Introduction
Low resistance measurements offer a good way to identify resistance elements that have changed over time. Often, these types of measurements are used to evaluate if a device or material has degraded due to environmental factors like heat, fatigue, corrosion, vibration, etc. For many applications, these measurements are typically lower than 10Ω. A change in resistance value is often the best indicator of some form of degradation between two points of contact. Low resistance measurements performed using high currents are commonly used to evaluate high power resistors, circuit breakers, switches, bus bars, cables and connectors, and other resistance elements.
Most digital multimeters (DMMs) lack the ability to make low resistance measurements with high currents. A DMM combined with a power supply will work, but these instruments must first be integrated into a system in order to automate the measurement process, then the resistance must be calculated manually.
Source Measure Unit (SMU) instruments or SourceMeter® instruments can simplify making low resistance measurements with high current stimulus. A SourceMeter instrument is capable of sourcing and measuring both current and voltage. Keithley’s Model 2460 High Current SourceMeter SMU Instrument has the flexibility to source/sink high current and measure voltage and current, making it a perfect solution for measuring low resistance devices that require stimulus currents up to 7A. The Model 2460 automatically calculates the resistance, so there’s no need to make the calculation manually. Built-in features such as remote sensing and offset compensation help optimize low resistance measurements. The Model 2460 offers <1mΩ resolution.
Low resistance measurements can be made using either the Model 2460’s front-panel or rear-panel terminals, as shown in Figures 1 and 2. Note that either the front-panel terminals or rear-panel terminals must be used—the connections can’t be mixed.
When the leads are connected to the device under test (DUT), note that the FORCE LO and SENSE LO connections are attached to one of the DUT leads and the FORCE HI and SENSE HI connections are attached to the other lead. The sense connections should be connected as close to the resistor under test as possible. This four-wire measurement cancels out the resistance of the test leads in the measurement.
Figure 1 illustrates the front-panel connections, which can be made with four insulated banana cables that are rated to the maximum current (7A), such as two sets of Keithley’s Model 8608 High-Performance Clip Lead Set.
Figure 2 illustrates the rear-panel connections, which can be made with either the Model 2460-KIT ScrewTerminal Connector Kit (included with the Model 2460) or a Model 2460-BAN Banana Test Leads/Adapter Cable with appropriate cabling.
Common Sources of Error for Low Resistance Measurements
Lead Resistance
As shown in Figure 3, all test leads have some level of resistance, some as high as hundreds of milliohms. This can result in an incorrect measurement if the lead resistance is high enough.
Thermoelectric Voltages
Thermoelectric EMFs or voltages are generated when different parts of a circuit are at different temperatures and when conductors made of dissimilar materials are joined together. A few microvolts can be generated by temperature gradients in the test circuit caused by fluctuating temperatures in the lab or a draft near the sensitive circuitry.
Non-Ohmic Contacts
Non-ohmic contacts are evident when the potentialdifference across the contact isn’t linearly proportional to the currentflowing through it. Non-ohmic contacts may occur in a low voltage circuit as aresult of oxide films or other nonlinear connections. To prevent non-ohmiccontacts, choose an appropriate contact material, such as indium or gold. Makesure the SMU instrument’s compliance/limit voltage is high enough to avoidproblems due to source contact non-linearity. To reduce error due to voltmeter non-ohmiccontacts, use shielding and appropriate grounding to reduce AC pickup. (Referto the latest edition of Keithley’s Low Level Measurement Handbook forfurther information on non-ohmic contacts.)
Device Heating
The test currents used for low resistancemeasurements are often much higher than the currents used for high resistancemeasurements, so power dissipation in the device can be a consideration if itis high enough to cause the device’s resistance value to change. Powerdissipation in a resistor is given by the formula:
P=I2R.
From this relationship, we can see that the powerdissipated in the device increases by a factor of four each time the currentdoubles. Therefore, one way to minimize the effects of device heating is to usethe lowest current possible while still maintaining the desired voltage acrossthe DUT. If the current level cannot be reduced, consider using a narrowcurrent pulse rather than a DC signal.