There are many resistor technologies available, but the most common 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.
Resistor Construction Comparison
The Thick film resistor manufacturing process involves depositing a resistance film pattern onto a flat substrate (usually Alumina). The assembly is then fired at a high temperature.
The resistor elements are often applied in spiral patterns. They consist of metal alloy or oxide particles, functional elements and a glass particle binder.
As the name suggests Wirewound resistors manufacture involves winding wire around a core. The resistance value is a function of the length of the wire, its cross-section and its resistivity.
Total resistance is determined by the resistivity of the resistor material, and the resistor track length, width and thickness. The resistor material for thick film power resistors is the resistor film and for Wirewound, the resistor wire material.
As resistor length, width and thickness all have limits, resistor material selection is critical. The long term stability of the resistor material and the impact of temperature are also key considerations.
Resistor Performance Comparison
Resistance value. – Wirewound resistors are somewhat limited in resistance range. They do not have the high Ohmic range offered by thick film 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 thick film. Hence Wirewound devices are generally avoided in high-frequency applications. But 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. 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 (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, the materials selected and their susceptibility to the applied voltage and thermal stresses.
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 technology.
Temperature Effects. – Temperature effects can degrade the resistor performance or, in extreme cases, cause resistor failure.
In the thick film resistors, the impact of temperature is minimised by the selection of appropriate substrate material and thickness. For Wirewound resistors, the wire type and diameter, the core 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.
In thick film resistor devices, TCR is normally specified as 50 ppm/°C minimum in HV applications. 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
Heat Dissipation. – A major advantage of thick film resistor technology is the high thermal conductivity of the substrate. The characteristics of the substrate material mean it is an excellent match with the aluminium material in heat sinks.
Cooling techniques can significantly increase the power handling capability. For example, 400W thick film power resistors are available for oil-cooled (constant 50°C) applications.
Heat dissipation in Wirewound resistors is a major issue that can compromise overall performance.
Pulse and Surge survivability. – The choice of resistor materials and manufacturing method impacts surge survivability. The thick film construction delivers superior surge performance to Wirewound resistors in many applications. However, using special materials and winding techniques (at increased cost) can improve the surge survivability of Wirewound devices.
Both thick film resistors and Wirewound devices have high power handling capabilities. The key issue in many high power applications is how to dissipate the heat generated by the surge event.
Dimensions. – The key advantages of thick film resistors over Wirewound are 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. Whereas the construction method makes Wirewound resistor damage more likely via packaging, insertion, and lead forming processes.
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.
Wirewound resistors do offer higher precision (improved tolerance), higher stability and lower values. But this is often at the expense of a much larger footprint and higher inductance than an equivalent thick film resistor device.
A Wirewound resistor is fundamentally a coil of wire (an inductor). Although the choice of material can limit the impact of the high inductance, it remains a major disadvantage. Addressing heat dissipation from any Wirewound resistor is a key issue. This often drives the decision to select a thick film resistor vs Wirewound alternative.