Thick Film High Power Resistor Cooling Considerations
Thermal issues have a direct impact on the operational life of any thick film high power resistor hence resistor cooling is always a key issue to consider. In this post we consider air and liquid cooling and the advantages and disadvantages of each method.
Resistors are specified at a specific operating temperature, usually ambient (25°C). If the resistor is exposed to higher temperatures then it will not operate as expected and its maximum load must be derated accordingly. Heat generated as current passes through the resistor element must be dissipated to prevent damage to the resistor materials.
The general solution is to size the resistor appropriately (sufficient mass) to dissipate the heat generated but this may compromise the given system dimensions and impact on the performance of adjacent electronic devices. The solution is to cool the resistor device in some way. The main options are air cooling or liquid cooling, with or without the use of a heat exchanger.
Air cooling is the cheapest method to employ, combining air flow management with the use of appropriate heat sinks, and is the process utilized in most high power systems. However there are applications where the heat capacity of air is simply inadequate to transfer the required amount of heat away from the high power resistor device.
Liquids (water and oil are the most common) are much denser than air and have a much higher thermal capacity. They are therefore far more efficient than air at transporting heat away from the resistor and / or at transporting heat to a secondary cooling surface such as a heat exchanger or heat plate. However, they add additional resources to the system and therefore come at a significantly higher cost than air cooling systems.
Placing the high power resistor device in an appropriate metal tube then immersing that tube in the path of the water flow is an efficient method to cool the resistor. Water is cheap and has a high heat capacity enabling it to transfer heat out of the system or to a heat exchanger or plate. It is also non-flammable and relatively easy to source when required which are key advantages over oil cooling.
Care must be taken to use deionised water to reduce the risk of short circuit and to avoid contamination and scaling that can decrease the insulating resistance of the water but the major disadvantage of water is its boiling point. Although pressurising the water can increase the boiling point, in some high power applications water is not a viable solution.
Oil is the best choice for some applications as it has a much higher boiling point than water and it is an electrical insulator. However, unlike water, it can degrade (particularly at higher temperatures), it is flammable and it has only around half the specific heat capacity of water. Like water the oil must be checked and changed as required as its insulation resistance degrades over time.
Thick film high power resistor cooling is a key issue in many electronic systems that should be considered as soon as practical in the system design phase. Cooling systems, be they air or liquid, require additional mechanical and electrical resources and impact on the resistor design.