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Choosing a thick film high voltage resistor always involves compromises. Rarely is the perfect fit for the application available as a standard product.

In this post, we review the key elements of a thick film high voltage resistor specification. We discuss the primary factors that limit these elements and their interrelationships.

Resistance Value

The resistance value of a thick film resistor is a function of the resistor material and the track geometry (length, width, thickness.) Resistor materials have varying resistivity, temperature coefficients, and stability characteristics.

Manufacturing processes, such as screen printing and curing, are also a factor. These can lead to slight variations in resistance values between individual resistors in the same batch.

It is important to remember that the resistance value specified on datasheets is only valid given a specific set of conditions. Ambient temperature (TCR), applied voltage (VCR), power dissipation, environmental conditions and ageing effects can all affect high voltage resistor stability over time.

Depending on the high voltage resistor application and performance specification, the resistor manufacturer will decide on the thick film resistor material, track dimensions and manufacturing process.

Resistor Tolerance

Thick film high voltage resistor tolerance is expressed as a variation from nominal resistance value as a percentage. For thick film high voltage resistors the tolerance is typically between 0.5 and 10% at 25C.

The variance in resistor value can be due to a variety of factors. These include resistor thickness variability during print, variance in resistance of the resistor material and the impact of the thick film resistor firing process during manufacture.

The resistor manufacturer can make design and manufacturing choices to influence each of these factors (to a point). But there is always a trade-off between the resistor specification and cost.

Maximum Power Rating

It is important to select a high voltage resistor with an appropriate power rating. The resistor device must be capable of dissipating the heat generated at a given voltage. Higher resistance values result in higher power dissipation.

The construction and design of the thick film high voltage resistor can influence its power rating. Material selection, resistor size, and internal structure impact a high voltage resistor’s ability to dissipate power.

The operating environment, such as ambient temperature and airflow should be considered. Higher temperatures reduce the resistor’s ability to dissipate heat. Thermal management is crucial to ensure the resistor operates within its specified power rating. Manufacturers often provide derating factors to ensure the reliable operation of thick film high voltage resistors over the long term.

The final consideration is the Voltage Coefficient of Resistance (VCR). Applying a high voltage across a thick film high voltage resistor can cause a significant change in resistance. The impact can be minimised by using resistor materials with a low VCR.

Maximum Voltage Rating

The maximum voltage limit is directly related to the resistor’s ability to dissipate heat (see above). For a given resistance value the higher the voltage, the higher the current and, therefore, the more heat generated as current flows through the resistor.

One common solution is to increase the thickness of the resistor track, but that impacts on VCR. Another is to increase the mass of the resistor, but that increases its dimensions. Or, the resistor can be cooled using a heatsink or some other means, but that increases cost and complexity. Selecting alternative materials can help, but again that increases the cost.

The physical size of the resistor can also be an issue in high voltage applications. There is a risk of surface tracking between the terminations on a small device. Larger devices will give the resistor designer more track spacing options to prevent breakdown between resistor tracks within the resistor device. Of course, in some applications, increasing the resistor size may not be an option.

Choosing a high voltage resistor is a compromise. Resistance, tolerance, maximum power and voltage are all interrelated. A wide range of standard thick film high voltage resistor devices are available. If no standard resistor specification meets the system requirements, then a specialist thick film high voltage resistor manufacturer may be able to help.