In this post, we consider the use of Surge resistors to maintain operator safety in applications with the potential for high ESD (Electrostatic Discharge).
The term Pulse generally refers to an overload condition with high power and long duration whereas surge generally means an overload of high voltage and with short duration (as in ESD applications).
The paper manufacturing industry noticed a potential problem with fast moving webs of material back in the mid 1800s. Printing, plastics, textiles and pharmaceutical industry (among others) manufacturing processes can generate high level static charges.
In other applications, such as paint spraying or food product labeling, static fields are a necessary part of the production process. In all cases, high static fields can be a serious risk to personnel that must be managed appropriately.
Shielding, grounding, correct safety wear and personnel training can address many of the risks but specialist surge resistors can also be employed as a safety measure. The design, manufacture and installation of these devices are obviously of critical importance.
Choosing A ESD Resistor
Based on cost vs performance thick film surge resistors tend to dominate the ESD suppression marketplace. Thick film technology is space efficient and relatively low cost and other technologies tend to be only used in a very limited number of specialist ESD management applications. Resistor values tend to range from 100 MOhm to 500 MOhm. Tolerances tend to be 5% at best with 10% to 20% more common.
To survive a ESD surge condition a high-power resistor must be able to dissipate the surge energy. Design for surge conditions involves choosing appropriate dimensions for the resistive element and selection of the best (performance vs cost) resistive material. Selection of the substrate (size and material) is important to ensure its mass can dissipate the pulse energy.
Surge Resistor Design And Manufacturing Considerations
ESD pulses can have various durations (typically 10 μs to 100 ms) and a wide range of amplitudes. Before attempting to choose a surge resistor for ESD management applications it is important to understand the ESD pulse. Using this information the peak power specification of the resistor may be calculated.
For extreme applications, a surge resistor manufacturer may decide to modify the resistor paste materials or manufacturing process (or both) to achieve the required specification.
The choice of thick film paste can directly influence the ESD performance of the resistor device. The amount of potential damage to the conductive mechanism of the resistive material is directly related to the composition of that material. There are many resistive materials available each with their own design tradeoffs.
The thick film resistor manufacturing method and choice of materials are also crucial. The firing method is particularly important.
An electrical pulse can damage or degrade the resistive element when the voltage per unit length exceeds the design limit of the resistive material. The solution is often to increase the total length of the resistive element and therefore provide a proportional decrease in the voltage stress per unit length.
This technique delivers a higher voltage handling capability for any given package size. Trimming the resistor track is usually avoided to avoid potential hot spots as resistor value and tolerance are not generally important in surge management situations.
There are many standard surge resistors for ESD applications available but for specialist applications where personnel safety is of critical importance, it is often best to consult a specialist thick film surge resistor manufacturer.