A power resistor may be used in motor control circuits and is therefore likely to be subject to repeated short term (transient) surge conditions. It is essential to choose the correct thick film power resistor to ensure it will operate, without degradation, over the required lifetime of the system. Many designers underestimate the potential damage to resistors and other components caused by a power surge hence in this article we cover the key issues to consider.
Surges are typically caused by switching events and are of short duration which does not allow the resistor the time to dissipate the energy generated as heat. Where the resistance is relatively low (<250 Ohm) surge current tends to be the major issue. The heat rise generated by a current surge condition may be calculated using – Rise in temperature = (Energy/Specific heat) x Mass. The temperature rise must not exceed the maximum temperature of the resistive material.
Calculating this maximum temperature can be difficult for thick film resistors due to hot spots (more below) so it is often best to choose a maximum temperature below what the resistor material specification document may show. The specific heat value is generally available from the resistor material specification document.
Where resistance is higher (>250 Ohm) over voltage surge can be the major issue causing a degradation in resistance value. An over voltage condition can cause usually non-conductive elements in the resistive material to conduct and they do not necessarily return to their original state when the surge condition is removed.
The geometry of a resistor also affects its surge withstand capability. A larger surface area (example a cylindrical rather than flat resistor) delivers a higher film mass, and an improved surge performance. This increased surface area allows more heat dissipation which is important for power applications.
The final factor contributing factor to a thick film power resistor surge survivability is the component trimming process for final resistance value. Resistor trimming and the method used for trimming can create weak spots (reduced resistor material thickness in specific areas) that cause hot spots and failure under surge conditions.
To avoid premature failure or degradation of resistor performance it is therefore essential to quantify the level of a potential surge condition and to understand the performance characteristics and properties of the resistor material. The mass of the resistor, its form factor and geometry all have an impact as does the manufacturing process (trimming). For application specific resistor requirements it is often best to work closely with a specialist resistor manufacturer so all design issues may be considered.