No reputable manufacturer wants to sell an electronic product that could overheat, burst into flames and injure the user. So to avoid this scenario, most of today’s electronic products undergo rigorous temperature testing as part of compliance with standardized UL or IEC safety regulations.
To understand the importance of temperature testing, let’s start with the obvious: Each component used in an electronic product has unique thermal characteristics and temperature ratings specified by the manufacturer. The absolute temperature of each component must be well within its manufacturer-defined temperature range during all phases of equipment operation.
But what if this isn’t true? Several variables affect the absolute temperature of a component. These include the component’s location in the enclosure, its proximity to other warm components and the amount of air flowing across it.
Because circuits and components are becoming smaller, the art of managing heat within electromechanical products is becoming more difficult. For example, today’s high-end 120-MHz Pentium laptops have an internal temperature in excess of 30°C, which makes them too hot to comfortably sit on your lap. The ultra-compact DC-to-DC converter power supply is the primary cause of this high internal temperature.
The less efficient the power supply and charging circuit, the more heat the power supply dissipates within the enclosure. This results in an internal temperature that is less than optimal for other circuitry.
So to determine the internal temperature of a product and validate a heat-management design, temperature test your product. Temperature testing quantitatively demonstrates that your electronic product operates within specific temperature parameters. It also helps you to assess the effects of convection, radiation and conduction of materials or components you have chosen to include in your product design.
To avoid costly re-engineering work later, begin temperature testing as soon as possible during the design phase. A typical temperature test should include the measurement of key points and components in and around the circuits using thermocouples. These points may range from ten to several hundred.
Often these key temperature points are measured from the moment power is applied to the equipment until the internal temperature reaches a plateau. Be sure to measure transformers, magnetics, voltage regulators, heat sinks, user-accessible panels and handles, high-voltage capacitors and other high-current circuits.
Some safety directives require that you validate fail-safe temperature-detection circuits by temperature recording. If your fail-safe temperature- detection circuit works correctly, it will reduce power levels or shut down the power supply when the temperature exceeds a predetermined level. Recording the temperature and output voltage helps you to prove that no temperature rises above safe levels before or after the fail-safe circuit shuts down the system.
By attaching thermocouples to specific locations within a device and by using the appropriate thermocouple wire gauge, you obtain accurate temperature readings. For example, when you attach thermocouples to very small components, you use higher-gauge thermocouple wire to avoid making the thermocouple act like a heat sink.
Also, consider the affect thermocouple location has on the accuracy of your readings. If you attach thermocouples to conductive panels, the difference between the ground of the device-under-test (DUT) and the test instrumentation ground can cause ground loops that create noisy or erroneous readings.
Temperature recording is the best method of temperature testing. Many methods exist for recording temperature, ranging from manual via hand-held meters to setting up your PC to record data automatically via a temperature- recording instrument.
The most labor-intensive method for recording temperature data uses hand-held or benchtop meters. You visually monitor the meters at specific time intervals and manually record the data. The hardware required for this method is economical compared to other alternatives, but it requires more time to set up, especially if you are measuring many channels.
To automate this procedure, use a strip-chart recorder to continuously chronicle the temperature data. This method, however, is expensive and generally you cannot access the data to generate a report.
These methods are adequate if you have an application that is short, records only a few channels of temperature and is not likely to be repeated. But once an application exceeds eight hours or 30 channels or is repeated often, these methods become difficult to manage.
For applications that require recording many channels, PC-based benchtop temperature-recording instruments provide you with the best value with a cost as low as $29 per channel. By using the PC to control your temperature-collection activity, you automate the test process, saving valuable time. Your PC also provides a cost-saving method to set up the test hardware, collect the data and generate a report.
For example, you need to monitor the temperature of a specific power supply that you intend to include in your product. Program your PC to control an instrument that simulates a variety of operating conditions that your power supply may encounter, such as low-line, high-line, full load, overload, different combinations of loads on different outputs, blocked fan, inadequate ventilation, high ambient, low ambient and high humidity. Generally, the first round of tests uncovers circuit problems that require correction.
Each time the circuit or the mechanical design changes, you can easily duplicate an automated temperature test. Using a PC to control your temperature test helps you to effortlessly repeat your application by changing parameters, collecting data and generating reports. Especially in cases where the testing is destructive, automated testing assures that you record the data at the proper intervals and that the data is easy to retrieve.
One feature to consider in a PC-based temperature instrument is the instrument’s capability to provide stand-alone operation. For applications where connection to a PC is inconvenient or not desired, some PC-based instruments gather the data in the instrument’s onboard buffer. Later, you can download the data to your PC to analyze it and to generate reports.
For applications that require unattended monitoring, the capability to set alarm conditions is important. Your PC-based instrument must output a voltage based on alarm conditions you specify. Once you set up your alarm conditions, the instrument automatically shuts off the DUT when a particular channel exceeds its maximum rated temperature.
Some instruments feature modem support for applications when it is impractical to leave a dedicated computer for security or other reasons. With modem support, you monitor the unit’s operation and retrieve data without traveling to the remote site.
Ideal for meeting UL temperature directives, PC-based recording offers you the most economical means of automating temperature testing because it ensures that data is collected at precise intervals. PC-based temperature recording also makes it easy for you to analyze your data and generate reports. Even if you are not required to meet UL or IEC directives, PC-based temperature recording ensures that your electronic products are both safe and reliable, which benefits you in the long run.
About the Author
Steve Lekas is the vice president of new products at IOtech. Formerly IOtech’s director of research and development, Lekas has more than 15 years experience in the test and measurement industry, and also has worked for Gould’s Recording Systems Division and Keithley Instruments. He holds B.S.E.E. and M.S.E.E. degrees. IOtech, 25971 Cannon Rd., Cleveland, OH 44146, (216) 439-4091.
Copyright 1997 Nelson Publishing Inc.
April 1997