Key Power Resistor Application Considerations
When choosing a power resistor for any application it is important to consider more than the resistance value, power rating and tolerance. There any many factors that can affect the performance of the resistor, and the system as a whole, including thermal and mechanical issues and operational life.
Thermal Issues In Power Resistor Applications
Thermal performance has a direct impact on both mechanical issues and operational life and should, therefore, be the first issue to consider.
Resistors are rated at a set operating temperature, usually ambient (25°C). If the resistor is exposed to higher temperatures its maximum load must be derated accordingly. In power applications, a heat sink may be utilised to dissipate heat and/or the temperature in the area surrounding the resistor may be managed appropriately to ensure the ambient temperature is not exceeded.
The distance between any high power resistors and other high power components should be maximised. Also, the size, shape, wall material and thickness and finish of the system enclosure all affect the temperature rise of the resistor and should be designed accordingly.
Air flow may be increased using appropriate openings in the enclosure and/or by using a fan. Increased air flow can also minimise the size of any heatsink that may be required by reducing its thermal resistance. Heat sink thermal resistance depends on the surface area in contact with ambient air, and the amount of air flow. Increased airflow, therefore, has a direct impact on the surface area and overall size of the heat sink.
In most applications, the objective will be to minimise the power resistor dimensions, without compromising on reliability or performance. When choosing the most appropriate size and footprint of the resistor device two key factors come into play.
The first issue to consider is thermal performance as, given the environment, the resistor must be able to dissipate the required amount of heat. It is also important to note any mechanical issues such as shock or vibration that may impact on the performance and long term reliability of the resistor.
For example to maximise airflow over a resistor device and to minimise the footprint on the system board a vertical in line footprint device may seem the best choice. However, if the system is subject to mechanical shock or vibration then the chances of fracture or mechanical failure will rule this out as an option.
The resistor substrate, tracking material and manufacturing methods must be appropriate for the application. Vibration or mechanical shock, thermal factors, moisture, altitude and quality specifications can all influence the decision making process. Taking advice from a specialist manufacturer is often best before making a final decision.
As discussed above thermal and mechanical issues can both impact on the performance of the power resistor in the final application. Failure to consider these issues during the design phase may not only impact on the performance of the resistor it could also dramatically shorten the effective in service life of the device.
Choosing a power resistor for any application is not as straightforward as may be imagined. There are many issues to consider other than the resistance value and tolerance. A wide range of resistors are available as standard products from the major resistor manufacturers. However, when performance is critical or no standard device is fit for purpose it can be best to consult a specialist supplier expert in producing application specific power resistor devices.