Engineers must carefully balance performance, cost, and technological constraints when choosing high-voltage resistors for industrial instrumentation applications. In this post, we outline the importance of resistor power and voltage ratings, discuss potential failure modes, and compare high-voltage resistor technologies.
High-voltage resistors are used in a wide variety of industrial instrumentation applications. These include:
High-Voltage Dividers: For voltage measurement in HV power supplies, test equipment, and power transmission systems.
Current Limiting and Surge Protection: Protecting sensitive instrumentation from overcurrent and voltage surges. High-voltage resistors are a key element in snubber circuits.
High-Voltage Sensing and Feedback: For regulation and control of high-voltage power supplies.
Medical and Scientific Instrumentation: Including X-ray equipment and mass spectrometers.
Control Systems in Manufacturing Lines: These include robotics and PLC control.
Sensing and Measurement in Harsh Environments: Chemicals, oil, and gas.
High-Voltage Resistor Power and Voltage Ratings
When selecting a high-voltage resistor, engineers must ensure that the resistor operates within both its specified power and voltage ratings. A resistor can fail due to excessive voltage even if the power dissipation is low, and vice versa. Hence, low-value resistors are often limited by their power rating, while high-value resistors are limited by their voltage rating.
Voltage Rating indicates the maximum voltage the resistor can safely withstand without experiencing dielectric breakdown. This rating is related to the resistor’s physical construction and materials.
Power Rating specifies the maximum power the resistor can dissipate as heat without damage. It is determined by the resistor’s size, material, and heat transfer capability.
Resistors often have a lower AC voltage rating compared to DC. The DC voltage rating is the maximum continuous direct voltage a resistor can withstand without breakdown or excessive leakage. The AC voltage rating is the maximum continuous alternating voltage a resistor can handle, typically specified as the RMS (Root Mean Square) value.
In a DC circuit, the voltage stress on the resistor’s materials and insulation is constant. This can cause long-term degradation but is generally predictable. In an AC circuit, the alternating voltage causes cyclical stress on the resistor’s materials and insulation, which can lead to an increased risk of dielectric breakdown.
In industrial high voltage resistor applications, ratings are also impacted by environmental factors, including temperature, moisture, humidity, chemical exposure, dust, and extreme altitude. It is important to consider these factors and derate the resistor accordingly.
Typical Failure Mechanisms of High-Voltage Resistors
High-voltage resistors in industrial applications can operate under extreme electrical stress, making them susceptible to several potential failure mechanisms:
Dielectric Breakdown (Insulation Failure): This occurs when the electric field exceeds the dielectric strength of the resistor material. This can lead to a permanent open or short circuit.
Surface Tracking: Contaminants on the resistor surface allow leakage currents to form conductive paths, leading to gradual failure. High-voltage resistors for AC applications require longer creepage distances and special coatings to prevent arcing.
VCR Effects: Resistance changes with applied voltage due to the non-linear behaviour of resistive materials.
Thermal Issues: If the resistor cannot dissipate heat to the environment, it can cause resistance drift or, over time, complete failure.
Energy Dissipation: AC applications cause continuous power cycling in the resistor, leading to localised heating and thermal fatigue.
Mechanical Stress: Thermal cycling, vibration, or mishandling can damage the resistor, leading to resistance drift, intermittent operation, or open circuit.
Moisture/Humidity Ingress: Humidity or contaminants can enter the resistor body, leading to resistance changes or electrical leakage.
High-Voltage Resistor Selection
To maintain measurement accuracy, it is important to minimise the impact of the factors listed above and ensure the high voltage resistor remains within its specification limits. Wherever possible, environmental, thermal, and mechanical stressors should be reduced. Unfortunately, in most industrial applications, these factors cannot be eliminated entirely. Therefore, it is essential to choose the most appropriate resistor technology to match the technical and financial brief.
There are many resistor technologies, each with its advantages, disadvantages, and application limitations. Carbon film, metal film, and thin film resistors are generally not suitable for high-voltage applications. Hence, the designer often must choose between wirewound, high-voltage thick film, and metal oxide film resistors.
The general performance of thick film resistors versus wirewound, thick film versus metal oxide, and thick film versus ceramic resistors are discussed elsewhere in this blog. The specific high-voltage resistor application issues to consider include:
- Voltage rating
- Voltage coefficient of resistance (VCR)
- Power dissipation
- Pulse and surge handling
- Tracking and insulation resistance
Stability over time
A high-voltage resistor technology performance comparison follows:
- This table provides a general overview. The specific performance characteristics of any given resistor will depend on its exact design, materials used, manufacturing process, and intended application.
- There can be significant variations within each resistor technology based on specific product lines and manufacturers. Customised resistors delivered by specialist manufacturers can deliver improved performance for specific parameters.
The trade-off is between cost, performance, and size. In many applications, thick film resistor technology is perceived as the best compromise delivering acceptable performance at a reasonable cost.
In high-voltage circuits, resistors are subjected to substantial voltage stress. If a resistor has a high VCR, its resistance can deviate significantly from its nominal value. High voltage applications also generate heat, this can cause the resistor’s temperature to rise, and if the TCR is high, the resistance value will change significantly.
TCR and VCR depend upon the resistor material and doping to achieve specific characteristics. Thick Film Resistor design can impact TCR and VCR somewhat, but there are limits.
VCR depends on the voltage change over time and is impossible to predict until the resistor is in the end application. However, it is possible to establish the best possible compensation by working through a prototype sampling process with a specialist resistor manufacturer.
Selecting the most appropriate high-voltage resistor demands an understanding of electrical, thermal, and mechanical challenges. While no single technology offers a perfect solution, engineers can mitigate risks by carefully evaluating voltage ratings, power dissipation, and environmental constraints.