Precision resistance measurement - small resistance test

When the current to be measured is large, the resistance R value is often required to be very small to avoid negative effects such as excessive voltage division and resistance heating. At this time, the sampling resistor selected may only be milliohm or even microohm level, and the resistance error directly affects the current measurement accuracy.

Measuring ultra-large resistors is not easy, and measuring tiny resistors is equally difficult

With the application of high-power devices, such as photovoltaics, wind power, electric vehicles and other new energy fields, the precise monitoring of high voltage or high current in the line poses a higher challenge to engineers.

"Sampling resistance" is a common current test method. According to Ohm's law R=V/I, it is only necessary to accurately measure the voltage V on the known sampling resistance "R". Since the voltage measurement technology is very mature, it is not difficult to obtain the current value in the circuit.

When the current to be measured is large, the resistance R value is often required to be very small to avoid negative effects such as excessive voltage division and resistance heating. At this time, the sampling resistor selected may only be milliohm or even microohm level, and the resistance error directly affects the current measurement accuracy.

For example, if a 10mΏ resistor is used to measure a 100A current, even if the resistor measurement error is 1mΏ, it will result in a current error of 10A.

Digital multimeter is a common resistance measuring device

Which parameters can improve the test accuracy of small resistors?  

The principle of multimeter resistance test is similar to the above current measurement. As shown in the figure below, the multimeter applies a current to the resistor and then measures the voltage of the resistor.

Similarly, according to the formula R=V/I:  

1. Current source accuracy - appropriately increase the current and test aperture;

2. Voltage measurement accuracy - voltage range and test aperture;

3. Test lead resistance - must use 4-wire measurement;

Let's take the industry's most popular 6 ½-digit DMM, the Keysight 34461A, as an example.

The following are its resistance measurement specifications: minimum range 100Ω, resistance test drive current 1mA, test accuracy 100Ω x 0.01% approximately 10 mΩ.

Obviously, the error is obvious when using 34461A to measure milliohm-level resistance.

In order to accurately measure small resistors, an effective method is to use a high-precision source meter. For example, the Keysight B2912A precision source meter has a voltage and current test resolution of 6½ digits, which is the same as the 34461A, but its resistance measurement specification is much better than the 34461A, with a minimum range of 2Ω, a resolution of 1uΩ, and a test error of 2Ω x 0.2%, which is about 4mΩ.

By comparison, we can find that the B2912A has a larger current of 1A = 1000mA, but the 34461A has a smaller voltage measurement range of 100mV.

If you need to accurately measure u resistance, you need a combination that takes the best of both worlds, i.e. B2961A + 34420A 7 ½ nanovolt micro-ohm meter.

The high-precision current source of the B2912A is used in combination with the high-precision voltage measurement of the 34420A to achieve higher accuracy in measuring small resistances.