The most common power resistor technologies are Wirewound and Thick Film. In this post, we compare thick film resistors vs Wirewound equivalents. We discuss the advantages and disadvantages of each type.
When comparing various types of resistors, it is important to consider:
- The resistance value.
- The resistor tolerance.
- Electrical performance.
- Stability of the resistance value over time.
- Changes in resistance value with temperature.
- Heat dissipation.
- Power (and surge) handling capability.
- Dimensional constraints.
- Mechanical robustness.
- Cost.
Resistor Construction Comparison
The Thick film resistor manufacturing process involves screen-printing a resistive paste onto a flat ceramic substrate (usually Alumina), The paste consists of metal or metal oxide particles, additives and a glass binder.
The assembly is then fired at high temperatures causing the paste to adhere to the ceramic surface and form a durable resistive layer. The resistance pattern is often printed with a serpentine or meandering path to control resistance and optimise thermal characteristics.
As the name suggests Wirewound resistor manufacture involves winding wire around an insulating core made from materials such as ceramic or fiberglass. The resistance is determined by the length, diameter, and resistivity of the wire. After winding, the assembly is coated or encapsulated to protect the wire and improve heat dissipation.
In thick film resistors, the resistive element is the printed film layer, while in Wirewound resistors, it is the metallic wire. The total resistance in both cases depends on the material’s resistivity and the geometry of the resistive path (length and cross-sectional area).
However, as resistor length, width and thickness all have limits, resistor material selection is critical. The long-term stability of the resistor material, the impact of temperature and resistance to environmental factors are key considerations.
Thick Film vs Wirewound Resistor Performance Comparison
Resistance value. – Thick film resistors offer a greater range of resistor values. The thick film resistive element can be precisely patterned into long, narrow, serpentine paths that maximise resistance within a compact footprint. Additionally, resistive inks can be formulated with high-resistivity materials. Combined, these factors enable thick film resistors to deliver resistance values roughly 10x higher than Wirewound resistors.
Resistor Tolerance. – Tolerance values of Wirewound resistors are generally superior to those available from thick film. Tolerance values for Wirewound devices can be as low as 0.05% compared to 0.5% for thick film resistors.
Electrical performance. – The inductance of Wirewound resistors is significantly higher than that of thick film. Hence Wirewound devices are generally avoided in high-frequency applications. In contrast, thick film resistors generate more electrical noise than their Wirewound equivalents.
Thick film resistors can withstand higher voltages (up to 100KV as single elements) than Wirewound. Although Wirewound resistors are available with higher power (higher current) ratings than thick film. In practice, this power rating advantage is often limited by heat dissipation issues (see below).
Resistance Stability. – 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, material selection and material susceptibility to the applied voltage and thermal stresses.
The choice of materials impacts on cost. It is, therefore, difficult to make a direct comparison between thick film resistors and Wirewound.
Temperature Effects. – Temperature effects can degrade the resistor performance or, in extreme cases, cause resistor failure.
In thick film resistors, the impact of temperature is minimised by the selection of appropriate substrate material and maximising its thickness. For Wirewound resistors, the wire type and diameter, the core material and the construction require careful consideration.
The lower the TCR (Temperature Coefficient of Resistance) the better a resistor will maintain its resistance regardless of ambient temperature variations and self-heating effects.
For thick film resistor devices, TCR is normally specified as 50 ppm/°C minimum. However, 25 ppm/°C is possible if resistor elements are limited to square or short rectangular dimensions.
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. Hence, high-temperature resistance drift is lower for thick film resistors than Wirewound.
Heat Dissipation. – A major advantage of thick film resistor technology is the high thermal conductivity of the substrate. The substrate characteristics mean it is an excellent match with the aluminium material in heat sinks. Wirewound resistors, in contrast, rely on bulky aluminium fins for heat management.
Effective cooling techniques can dramatically enhance power-handling capabilities across both technologies. For instance, thick film power resistors can handle up to 400W in oil-cooled applications operating at a constant 50°C.
Pulse and Surge survivability. – The choice of resistor materials and manufacturing method impacts surge survivability. The construction of Wirewound resistors means they excel in continuous high-power (hundreds of watts) and high-energy pulse situations. The pulse performance of thick film resistors is limited by film thickness and substrate area.
While thick-film resistors deliver acceptable pulse performance, they excel in moderate-power, high-voltage, low-inductance applications where their compact design and cost-effectiveness provide clear advantages. Wirewound resistors can achieve superior pulse performance, but this capability comes with trade-offs in both physical size and cost,
For surge events, the dominant limit is energy absorption (Joule rating) and peak internal temperature. For the same surge energy, a Wirewound resistor usually runs cooler because its wire mass is orders of magnitude larger than the thick film resistive element, but again at the expense of size and cost. The performance of thick film resistors can be improved via appropriate heatsinking.
Dimensions. – The key advantages of thick film resistors over Wirewound is its smaller size (high resistance density). The high integration density of the substrate makes it ideal for high power dissipation applications in a small area.
Mechanical Issues. – Thick film technology is mechanically robust. Through-hole thick-film resistors generally offer superior mechanical robustness due to their simpler and more rigid construction. Wirewound resistors are mechanically robust in many applications but are more vulnerable due to their heavier mass and internal coil structures
Thick film resistors have superior vibration and shock resistance. They also possess better thermal cycling resistance and environmental factor resistance.
Cost. – For a given resistance value and specification, the cost of a thick film resistor tends to be significantly lower than Wirewound. Tooling costs for custom thick film resistors are also lower than their Wirewound equivalents.
In Conclusion, the choice between thick film and Wirewound resistors depends on the specific application. Thick film resistors excel in high-voltage, high-frequency applications where compact size, low inductance, and cost efficiency are priorities. They offer superior heat dissipation, mechanical robustness, and resistance values up to 10x higher than Wirewound alternatives.
Wirewound resistors provide superior tolerance (0.05% vs 0.5%), load life stability and pulse handling capability for high-power continuous applications. However, their higher inductance limits high-frequency performance, and larger size increases system complexity.
For most moderate-power applications requiring high integration density and environmental resilience, thick film resistor technology offers the optimal balance of performance, size, and cost.