What is Voltage Coefficient of Resistance?
VCR (Voltage Coefficient of Resistance) is defined as the change in resistance over a specified change in voltage. This means the stability of the resistance value under different voltages. You (include me) may not realize that VCR even exists, but every resistor has some level of instability over voltage. For applications where voltage levels are stable or are regulated and predictable, VCR is a non-issue. For other applications such as power delivery systems or test equipment, changes in voltage can lead to unacceptable changes in resistance.
How to minimzie VCR effect?
By choosing the correct resistor and understanding the factors that affect the VCR, you can minimize the VCR. Resistors that utilize standard thick film deposition processes will have poorer VCR than those that utilize direct writig process. This is due to improved resistance definition and reduced edge effects. Actually a larger size resistor will have a better VCR because a lower ohmic value material can be used to achieve a high resistance value. Below example is a typical VCR by different case sizes of UHV series of Stackpole chip resistors.
When manufacturing resistors, laser or mechanical trimming processes are commonly used to adjust the resistance value within the specified tolerance. Its resitance tolerance is commonly around 1% or less. However, trimming the resistance element during the manufacturing may adversely affect the VCR of the resistor. For thick film chip resistors, this calibrated trimming of the resistance element will cause microcracks because the thick film material will cool after trimming. This microcracks will lead parasitic impedance changes, increased electrical noise, and increased VCR.