Three ways to measure resistance with a multimeter
1. Two-line method
The two-wire method is a commonly used method of resistance measurement, as follows:
Connect the V+ end of the multimeter to one end of the resistor, and the V- end to the other end of the resistor, and then set the multimeter to measure. A multimeter can determine resistance through Ohm's law by supplying a source current to a resistance and then calculating the voltage across the resistance.
With the simplified example above, the lead resistance R causes a bigger problem because the voltage is the voltage of the three resistors above. This effect is greater in the case of small resistance, generally in the case of 30KΩ, this effect is very obvious. Of course, these are for high-precision situations. If the accuracy requirements are not high, this method can be used.
This effect caused by the wire resistance R can be eliminated by some relative value measurement functions of the multimeter. In order to eliminate these problems, the first thing that needs to be determined is where these problems are coming from. This can be achieved by setting the resistor to 0Ω.
If you put all the resistances on both ends of the test terminal leads, you can then measure through the two wires for relative value measurement.
2. Four-line method
The four-wire method is ideal for low-resistance measurements because it eliminates the effects of lead wires without the aid of the relative value measurement function. These calibrations are all automatic.
In the four-wire method, the V+ and V- terminals of the multimeter still supply current to the resistor through the leads. The voltage drop here is the sum of the lead resistance and the resistance under test.
The lead is connected to both ends of the resistor, and the voltage across the resistor is measured. The voltage of this part does not include the part of the switch system connected to the DUT through the test lead (or through the multimeter. For details on the switch system, please refer to other related articles), the input impedance of the voltmeter is large enough so that it doesn't transfer any voltage or create an erroneous voltage on the lead resistance.
All of these reading feedbacks are based on resistance, and in fact, the resistance of the test leads. The four-wire method is very accurate, repeatable and stable resistance measurement method, and is particularly suitable for measuring low value resistances, even down to 10 milliohms. But for high resistance measurement, this method is not suitable, because the input resistance and leakage current of the voltmeter will affect the reading. In general, the four-wire method is not recommended.
3. Six-line method
Six-wire is a resistance value suitable for measuring the resistance of the part of the resistance that has a shunt structure. For example, in an automated test system, the resistors that need to be tested are all soldered on the PCB, which will be affected by other components in the surrounding circuit.
In order to isolate the resistance under test, a protection voltage is generally added to the node defined by the user, and this protection voltage is driven by the voltage buffer area of the V+ terminal. This protection voltage can ensure that the voltage from the multimeter will leak into other paths.
The following example explains how the six-wire method works:
As shown in the picture above, in parallel with the 30KΩ resistor are two resistors, one is 510Ω and one is 220Ω. In a normal resistance measurement, this 510Ω and 220Ω would dissipate the source current from the multimeter, which would produce a false reading. By sensing the voltage across this 30KΩ resistor, and then connecting the same voltage to the 510Ω and 210Ω resistors, there will be no current through the bypass. The protection voltage can ensure that the voltage is the same as the voltage of the V+ terminal, and the current through 220Ω is provided by the protection source. In this case, the multimeter can accurately test the resistance of the 30Ω resistor.
The current carrying capacity of this protection terminal is limited by the classic DMM (with short-circuit protection), so there will be a limit on the number of drives.
The resistor is connected to the low voltage side of the 4-wire terminal, and the protection terminal is a thermal fuse resistor or Rb. Due to the existence of the protection source current, this resistance cannot be smaller than the resistance of Rbmin, because:
Rbmin=Io*Rx/0.02
Here Io is the selected source current and Rx is the resistance under test.
For example, if a 330Ω resistor is selected and a 300Ω resistor is tested, the minimum resistance value of Rb used is 15Ω.
Because of the maximum load resistance Ra, there is no limit, as long as the polarity of the measurement is selected, it will be effective, because Ra can become Rb and the opposite. It is best to set the polarity of the measurement because Ra is higher than both load resistors.
The six-wire method of measuring resistance is specially designated to measure the resistance of 330KΩ. For the case where the resistance value is larger than this, the configuration of the six-wire method can be used, but the multimeter should be set to the two-wire mode (this will have a lower source current).
