There are many resistor technologies available, but the most common are wirewound and thick film. In this article, we compare thick film resistors with wirewound resistors. We discuss the advantages and disadvantages of each type.
When comparing various types of resistors, it is important to consider:
Resistance value. Resistance tolerance.
Electrical properties. The stability of the resistance value over time.
The change in the resistance value with temperature.
Cooling. Power (and surge) handling capacity.
Dimensional constraints. Mechanical robustness.
Cost. The EAK high-power thick film resistor manufacturing process involves depositing a resistive film pattern onto a flat substrate, typically alumina. The components are then fired at high temperatures.
Resistive elements are typically applied in spiral mode. They consist of metal alloys or oxide particles, functional elements, and glass particle binders.
As the name suggests, the manufacture of wirewound resistors involves winding wires around a magnetic core. The resistance value is a function of wire length, cross-section, and resistivity.
The total resistance is determined by the resistivity of the resistive material and the length, width, and thickness of the resistance track. The resistor material of a thick film power resistor is a resistive film, whereas the resistor material of a wire wound is a resistive wire material.
Since resistors are limited in length, width, and thickness, the choice of resistor material is crucial. The long-term stability of the resistive material and the influence of temperature are also key considerations.
Resistance value. Wirewound resistors have a limited range of resistance. They do not have the high ohm range that thick film resistors provide.
Resistance tolerance. The tolerance values of wirewound resistors are generally better than those of thick film resistors. Tolerances for wirewound devices can be as low as 005%, while the tolerance value of a thick film resistor is 05%。
Electrical properties. The inductance of wirewound resistors is significantly higher than that of thick films. For this reason, wirewound devices are often avoided in high-frequency applications. However, thick-film resistors produce more electrical noise than wirewound resistors.
EAKEAK high-power thick film resistors can withstand higher voltages (up to 100kV per element) than wirewound resistors. Wirewound resistors have a higher power rating (higher current) 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 a resistor to maintain its initial resistance value within a specified range over its lifetime. The stability of a resistor is directly related to the resistor's manufacturing process, the material chosen, and its sensitivity to applied voltage and thermal stress.
The choice of material has a direct impact on the cost. As a result, it is difficult to directly compare thick-film high-voltage resistors with wirewound resistors. However, in general, winding technology has superior load-life stability over thick-film technology.
Temperature effects. Temperature effects can degrade resistor performance or, in extreme cases, lead to resistor failure.
In thick film resistors, the influence of temperature is minimized by selecting the appropriate substrate material and thickness. For wirewound resistors, careful consideration needs to be given to the wire type and diameter, core, and construction.
The lower the TCR (temperature coefficient of resistance), the better the resistor retains its resistance, regardless of ambient temperature changes and self-heating effects.
In thick film resistive devices, TCR typically has a minimum value of 50 ppm °C in high-voltage applications. However, if the resistive element is limited to a square or short rectangular size, it is possible to reach 25 ppm °C.
The TCR of wirewound devices varies depending on the wire type and ranges from 25ppm °C to 1,000ppm °C for precision devices.
Cooling. A major advantage of thick film resistor technology is the high thermal conductivity of the substrate. The nature of the substrate material means that it is a good match for the aluminum material in the heat sink.
Cooling technology can significantly improve power handling capabilities. For example, 400W thick film power resistors can be used for oil-cooled (constant 50°C) applications.
Heat dissipation of wirewound resistors is a major concern and can affect overall performance.
Pulse and surge survivability. The choice of resistive material and the method of manufacturing affect the survivability of the surge. The thick film construction provides superior surge performance for wirewound resistors in many applications. However, the surge survivability of wound devices can be improved by using special materials and winding technology (increased cost).
Both thick-film resistors and wirewound devices have high power handling capabilities. In many high-power applications, the key issue is how to dissipate the heat generated by a surge event.
Size. The main advantage of thick film resistors compared to wirewound resistors is their smaller size (high resistance density). The substrate's high integration density makes it ideal for small-area, high-power dissipation applications.
Mechanical problems. Thick film technology is mechanically robust. This construction method makes wirewound resistors more susceptible to damage through packaging, insertion, and wire forming processes.
Cost. For a given resistance value and specification, thick film resistors tend to cost significantly less than wirewound resistors. Custom thick film resistors also have lower tooling costs than their wirewound resistors.
Wirewound resistors do have higher accuracy (improved tolerances), higher stability, and lower values. But this usually comes at the cost of larger size and higher inductance than equivalent thick-film resistive devices.
A wirewound resistor is basically a coil (inductor). While the choice of material can limit the effects of high inductance, it is still a major drawback. Solving the heat dissipation problem of any wirewound resistor is a key issue. This often leads to the decision to choose between thick film resistors and wirewound resistors.