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Electrostatic Discharge (ESD) can wreak havoc on sensitive electronic components and circuits. To safeguard electronic devices against ESD, system designers employ many protective measures, including surge resistors. This article explores the role of surge resistors in ESD management applications.

First a definition, the term pulse generally refers to an overload condition with high power and long duration. In contrast, surge refers to an overload of high voltage and a short duration (as in ESD applications).

Surge resistors dissipate harmful energy surges and protect devices from ESD events. They are strategically placed in circuits to absorb and attenuate high-voltage transients.

In this article, we cover the fundamentals of surge resistors, the types available and selection criteria.

Electrostatic Discharge Overview

Static fields pose a serious risk to personnel. In some applications, a static field is generated by the manufacturing process. Other production processes, including paint spraying and food product labelling, require static fields to function.

ESD Protection Strategies

Shielding, grounding, correct safety wear and personnel training can address many risks. Where these precautions are inadequate surge resistors act as an extra safety measure.

The design, manufacture and installation of these devices are of critical importance. It is important to follow the relevant industry specifications.

Surge Resistor Technologies

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.

The resistor must dissipate the surge’s energy, hence the designer must carefully consider the nature of the ESD pulse. From this information, the peak power specification can be calculated.

Surge resistor selection tends to be based on a cost vs performance calculation. Thin film resistors are not suitable due to their power-handling capabilities. Ceramic resistors are only used to protect against short surge as they have limited energy handling capability. This leaves thick film, metal foil and wirewound resistors.

The lower cost (Than metal foil), smaller size (than wirewound) and lower inductance (than wirewould) mean thick film surge resistors tend to dominate the ESD suppression marketplace. Metal foil and wirewound are utilised in a limited number of specialist ESD management applications.

Thick Film Surge Resistor Design

Many factors can cause an ESD event. Understanding the nature of the ESD pulse is crucial for selecting an appropriate surge resistor. ESD pulses can have various durations (typically 10 μs to 100 ms) and a wide range of amplitudes.

To survive an ESD surge event, a thick film resistor must be able to dissipate the surge energy. Design for surge conditions involves choosing appropriate dimensions for the resistive element and selecting the best (performance vs cost) resistive material. Selection of the substrate (size and material) is vital to ensure its mass can dissipate the pulse energy.

For extreme applications, a surge resistor manufacturer may modify the resistor paste materials or resistor manufacturing process (or both). The firing process used during resistor manufacture is particularly important.

The amount of potential damage to the conductive mechanism of the resistive material depends on its composition. Hence, the choice of thick film paste can directly influence the ESD performance of the resistor device. There are many resistive materials available, each with their design trade offs.

Thick film surge resistors have emerged as the dominant choice in the ESD suppression marketplace. However, it’s essential to recognise that surge resistor design is not a one-size-fits-all solution. Resistor customisation, including selecting resistive and substrate materials, may be necessary for extreme applications.