Thick film power resistors are used in a wide range of applications. These include power supplies, motor control, load banks, braking control, IIoT and ESD management.
Common circuit applications include surge and pulse suppression, measurement circuits, snubber circuits and filters. In this post, we discuss the main categories of thick film power resistor applications.
Inrush Current Limiting
An input current surge (inrush) can occur when electrical equipment is first switched on, often as input capacitors charge or transformers and motors are energised. Although brief, this surge can be significant and may place short-term high levels of stress on the current-limiting resistor.
When choosing a current-limiting resistor, the first step is to define the surge event. This includes pulse width and applied voltage/power.
During a short surge, the resistor doesn’t have time to conduct heat away to its surroundings (PCB, air, heatsink). Hence, the temperature rise is dominated by the resistor’s thermal mass and its short-term thermal impedance. Hence, for short pulses, the limiting factor is not the resistor’s steady-state power rating, but its ability to absorb pulse energy without excessive temperature rise or long-term drift.
Pulse Protection
A “pulse” is a short-term overload event that may occur during normal operation or under abnormal conditions. Pulses can vary widely in peak level and duration. The function of a pulse resistor is to withstand pulse energy and/or limit current, thereby protecting downstream parts.
When selecting a pulse-rated resistor, it’s important to identify the pulse amplitude, shape, and duration and the time between pulse events. Environmental conditions, including ambient temperature, should also be considered, as they can impact recovery between pulses.
Manufacturers typically provide pulse/overload curves for common waveforms. If the actual pulse shape differs, it’s important to calculate an equivalent (conservative) approximation to those curves and rate accordingly. Selecting an inadequately rated resistor can lead to resistance drift, resistor cracking, and eventual failure.
Load Banks
Resistor load banks are used to test and commission power equipment, including generators, UPS systems, motor drives and solar inverters by providing a controlled, repeatable electrical load. The load is often adjustable using switchable resistor stages.
Typical applications include minimum-load/bleeder loads for stability in some supplies, burn-in rigs, production test fixtures, and acceptance testing during installation and maintenance.
When designing or selecting a load bank, the key issues to consider are the power level and operating range (voltage/current), the duty cycle, and the resolution of the load steps.
As most of the input power is converted to heat, thermal management is critical. Most applications rely on forced-air cooling or, for higher power densities, liquid-cooled solutions utilising de-ionised water or oil.
Safety features, such as temperature monitoring, over-temperature shutdown, safe enclosures and robust switching/connection hardware, are as important as the resistor elements themselves.
Snubber Resistor
Snubber resistors protect switching devices by damping voltage overshoot and ringing that can stress components and increase EMI. A common approach is to utilise an RC snubber circuit, where a resistor and a capacitor are connected in series, and this series network is placed across the switching device or the inductive load.
The resistor must handle the repetitive pulse current and energy associated with each switching event, and it must also be rated for the average power it will dissipate over time. The snubber resistor value is chosen to provide effective damping while keeping losses and peak current within acceptable limits. Low inductance ensures the snubber works as intended at fast switching edges.
For compact snubbers circuits, low-inductance power resistors (thick film is ideal) and good pulse handling are typically more important than continuous wattage alone.
Current Sense Resistor
A shunt (current-sense) resistor creates a small voltage drop that is proportional to the current flowing through it. By measuring this voltage, the current can be calculated and used for control or protection.
Key current sense resistor selection factors include signal level vs. power loss (I²R), tolerance, temperature effects (TCR/self-heating) and stability/drift. In high-voltage or divider applications, the Voltage Co-efficient of Resistance (VCR) can introduce a gain error.
If the voltage drop is too low, amplifier offset and noise can dominate; if it is too high, power loss and reduced load voltage increase. Wide tolerance and high TCR (or significant self-heating) increase measurement error.
Harmonic Filters
Harmonic filters reduce harmonic distortion by attenuating harmonic currents/voltages and controlling resonance.
Selecting resistors for harmonic filter networks (for damping, snubber, or discharge functions) requires an understanding of the system impedance, harmonic spectrum, expected RMS and transient stresses, and thermal constraints to ensure stable performance and reliable operation.
Key selection factors include the resistor’s intended role (damping/snubber/discharge), RMS and pulse power handling and derating, working/peak voltage limits, frequency parasitics, stability (TCR/self-heating/drift), and safety issues.
Grounding Resistor
Grounding (neutral grounding) resistors limit earth-fault current by connecting the system neutral to earth through a specified resistance. Selection depends on the system voltage and grounding configuration, the target fault-current limit, the short-time duty (I²Rt) and temperature rise for the expected fault-clearing time. Insulation, enclosure, and environmental requirements are also important considerations.
Bleeder/discharge resistors are chosen to meet a specified discharge time while keeping continuous V²/R loss and temperature rise within acceptable limits. Working voltage/spacing and transient overload capability must also be considered.
Grounding (neutral grounding) resistors are safety-critical because they sit in the earth-fault path and must reliably limit fault current without failing during the time it takes protection devices to clear the fault. Selection should be based on the required fault current and clearing time, plus adequate insulation, creepage/clearance, and enclosure protection.
Braking Resistor
Braking resistors slow down or stop motors by dissipating regenerated energy as heat, preventing DC-link overvoltage during deceleration. There are a wide variety of loads and, therefore, a variety of resistor technologies used in braking applications.
The Resistor technology is chosen based on braking energy, peak power, duty cycle, and installation constraints. Thick film parts are typically used in lower-energy, compact applications, while higher-energy systems often employ wirewound or grid assemblies.
Key considerations include pulse/overload capability, heat dissipation (mounting, airflow, surface temperature) and working voltage on the DC link.
The resistor should be fail-safe and have protective features such as thermal cut-outs and predictable failure behavior with drive-side fault detection.
Thick Film Power Resistor Selection Criteria
When choosing a thick film resistor for power applications, it is important to consider:
* Required resistance value and tolerance
* Continuous power rating vs. ambient/case temperature
* Maximum working voltage and overload limits
* Pulse/overload profile (shape, amplitude, duration, repetition)
* Operating temperature range and cooling
* Package and mounting limitations (including heatsinks)
* Environmental factors: humidity, dust and chemicals, mechanical shock/vibration.
Conclusion
An extensive range of resistor technologies and devices is available from a range of manufacturers. When choosing a power resistor for a particular application, it is useful to utilise the manufacturer’s application and sales support teams. This is particularly important where safety is an issue.
If a standard device is not adequate, application-specific thick-film power resistor manufacturers can produce a custom device.