit. Often, the easiest way to determine this is to impose a resistor in the pathway of the rest of the circuit to that node. There will be a voltage drop across the resistor that can be measured, perhaps with an op amp. The circuit’s microcontroller can then compute the current by Ohm’s law, I = E/R. So as not to either waste power, or to alter the function of the circuit, that resistor—the current sensing resistor—must be exceedingly small, generally in the order of hundredths (0.01) of ohms.
This method is most often used to measure very small currents, so the amount of power dissipated in the resistor will be tiny. The difficulty with this method is that for resistors this small, the resistance in the wire leads to and from the component is actually significant compared to the resistor itself, and there is a likelihood that it will be enough to throw off the measurement.
If, for example, the stated value of the resistor is 0.001 ohms, the added resistance in the leads may be an additional 0.002 ohm. In this case, the system believes it is reading the voltage drop across 0.001 ohms, but it is, in fact, reading the drop across 0.001 ohms plus 0.002 ohms, or 0.003 ohms. Because of the inverse relation between current and resistance (I = E/R), the value of the current computed is much greater than the actual value.
Why Specialized Resistors are Needed for Current Sensing
The way around this dilemma is to have an extra pair of leads, specifically devoted to voltage measurement, in addition to the original two leads used to carry current into and out of the node.
These extra two leads are the ones that are used to measure the voltage drop. And, because the op amp—or any other device that reads the voltage is assumed to have a very high input resistance—the voltage drop across this second pair of leads will be negligible, and the actual value of the current can be accurately computed. This specialized resistor configuration, with four terminals rather than two, is described as a Kelvin Connection.
Ohmite’s LVK series of four-terminal current sensing resistors are available from Arrow Electronics. The datasheet reveals a series of surface-mounted resistors. The illustration below reveals the four separate pads, two for current, and two for voltage measurement.
Members of this series are available over a range of power dissipations, resistance ranges and tolerances. These devices have solid temperature coefficients despite the sub-ohm resistance values they offer.
It’s hard to get one’s hands around the concept that the voltage drop across a small length of wire lead can throw off a current measurement, but with the tiny resistance values employed for the purposes of current measurement, the effect can be quite pronounced. Fortunately, designers have an inexpensive way to avoid the difficulty.
