Current flow through a resistor generates heat, which must be dissipated to the environment. Without appropriate thermal management, a power resistor will not operate to specification. Over the medium to long term, it will probably fail.
Before deciding what action to take, it is important to understand both the resistor heat dissipation profile and the temperature of the ambient environment. A low-power resistor’s primary heat dissipation mechanism is via conduction through its leads or connections. A high power resistor tends to dissipate heat via radiation.
Power resistor ratings assume an ambient temperature of 25C. If the ambient temperature and/or resistor heat dissipation is not controlled, the only solution is to derate the resistor. The implications of resistor derating are covered elsewhere in this blog.
In this post, we consider the alternatives to resistor derating. These include controlling the ambient temperature, cooling the resistor, or choosing appropriate resistor materials.
Controlling Ambient Temperature
Common factors affecting the ambient temperature include:
- Surrounding components
- Enclosure volume and material
Altitudes below 5,000ft are generally not an issue, but if equipment must operate above this level, there is no choice but to derate accordingly. However, the other three key factors are all manageable to some extent.
Air temperature drops by approximately 2℃ per 1,000ft, which helps with resistor cooling. But reduced atmospheric pressure at (and above) 5,000 ft degrades the impact of airflow over the resistor or cooling surfaces.
The system designer should consider the enclosure design and maximise the distance between heat-generating components. It is important to avoid hot spots on the power resistor due to poor airflow over some areas with respect to others.
The enclosure material, its thickness and finish will affect heat conduction to the outside environment. The position of enclosure openings and vents can maximise airflow over the resistor components. Maximising the enclosure volume can also help.
If necessary, a fan can boost airflow. Appropriately positioned heat sinks and vanes can maximise heat transfer from resistor components.
Cooling The Power Resistor
One solution is to size the resistor appropriately (sufficient mass) to dissipate the heat generated. But, using a larger power resistor device is more costly and compromises the system board area available for other components.
The alternative is to cool the resistor device in some way. The main options are air cooling, or liquid cooling, with or without using a heat exchanger.
Where the heat capacity of air is inadequate to transfer heat away from the power resistor device, liquid cooling is the preferred option. Liquids are 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. But, they add resources to the system and cost significantly more.
Placing the power resistor device in a metal tube and then immersing that tube in the path of deionised water flow is a common cooling method. 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.
Water is non-flammable and easy to source. Deionised water reduces the risk of short circuits. It also reduces contamination and scaling, which can decrease the insulating resistance.
The major disadvantage of water is its boiling point. Pressurising the water can help raise the boiling point, but in some applications, water is not a viable solution.
Oil is the best alternative as it has a much higher boiling point than water and is an electrical insulator. However, it can degrade at higher temperatures, it is flammable and has only around half the specific heat capacity of water. The oil must be checked and changed as required, as its insulation resistance degrades over time.
Choosing The Correct Resistor Technology
Thin film and bulk metal foil resistors are generally not suited to high-power applications. The most common resistor types in power applications are wirewound and thick film power resistors.
The heat dissipation properties of thick film technology give it a distinct advantage over wirewound in power applications. For a given power rating, thick film resistors tend to be smaller than their wirewound equivalents. Flat thick film resistor substrates are relatively easy to interface with heat management devices such as heat sinks.
Choosing thick film resistor materials with relevant TCR and PCR can reduce the impact of temperature. Various substrate materials are available. Some are capable of operating at extreme temperatures.
Power resistor thermal management is a complex subject that is often overlooked. This can result in a reduction in resistor performance, a high resistor failure rate, or the installation of larger and more expensive resistor components than necessary.
In some applications, power resistors are safety-critical devices. If in doubt, consult a manufacturer with long-term resistor design, manufacturing and application expertise.