Temperature Sensors Testing Application Note

General

It is often desirable to verify the proper operation of thermistors in the various stages of manufacturing and assembly. Additionally, it is beneficial to subject the devices to varying the environmental conditions while performing this testing. Many thermistor devices are built in large quantities and the need for a effective testing capability is critical. This application note will give you ideas on how to apply the Thermotron Product Test System (PTS) to testing thermistor devices. The ability to handle large numbers of products in an economical manner indicate that the PTS is an excellent choice for this task.

Typical Thermistor Device

A thermistor is a device that makes use of the change of resistivity of semiconductor with a change in temperature. Most thermistors exhibit a negative temperature coefficient; that is, their resistance decreases with increasing temperature. The negative temperature coefficient can be as large as several percent per degree, but is non-linear in nature. In this example, this non-linearity is of no consequence since we are performing functional testing, not temperature measurement. A typical thermistor has a value of 5000 ohms at 25° C with a typical temperature coefficient of 4% per degree. Thermistors are often utilized due to their high output and fast response while only requiring two wires for hookup.

General Testing Strategy

Figure 1

The general strategy in testing the thermistor device is to monitor the resistance continuously over a range of temperatures. If the resistance of the device varies outside of defined limits, it is considered a failure. Open or shorted conditions must be checked continuously as intermittent failures may occur when cycling temperature. Note that the device under test may also be subjected to humidity, vibration or other physical phenomena during the testing. Also, the pass/fail conditions must change in synchronization with the change in environmental conditions. A general block diagram of such a configuration is shown in Figure 1. This configuration allows testing of "N" resistive devices.

Measuring Resistance

A PTS Analog Input Module (AIM-32) can be utilized to measure the changing (or steady) resistance values of thermistors being tested. The AIM-32 can monitor 32 single-ended or 16 differential analog signals. As a result, a large number of devices can be monitored efficiently and continuously. The AIM-32 also has the capability of using a "golden" or known good product on the first channel, and comparing the remaining channels to it. If the other channels vary more than a preset amount from the "golden" product, a failure is noted.

The Product Test System software allows for the setup and control of a environmental chamber temperature and/or humidity profile in synchronization with the AIM-32 operation. The use of the Product Test System with an environmental chamber is optional, but obviously very useful.

Load Boards

Load boards provide an interface between the PTS and the product(s). In some cases it may be necessary to "amplify" the varying resistive changes of a product to make them usable by the product test system. Such would be the case if the products being tested were devices like an RTD, which exhibit a much smaller change in resistance with a change in temperature. In the case of a RTD a wheatstone bridge may be used along with an op-amp. The thermistor discussed previously did not require this since its resistance change was greater over temperature and the voltage divider capabilities built into the AIM-32 would suffice. Load boards can additionally provide a convenient location for test points, diagnostic displays and configuration jumpers. As load boards are removable, extensive changes to the products can be accommodated by creating new load boards.

PTS Profiles

Once the PTS equipment is set up, the PTS software must be configured. To configure the PTS software, profiles are defined for the function modules that are used. In this case only the AIM-32 and temperature/humidity profiles need to be defined. No programming is required, only high and low limit definition is needed.

Possible Thermistor Tester Physical Configuration

Figure 2

A 19 inch console is typically used for PTS equipment mounting. AC power is normally distributed with an outlet strip and excess heat is dissipated with a blower located at the top of the console. Casters on the bottom of the console will contribute to convenience. Figure 3 illustrates a possible PTS configuration.

Load boards are typically located toward the top of the console and the PTS base unit is located toward the bottom, near the power supply. The PTS PC is typically located at a height that allows for easy operator access.

Connections between the PTS console and the products can be accomplished via cables that connect to the back of the PTS cabinet. These cables can connect directly to the air bag controllers, or to replaceable adapters. Note that is often desirable to locate PTS and product connections inside the console (as opposed to the front of the console). This increases the cabling protection, limits cable lengths and reduces the chance of tampering.

Conclusion

This application note demonstrates the possibility of creating a capable, cost effective, flexible method of testing thermistors, and thermistor-like devices with the Thermotron Industries PTS. This testing can be done in either production or laboratory environments. Contact Thermotron to have your testing needs analyzed and solved.