The design of a high voltage resistor can seem trivial but if the resistor is to perform to specification over an extended period of time there are actually a number of key design issues to consider. The impact on high voltage resistor performance of external thermal and mechanical issues should not be underestimated. In this article we cover thick film resistor technology limitations and the impact of thermal and mechanical stress.
To understand the key design issues it is important to first understand how thick film high voltage resistors are manufactured. The process usually involves depositing resistor material onto a flat substrate (usually Alumina) and then firing at high temperature. The resistor element is then often be modified (abraded) to deliver the required resistance value and a coating added for environmental protection.
The resistor elements are applied in spiral patterns as long tracks or as short but wide elements depending on the specification and application. The resistor element consist of metal alloy or oxide particles, functional elements and a glass particle binder. The glass binder melts in the firing process, adheres to the substrate and binds the granular active particles together. Resistor performance issues including resistor tolerance temperature coefficient of resistance drift and power rating are all directly linked to the appropriate choice of materials.
When designing high voltage resistors one of the most important parameters to consider is load life stability. This defines the ability of the resistor to retain its initial resistance value, within specified limits, over a long period of time. Resistor stability is directly related to the manufacturing process (see above), the resistance film and its susceptibility to applied voltage and thermal stresses.
The key issue to consider is the granular structure of the resistor film as it this that makes it susceptible to both thermal and mechanical stress damage during operation. If any damage does occur it is often not catastrophic but causes either changes in performance and/or premature failure.
It is therefore important for the designer to consider both self-heating effects (current related) and the temperature of the environment. Any sudden increase or decrease in either self-heating or external temperature should be avoided. Although the impact of mechanical stress on the resistor can be minimized by appropriate resistor substrate selection and design any excess mechanical stress should be avoided where possible.
Although the granular structure of the resistor film material does make thick film technology more susceptible than some other resistor technologies a specialist manufacturer can reduce the impact of the problem as careful resistor design and selection of materials can minimize the potential impact of thermal and mechanical events.
High Voltage Resistor Design – Key Issues
Last Updated on August 3, 2022