Low resistance measurement provides a good way to identify the elements of resistance that change with time. Generally, this kind of measurement is used to evaluate whether the device or material is degraded due to environmental factors (such as heat, fatigue, corrosion, vibration, etc.). For many applications, these measurements are usually less than 10 Ω. A change in resistance is often the best indication of some form of degradation between two contacts. In order to evaluate high power resistors, circuit breakers, switches, buses, cables, connectors and other resistance components, high current is usually used for low resistance measurement.
Most DMMs do not have the ability to measure low resistance through high current. It can be measured with DMM and power supply, but in order to realize the automation of the measurement process, these instruments must be integrated into the system first, and then the resistance must be calculated manually.
Using SMU instrument or digital source meter instrument, the low resistance measurement excited by high current can be simplified. Digital source meter instrument can supply and measure current and voltage. Model 2460 high current digital source meter source measurement unit (SMU) has the flexibility to pull / fill high current and measure voltage and current, making it a perfect solution for measuring low resistance devices (up to 7a excitation current). The 2460 instrument can calculate the resistance automatically, so there is no need for manual calculation. Its built-in characteristics such as remote detection and offset compensation help to optimize low resistance measurement. The resolution of model 2460 is less than 1 m Ω.
Low resistance measurement can be carried out through the front or rear panel terminals of model 2460 instrument, as shown in Figure 1 and Figure 2. Note that you can use the front panel terminal or the rear panel terminal separately, but you can’t cross connect them.
When the lead is connected to the DUT, note that forcelo and senselo are connected to one end of the DUT lead, forcehi and sensehi are connected to the other end of the DUT lead. testing. The connection should be as close as possible to the resistance to be measured. This 4-wire measurement eliminates the influence of the test lead resistance on the measurement.
Figure 1 shows the connection of the front panel, which can be connected by four insulated banana cables with the maximum rated current of 7a, such as two sets of high-performance alligator clip test line group of giseley 8608.
Fig. 1 connection diagram of front panel of 2460 instrument for low resistance measurement
Figure 2 shows the rear panel connection, which can be connected through the 2460-kit screw terminal connector kit (including the 2460 instrument) or the 2460-ban banana test lead / adapter cable.
Fig. 2 connection of rear panel of 2460 instrument in low resistance measurement
Common error sources of low resistance measurement
There are many error sources for low resistance measurement, including lead resistance, non ohmic contact and device heating.
As shown in Figure 3, all test leads have a certain resistance, some of which are as high as hundreds of milliohm. If the lead resistance is high enough, incorrect measurement may result.
When different parts of a circuit are at different temperatures, or when conductors of different materials contact each other, thermoelectric EMF or thermoelectric voltage will be generated. Temperature fluctuations in the laboratory or airflow near the sensitive circuit may cause temperature gradient changes in the test circuit, which may produce several microvolts of thermoelectric voltage.
Non ohmic contact
When the potential difference at both ends of the contact is not proportional to the current flowing through the contact, non ohmic contact occurs. Non ohmic contacts may occur in low voltage circuits or other nonlinear connections formed by oxide films. In order to avoid non ohmic contact, appropriate contact materials, such as indium or gold, should be selected. Make sure that the clamping voltage at the input is high enough to avoid the problem caused by the nonlinearity of the source contact. In order to reduce the error caused by non ohmic contact of voltmeter, shielding and proper grounding measures are adopted to reduce AC interference.
The current used for low resistance measurements is often much higher than that used for high resistance measurements. If the test current is high enough to change the resistance of the device, the power dissipation of the device should be considered. The power dissipation of the resistor is determined by the following formula:
It can be seen from this relationship that when the current is doubled, the power dissipation of the device will increase to 4 times. Therefore, one way to minimize the device heating effect is to use the lowest possible current while maintaining the expected voltage at both ends of the DUT. If the current level can not be reduced, narrow current pulse instead of DC signal can be considered.
How to measure low resistance and high current successfully
Lead resistance and 4-wire Kelvin method
The resistance is often measured using the two-wire method shown in Figure 3. We force the test current through the test lead and the resistance (R) being tested. Then the instrument measures the voltage at both ends of the resistance through the same set of test leads, and calculates the corresponding resistance value.
Fig. 3 2-wire resistance measurement with SMU instrument
The main problem of using two-wire measurement method in low resistance measurement is that the total resistance of lead wire (rlead) is added to the measurement result. Because the test current (I) produces a small but very important voltage drop on the lead resistance, the voltage (VM) measured by the meter will not be exactly the same as the voltage on the measured resistance (R), thus causing considerable error. The typical lead resistance is in the range of 1m Ω ~ 10m Ω, so when the measured resistance is less than 10 Ω ~ 100 Ω, it is difficult to obtain accurate measurement results by two-wire measurement method (depending on the value of lead resistance).
Due to the limitation of the two-wire method, people generally prefer the four wire connection method (Kelvin method) as shown in Figure 4 for low resistance measurement. In this configuration, the test current (I) is forced through a set of test leads to flow through the resistance under test (R), while the voltage at both ends of the DUT is measured through a second set of leads called test leads. Although there is a small current flowing through the detection lead, these currents can be ignored in all practical measurement work.
Fig. 4 four wire resistance measurement with source measurement unit (SMU) instrument
Since the detection lead voltage drop is negligible, the voltage measured by the meter (VM) and the voltage on the resistance (R) are actually the same. In this way, the resistance value can be determined with much higher accuracy than the two-wire method. Note that the voltage sampling lead should be connected as close as possible to the resistance to be measured to avoid including the resistance of the test lead in the measurement.
· thermoelectric voltage (thermoemf) and bias compensated ohmic method
Bias compensated ohmic method is a technique to minimize thermoelectric EMF. As shown in Fig. 5a, the source current is added to the measured resistance only for a part of the measurement cycle. When the source current is on, the total voltage measured by the instrument includes the voltage drop on the resistor and the thermoelectric EMF (Fig. 5b). Turn off the source current during the second half of the measurement cycle. At this time, the total voltage measured by the instrument is just the thermoelectric EMF in the circuit (Fig. 5C). If the vemf can be measured accurately in the second half of the measurement cycle, it can be subtracted from the voltage measured in the first half of the measurement cycle, so that the measurement result of bias compensation voltage becomes:
Similarly, we note that the thermoelectric electromotive force term (vemf) is eliminated in the measurement process. Instrument limitations
Even the image source measurement unit (SMU) instrument, which can provide up to 7a DC current, has limitations in terms of total output power, which may affect the measured resistance. This limitation originates from the design of the equipment, and usually depends on the design parameters, such as the maximum output of the internal power supply of the instrument, the safe working area of the discrete devices used in the equipment, the metal wire spacing on the circuit board inside the instrument, etc. Some design parameters are limited by the maximum current limit, some by the maximum voltage limit, and some by the maximum power limit (I × V).
Figure 6 shows the maximum DC current and maximum power of 2460 instrument at different working points. For example, the power envelope of the source measurement unit (SMU) has a maximum current of 7a (point a in the figure) and a maximum voltage of 100V (point d). The maximum output power of the source measurement unit (SMU) is 100W, which can be reached at point d (1a × 100V). At point a, the power is less than 49w.
Figure 5 bias compensation ohm method
Figure 62460 high current source measurement unit (SMU) instrument power envelope
Editor in charge: GT