High-voltage resistor selection is about more than resistance value and power rating. In high-reliability, high-voltage applications, voltage stress, long-term stability, dimensions, system board interface, and environmental constraints are often critical.
Wherever possible, a standard resistor device from one of the major resistor manufacturers should be the first choice. When a standard resistor cannot meet a precise specification and a compromise is not an option, a custom high-voltage resistor may be the most practical solution.
We cover the key factors to consider when specifying a custom high-voltage resistor device.
Why Thick Film Resistors For High Voltage Applications
There are many types of resistors, but selecting a high-voltage resistor is not determined by resistor technology alone. The choice of a resistor for a particular application depends on the specific working-voltage rating, overload rating, creepage/clearance, device coating, body length, and environmental factors.
Small SMD chip resistors are not suitable for high-voltage applications. Carbon film and carbon composition resistors also exhibit poor high-voltage performance. In general, thick and thin-film, wirewound or metal oxide film resistors are the best choice.
Thick film resistors are common in high voltage applications as they deliver
- High resistance values.
- Compact size relative to voltage capability.
- High pulse handling options.
- Customisation options.
They are well-suited to high-voltage dividers and sensing applications.
Leaded thick-film resistors have significantly higher resistance values than thin-film resistors and are generally lower cost. Low-inductance wirewound resistors or metal-oxide film resistors are best when energy absorption is critical.
Start With The Application
A resistor should be specified based on the application conditions. Trying to force a fit with a standard resistor can be counterproductive.
The system designer should define:
- Resistance value
- Tolerance
- Working voltage
- Operating temperature range
- Expected lifetime/reliability target
- Transient conditions, including continuous or pulsed loading
- Environmental conditions
- Any mechanical issues, including stress and vibration
- Compliance/reliability/sample approval needs
- TCR / VCR requirements
- Dimensional constraints
- Termination preferences
- Coating/insulation requirements
- Annual volume and prototype needs
Specifying many of those parameters is straightforward; others, less so. We cover these in the next section.
Defining High Voltage Resistor Parameters
Key parameters to define can be split into three categories.
- Electrical Performance Parameters
- Stability and Drift
- Physical and Environmental Constraints
Electrical Performance Parameters
Working voltage rating and VCR: A custom high-voltage resistor manufacturer will need to know the peak working voltage, including any known transient conditions and whether the voltage is DC, AC, or pulsed.
Voltage Coefficient of Resistance (VCR) describes how a resistor’s resistance changes in response to applied voltage. It is expressed as resistance change per volt, typically measured in parts per million per volt (ppm/V). It is important to specify the maximum allowable total change at the rated working voltage, e.g. ≤ ‘x’ ppm at ‘y’ V
TCR and self-heating: TCR is the relative change in a material’s electrical resistance per degree change in temperature. System designers should specify the required TCR across a temperature range, e.g., < ’x’ ppm between ‘y’ and ‘z’ C. The maximum ambient temperature and the expected temperature rise due to power dissipation are also important information for the resistor manufacturer to consider.
TCR vs VCR: TCR and VCR tend to work against each other, forcing design compromises Hence, most resistor manufacturers that offer very low VCR only state a small temperature window.
TCR and VCR are mostly determined by the thick film material. However, resistor geometry (track length and pattern) also matters. A longer resistor track lowers the electric field per unit length, which improves VCR. But making the resistive path longer can make the temperature behaviour worse, increasing TCR.
Pulse and transient performance: If a resistor is subjected to pulse events, then transient energy dissipation is a concern. In many applications, it will be difficult to quantify, but it’s vital to provide as much information as possible on the number of pulses ( thousands, tens of thousands?) a resistor will be subjected to over its lifetime, pulse waveform (square, exponential, other), pulse width amplitude and duty cycle to the high voltage resistor supplier.
Stability and Drift
Long-term resistance drift: System designers must decide on the target resistor lifetime and the maximum allowable resistance drift over that period. It is also important to consider thermal stress and voltage stress. Hence specify ‘x’ % resistance drift after ‘y’ hours at ‘z’ C rated power.
However, rated power can be ambiguous and often needs further clarification. For example, is it based on continuous load? What is the mounting method? These factors and more should be considered.
Ratio stability in divider applications: When high-voltage resistors are used in dividers, sense chains, feedback networks, or scaling elements, in industrial instrumentation applications ratio matching typically matters more than absolute resistance value. This is because when a circuit’s output depends on a resistor ratio, two resistors sharing the same resistance variance will still deliver the correct output.
In these applications, VCR, TCR, self-heating and long-term drift are all key issues to consider.
Physical and Environmental Constraints
Creepage and clearance: This can be critical for safety and regulatory compliance and is closely tied to physical dimensions. Creepage distance determines whether a given body size is viable at a given voltage.
Package temperature range: It is important to advise the high voltage resistor manufacturer of three temperatures, not just one:
- maximum ambient temperature around the resistor.
- maximum substrate / PCB temperature near the resistor.
- maximum resistor element or hot-spot temperature under load.
To ensure load-life stability, the manufacturer will need sufficient information to estimate the actual resistor film temperature.
Environmental protection: Specify the environment the resistor will actually see, so the manufacturer can recommend the right passivation, overglaze, encapsulation, coating, sealing, or termination protection.
In harsh environments elements to consider include humidity, chemicals and corrosive substances, salt, and dust. Remember, chemicals may only be present for a short time (for example, cleaning solutions), but can still cause damage.
Vibration and shock: Specify the vibration and shock levels present in the application, and classify each as either constant or intermittent.
Specifying a custom high voltage resistor is a system-level exercise involving electrical, mechanical, environmental, and reliability trade-offs. Thick-film technology offers strong flexibility, but only if the application requirements are clearly defined. The better the application brief, the better the resistor design.
Ultimately, a close working relationship between manufacturer and customer delivers the best outcome.