High performance contact resistance test system

Contact resistance is the resistance to current flow through a closed pair of contacts. This type of measurement is performed on devices such as connectors, relays , and switches. The resistance is usually very small, ranging from micro-ohms to a few milliohms, so a 4-wire measurement is required. Programmable scanners greatly reduce measurement time by switching a set of test instruments to multiple contacts. For example, when testing multi-pin connectors or multiple contacts in an automated test chamber.

Figure 1. Measurement of contact resistance of 20 contacts

As shown in Figure 1, to prevent the contact resistance of the test leads, vias, and switches from being added to the measured value, all four terminals of the ohmmeter are switched by switches. Note that the power HI and LO terminals are connected to one set of double-pole switches, while the measurement HI and LO terminals are connected to another set of double-pole switches. To measure the resistance of contact 1, close channels 1 and 21, then measure the resistance with the ohmmeter; to measure contact 2, open channels 1 and 21, close channels 2 and 22, and then measure the resistance again.

The Keithley 2750 Multimeter/Switch System can measure resistances below 1 mΩ and has offset compensation and dry circuit testing capabilities, so it can be used to measure contact resistance.

Contacts in low-power applications are usually tested under dry circuit conditions, where both current and voltage are limited to not cause changes in the physical and electrical conditions of the point contact junction. Typically, the open circuit voltage of the dry circuit is very low, typically 20 mV or less, and the short circuit current is limited to 100 mA or less. [page]

Figure 2: Measuring 32 contacts using a common terminal

In some cases, it may be necessary to reduce the number of switches by half in a unique series arrangement, as shown in Figure 2. In this example, all of the tabs are connected in series, and each base is connected to a double-pole switch. In this case, the Model 2750 Multimeter/Switch System with the Model 7701 Low Voltage 32-Channel Differential Multiplexer Module can test 32 contacts. With five Model 7701 modules in the Model 2750 mainframe, a maximum of 160 contacts can be tested.

Figure 3 shows a high-performance contact resistance system that can measure very low resistance (μΩ) with relatively low current (<100 μA). In this system, the Model 2400 SourceMeter outputs the test current and the Model 2182A Nanovoltmeter measures the voltage drop across each contact. Both the 2182A and 2400 are switched to each contact via the Model 7011 40-Channel Multiplexer Card. The Model 2400 provides a programmable voltage follow limit so the system can ensure dry circuit test conditions.

Figure 3, High-performance contact resistance test system

A 7011 switch card and a 7001 switch host can switch 20 contacts. The 7011 switch card is configured as two groups of 20 channels. Using the host's 4-way switch mode, two groups of relays can be opened or closed simultaneously.