In industrial environments, electrical systems are often exposed to transient over voltages. These events range from fast microsecond pulses to surges lasting hundreds of milliseconds. While a single surge can destroy high-value equipment, more often it’s the cumulative stress of repeated smaller surges that impacts on equipment life and reliability.
This article examines the functional differences between surge and pulse protection resistors, reviews available resistor technologies, and outlines design considerations for thick film resistors used in industrial surge conditions.
Surge Protection Resistors vs. Pulse Protection Resistors
Pulse and Surge resistors are both designed to handle high-energy transients in electrical systems, but there are fundamental differences. Surge resistors absorb high-energy overvoltage transients from a few milliseconds up to several hundred milliseconds duration. In contrast, pulse protection resistors are designed to withstand short pulses (microsecond (µs) to low millisecond (ms) duration).
The Threat – Industrial Surge Events
Surge events are common in industrial applications. They can be caused by external events such as lightning strikes, but those caused by events within an industrial facility are much more common. Industrial surge events include switching of inductive loads (motors, solenoids), arc faults, short circuits and breaker tripping.
Electrical surges can destroy semiconductors, control boards, sensors, and PLCs, accelerate wear in motors and transformers and corrupt control signals. In extreme cases, electrical surge events can cause fire or explosion.
Comparing Surge Resistor Technologies
Surge resistor selection tends to be based on a cost vs performance calculation.
Due to their construction, Thin film and metal foil resistors’ surge survival performance is poor. Carbon film resistors have moderate surge tolerance and are often found in consumer goods rather than industrial applications.
Metal Oxide Film resistors are an excellent choice for surge applications; they are generally more expensive than carbon film and thick film resistors, but less costly than Wirewound or metal foil types.
Wirewound resistors are the most rugged option for absorbing large surge energy. They can handle repeated surges, fault currents, and overloads, but they are large devices and more expensive than Thick film or Carbon film resistors.
Leaded thick film devices exhibit good performance under surge conditions and they are relatively small and cheap. In many surge applications, they are the best compromise selection.
Specifying Thick Film Surge Resistors
Surge resistor values tend to range from 100 MOhm to 500 MOhm. Resistor tolerance is not an important consideration and tends to be 5% at best, with 10% to 20% more common.
Effective surge protection depends on selecting resistors that can withstand the energy and duration of surge events. For thick film resistors, design choices such as resistive element geometry, paste composition, and substrate mass directly influence performance.
Since a thick film resistor must dissipate the surge’s energy, the surge waveform profile is a key input to the design process. Given the profile and energy rating (Joules), the design for surge conditions involves choosing appropriate dimensions for the resistive element and selecting the best (performance vs cost) resistive material.
The resistive material’s composition determines how much damage the resistive path will sustain during surge events. This makes the choice of thick film paste a key factor in surge resistor performance.
Multiple resistive materials are available, each offering different design trade-offs.
For extreme applications, surge resistor manufacturers may modify the resistor paste materials, thick film resistor manufacturing process, or both. The firing process during manufacture is particularly important.
Selection of the substrate (size and material) is vital to ensure it has sufficient thermal mass to absorb and dissipate the surge energy. For short surges, extra cooling (heatsinks, oil, forced air) usually is not needed, because there isn’t time for heat to transfer to the surroundings, the resistor just needs enough thermal mass to absorb the pulse.
However, if there are repetitive surges (e.g., pulsed power circuits, crowbar protection, motor drives), then average heating is a concern. In those cases, the duty cycle determines whether extra cooling or derating is necessary.
When properly specified, thick film resistors offer a practical balance of robustness, size, and cost, making them well-suited for protecting industrial electronics from damaging surge conditions.