When a standard power resistor will not match the demands of a particular application a custom power resistor can be one solution. To ensure the resistor performs to specification over its full in service life close co-operation is required between the system design engineer and the application specific power resistor manufacturer during the component design phase.
It is important the resistor manufacturer is supplied with to as much information as possible on the key custom resistor parameters and end application to ensure they have the best chance of delivering a device that is fit for purpose. Key application issues to consider include:
- Heat management.
- Surge and ESD protection.
- Mechanical constraints including mounting method.
- Environmental conditions.
It is important to specify the actual operating temperature, directions of airflow (if any forced cooling is in place), proximity of both temperature sensitive and heat generating components and the board area available to accommodate the custom power resistor (plus any heatsink). It can also be useful to specify the available height as a vertically mounted device can be an appropriate solution in some applications.
In many applications available system board area is limited however attempting to reduce the size of the resistor can have implications for heat dissipation, resistor functionality and long term reliable performance. An appropriate choice of resistor substrate material may be one solution maximising heat dissipation while minimising component size but only at an increased cost. Target power resistor pricing should, therefore, be discussed at an early stage in the design cycle.
Surge & ESD
One of the main reasons power resistors fail is the application of a single (or repetitive) surge or pulse condition. However, if quantified and fed into the design process at an early stage the impact of known surge conditions can be minimised. Any surge conditions that exist within the system should be identified and their shape, duration and amplitude provided to the resistor manufacturer. Surge survivability can be improved by selecting appropriate substrate materials (and dimensions) and by optimising the thick film firing process.
Mechanical & Environmental
Mechanical and environmental concerns include mechanical damage, vibration and/or shock conditions and moisture and corrosive elements that may damage the custom power resistor device. If the moisture content and the presence of any corrosive elements is known then a suitable coating may be applied to protect the resistor device.
Inappropriate handling can damage the substrate material and / or the interface or connection with the system board but may be minimised if any specific issues are known in advance. Vibration or shock may damage the resistor substrate and resistive elements but their impact may be reduced by modifying the resistor mounting, substrate type or dimensions.
Information To Be Delivered To Manufacturer
Based on the above the information to be delivered to the custom power resistor manufacturer should include (as a minimum)
- Available board area.
- Operating temperature range.
- Nominal resistance (with tolerances).
- Nominal power load.
- Power /current rating.
- Pulse withstand.
- Maximum Voltage.
- Operating Voltage.
- Operating Frequency.
- Termination type, position and material.
- Length of pin.
- Expected in service life.
- Target component cost.
- Quantity required.
- Required Date.