How to Select a High Voltage Resistor

1. Determine the Required Resistance Value

Start with the resistance value required by the circuit.

Depending on the application, a high-voltage resistor may be used for voltage sensing, voltage division, charging or discharging, current limiting, feedback, control or measurement functions.

The resistance value should therefore be considered together with the electrical function of the resistor rather than as an isolated specification.

For voltage-divider and precision measurement applications, the relationship between multiple resistance values may also be important, particularly where measurement accuracy must remain stable over temperature and operating voltage.

2. Check the Maximum Working Voltage

A resistor can satisfy the required resistance and power rating but still be unsuitable if the applied voltage exceeds its allowable working voltage.

Check the manufacturer’s specified maximum working voltage, rated voltage and applicable derating conditions.

The allowable voltage may be limited by the resistor’s maximum working-voltage capability even when calculated power dissipation appears acceptable.

This is why power rating alone should not be used to select a high-voltage resistor.

3. Evaluate Power Dissipation and Temperature

Power rating should be evaluated together with the actual thermal conditions in the application.

Important factors include:

• Continuous power dissipation
• Ambient temperature
• Case or heat-sink temperature
• Cooling conditions
• Mounting method
• Available thermal path

Published power ratings may depend on defined thermal conditions. As temperature increases, allowable power may need to be reduced according to the manufacturer’s derating information.

4. Identify Pulse and Surge Requirements

Many high-voltage applications do not operate under steady-state conditions alone.

A resistor may be exposed to switching transients, precharge events, capacitor discharge, repetitive pulses, short-term overload or surge conditions.

In these cases, continuous power rating alone may not indicate whether the resistor can withstand the load.

Consider:

• Peak voltage
• Pulse duration
• Energy per pulse
• Repetition rate
• Cooling time between pulses

Always compare the expected pulse profile with the manufacturer’s pulse-load data for the specific resistor series.

5. Consider Tolerance, TCR and Voltage Coefficient

For precision sensing and voltage-divider applications, resistance tolerance is only one part of the accuracy requirement.

Resistance Tolerance

Tolerance describes how closely the initial resistance value matches its nominal value.

Temperature Coefficient of Resistance — TCR

TCR describes how resistance changes as temperature changes.

A low TCR may be important where measurement accuracy must remain stable across a wide operating-temperature range.

Voltage Coefficient of Resistance — VCR

In some resistor technologies, resistance can also change as the applied voltage changes. This effect is described by the voltage coefficient of resistance, or VCR.

VCR can become particularly important when a large voltage is applied across the resistor, especially in precision high-voltage measurement circuits.

For demanding measurement applications, tolerance, TCR and VCR should therefore be considered together rather than evaluating tolerance alone.

6. Review Mechanical and Installation Requirements

Electrical specifications are only part of the selection process.

The resistor must also fit the mechanical and thermal design of the system.

Consider:

• Package dimensions
• Available installation space
• PCB, chassis or heat-sink mounting
• Terminal or connection type
• Cooling interface
• Mechanical environment
• Required insulation distances

For high-voltage circuits, PCB layout and insulation distances also require attention.

System-level insulation and safety requirements should be reviewed separately from the resistor’s component-level voltage rating.