Power resistors are key components in medium power (up to 200kVA) auxiliary power supplies. They provide overcurrent protection, limit inrush current, damp switching transients, and deliver harmonic damping.
Power resistors are used in power supply measurement and control circuits, dissipate heat in dynamic braking circuits, and improve operator safety by bleeding retained hazardous voltages down to a safe level.
Auxiliary Power Supply Applications
Auxiliary power supplies are “secondary” in function, but often critical in practice. They do not power the main asset (example: large motor-driven industrial machine) but power supporting infrastructure for that asset (example: cooling fans)
Power resistors are used in power supplies across a wide range of industries, including rail, automated production, mining, oil extraction, wastewater treatment, and data centres.
Power Resistor Selection Considerations
When considering resistors for power supply applications, the key design issues are not limited to resistance value and nominal power rating alone. The resistor must be selected according to operating factors, thermal behaviour, overload requirements, physical size and long-term stability, all of which interact in ways that can undermine a design if not addressed as a whole.
Central among these is the requirement to withstand not just steady-state current and voltage, but also short-duration overloads, startup surges and repetitive pulses. Resistor selection must therefore account for both continuous power dissipation and short-term energy absorption.
In particular, the maximum startup current and the energy dissipated during the charging period demand careful attention: this requires an understanding of peak current, pulse duration and repetition rate. As the resistor may not cool sufficiently between events, thermal accumulation compounds the challenge.
Where resistors are exposed to switching events, discharge cycles or abnormal operating conditions, pulse-handling becomes a further design constraint. Resistor selection for pulse events depends on the shape and severity of the pulse, peak power, pulse width, pulse energy and repetition frequency. These parameters that must all be evaluated together rather than in isolation.
Underlying all of the above is thermal performance. A resistor’s power rating is only valid under defined operating conditions, and in practice its temperature rise depends on enclosure design, airflow, nearby heat sources and mounting surface.
A part that appears adequate from a datasheet may be compromised by excessive temperatures inside a compact power supply casing. Physical size and mounting arrangement matter here too, since they directly influence both thermal behaviour and safety.
The resistor must also withstand the maximum continuous working voltage and any transient overvoltage that may occur. This is particularly relevant in damping, discharge or fault-limiting positions where high instantaneous voltages are common.
In transient and harmonic damping roles specifically, the resistor must perform correctly under dynamic conditions rather than simply dissipating steady power. The resistance value must deliver the required damping effect while tolerating the repetitive electrical stress produced by switching activity or harmonic currents.
Where a power resistor is used for current sensing accuracy becomes the dominant concern, making tolerance, long-term stability, and the temperature and voltage coefficients of resistance critical parameters, since all of these directly affect measurement accuracy.
All of the above impact on long-term stability. Elevated temperature, repeated pulsing, thermal cycling, humidity and contamination can all cause resistance drift and shorten service life.
Power Resistor Technologies Used In Power Supply Applications
The most common types of power resistors used in power applications are:
- Thick Film Resistors
- Wirewound Resistors
- Metal Film resistors
And for one specific application, metal element shunt resistors
No material or manufacturing method is perfect. Ambient temperature, moisture, transient circuit conditions and mechanical stress all impact performance.
The main issues to consider and the selection process are covered in detail in our selecting a high-power resistor post.
Thick film resistors are the most common type found in snubbers, damping, bleeders, voltage dividers, suppression networks, and other control/protection functions where size and pulse tolerance matter more than very high energy absorption.
Wirewound resistors are typically found in pre-charge, discharge, surge-handling and braking-related applications, where energy absorption is key.
Metal film resistors are used in precision and control circuitry. Sometimes thick-film power resistor devices are preferred in these applications, as they are lower cost.
Custom Power Resistors vs Standard Resistor Product
Our Custom Thick Film Resistor vs High Volume Resistor Manufacture post gives a general overview of the issues to consider.
When the requirement (pulse waveform, packaging, integration, thermal or compliance issues) fails to match the specification of an off-the-shelf part an engineer might select a custom power resistor device. Space constraints may make a customised device the only option.
A custom part may be chosen when a designer needs a non-standard resistance value or tolerance or has no choice but to match a specific resistor stability or drift target. Standard resistors may not survive in applications that are subjected to high temperature, chemicals and dust, high moisture levels, vibration and/or shock.
Conclusion
Selecting the right power resistor for auxiliary power supply applications involves more than matching a resistance value and power rating. Engineers must account for thermal behaviour, pulse handling, voltage stress, long-term stability, and the specific application, whether damping, protection, discharge, or measurement.
Thick-film resistors are well-suited to many of these applications, particularly where compact size, pulse tolerance, and cost matter. Wirewound and metal film types remain the better choice for high-energy absorption and precision measurement, respectively.
Where standard parts fail to match demanding specifications, a custom power resistor solution may be the only practical answer.