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 consider the key elements of a high voltage resistor specification. The main factors that limit each of those elements and how they are interrelated.
The resistance value is a function of the resistor material and the resistance track (length,width,thickness). The resistor manufacturer will decide on the resistor material and the track dimensions depending on the application and market requirements – The specification.
Resistor tolerance is expressed as a variation from nominal resistance value in per cent terms. The tolerance of thick film high voltage resistors is typically between 0.5 and 10%. The value is always specified at 25C.
The variance in resistor value can be due to a variety of reasons, including resistor thickness variability during print, the 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 ultimately there is a trade off to be made between the resistor specification and cost.
Maximum Power Rating
When designing high voltage resistors a key consideration is the Voltage Coefficient of Resistance (VCR). This defines the change in resistance with applied voltage and it is always negative. Applying a high voltage across a resistor can cause a significant change in resistance if steps are not taken to limit the VCR.
An appropriate choice of resistor materials and terminating materials is one part of the solution. The other is to increase the total length of the resistive element and therefore provide a proportional decrease in the voltage stress per unit length.
A serpentine pattern is often used to maximise resistor track length but this in turn limits the track width. The current carrying capability of a high voltage resistor must, therefore, be limited to prevent overheating and damage to the resistor track. This, in turn, limits the maximum power of the device.
Maximum Voltage Rating
The maximum voltage limit is usually directly related to the resistors ability to dissipate heat. For a given resistor value the higher the voltage, the higher the current and therefore the more heat generated as current flows through the resistor.
One solution could be to increase the thickness of the resistor track but that impacts on VCR (see above). Another could be to increase the mass of the resistor but that increases the dimensions of the resistor. Or the resistor could be cooled using a heatsink or some other means but that increases cost and complexity. Alternative materials could be used 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 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 resistor is a compromise. Resistance, tolerance, maximum power and voltage are all interrelated. A wide range of standard high voltage thick film resistor devices are available to give the designer options when choosing a resistor. If no standard high voltage resistor specification meets the system requirements then specialist resistor manufacturers may be able to help.