Two of the most common resistor technologies are thick film and wirewound. In this post we compare thick film high voltage resistors with their wirewound competitors discussing the advantages and disadvantages of each.
Thick film high voltage resistors are manufactured by depositing a resistance film pattern onto a flat substrate (usually Alumina). The assembly is then fired at high temperature. The resistor elements are often applied in spiral patterns and consist of metal alloy or oxide particles, functional elements and a glass particle binder.
As the name suggest all wirewound resistors are manufactured by winding wire around a core. The resistance value is a function of the length of the wire, its cross section and its resistivity. When comparing various types of resistor it is important to consider:
- The resistance value.
- The resistor tolerance.
- Changes in resistance value with temperature.
- Stability of resistance value over time.
- Power (and surge) handling capability.
- Heat dissipation
- Dimensional constraints.
- Mechanical robustness.
Depending on the application it may also be necessary to consider frequency performance, the voltage rating and the noise generated by the resistor.
Wirewound resistors are somewhat limited in resistance range and do not have the high Ohmic range offered by thick film. However, the tolerance values of wirewound resistors are generally superior to those available from thick film resistor technology. Tolerance values for wirewound devices can be as low as 0.05% compared to 0.5% for thick film.
TCR (Temperature Coefficient of Resistance – The lower a TCR the better a resistor will be at maintaining its resistance regardless of ambient temperature variations and self-heating effects) is 50 ppm/°C for thick film high voltage resistor devices. TCR for wirewound devices varies depending on the wire type and can range from 25ppm/°C for precision devices to 1,000’s ppm/°C
Load life stability defines the ability of the resistor to retain its initial resistance value, within specified limits, over its in service life. Resistor stability is directly related to the resistor manufacturing process, the materials selected and their susceptibility to applied voltage and thermal stresses. In wirewound resistors electrical and mechanical stresses cause random changes in the electrical parameters of the wire and hence the resistance. In thick film the granular structure of the resistance film can be affected by thermal and mechanical stresses causing a change in resistance value.
Given the relationship between load life stability, material selection and the manufacturing process the choice of materials and manufacturer is of vital importance. The choice of materials has a direct impact on cost, it is therefore difficult to make a direct comparison between thick film high voltage resistors and wirewound. However, in general, wirewound technology has superior load life stability to thick film.
The choice of materials and manufacturing method also has a major impact on surge survivability. The thick film construction delivers a superior surge performance to wirewound in many applications. However, using special materials and winding techniques (at increased cost) can improve the surge survivability of wirewound devices. Both thick film and wirewound devices have high power handling capabilities. The key issue in many high power applications is how to dissipate the heat generated.
The heat dissipation properties of thick film technology plus its capacity to interface directly with a number of heat management devices such as heat sinks give it a major advantage over wirewound technology. Thick film devices are much smaller than their wirewound equivalents and are available in a number of convenient footprints. Thick film technology is mechanically robust whereas the construction method makes wirewound resistors more likely to be damaged via poor handling or packaging, insertion, and lead forming processes.
There are always positives and negatives with any resistor technology and the important point is to select the most suitable, and cost effective, resistor for each application. In general a thick film high voltage resistor is cheaper than the wirewound equivalent.
Wirewound technology does however generate less noise which can be an advantage in some applications. Although wirewound resistors do offer lower tolerance values, higher stability and lower TCR it is often at the expense of much larger footprint, higher inductance and higher cost than an equivalent thick film resistor device. Addressing heat dissipation from any wirewound resistor is a major issue and often drives the decision to select the thick film alternative.