High Power Thick Film Resistors In High Temperature Applications
Resistors are used in a variety of demanding high power, high temperature applications. These applications include down hole, process applications and military systems. In this post, we discuss the impact of temperature on resistor performance, high power resistor technologies and design solutions to deliver long-term stable performance at temperature extremes.
The Impact Of Ambient Temperature On Resistor Performance
Resistance is directly proportional to temperature. An increase in ambient temperature will, therefore, cause a proportional increase in resistance. The level of that increase depends on the temperature coefficient of resistance (TCR) of the resistor device. The potential increase in resistance must be calculated and catered for in the system design.
When power is applied current flowing through the resistor device causes a self-heating effect proportional to its Power Co-efficient of Resistance (PCR). This heat must be dissipated to the surrounding environment or it will cause damage to the resistor element. A high ambient temperature will limit the resistors ability to dissipate excess heat although appropriate heat sinking can help.
Inappropriate management of temperature effects and/or a poor choice of resistor device can stress the resistor material and cause irreversible changes to the resistor value. In extreme cases, this can lead to complete resistor failure. Significant variations in the temperature of the environment can impact on the stability (change in resistance value over time) of high power thick film resistors.
Choosing The Correct Resistor Technology For High Temperature Applications
Of the most common types of resistor thin film and bulk metal foil are generally not suited for high power applications. The most common resistor types in power applications are wirewound and thick film resistors.
The heat dissipation properties of thick film technology give it a distinct advantage over wirewound in high power applications. For a given power rating high power thick film resistors tend to be smaller than their wirewound equivalents. Thick film has the added advantage of direct interface with a number of heat management devices such as heat sinks. The technology also has lower inductance than wirewound and superior high frequency performance.
As the temperature rises the maximum rated power rating of a resistor must be reduced accordingly. A linear relationship between temperature rise and derating factor is usually quoted on the resistor datasheet. It may be necessary to select a higher power (larger size) device than anticipated to cover the impact of a high ambient temperature.
The choice of appropriate thick film resistor materials with relevant TCR and PCR can reduce the impact of temperature. Thick film resistors may utilise a variety of substrate materials. Appropriate material selection can deliver thick film resistors capable of operating at extreme temperatures.
In high power applications where high temperature is an issue, Thick Film technology is generally an excellent choice. When a standard device is not available a specialist manufacturer can often offer a custom resistor solution.