A variable resistor is an adjustable electronic component consisting of a resistor element and a rotating or sliding system. When a voltage is applied between the two fixed contacts of the resistor element, by adjusting the position of the contacts on the resistor element through the rotating or sliding system, a voltage proportional to the position of the moving contact can be obtained between the moving and fixed contacts. It is primarily used as a voltage divider, as a potentiometer is a four-terminal element. A potentiometer is essentially a sliding resistor, available in various styles and commonly used in volume controls of speakers and for adjusting the power of laser heads. Here are some reasons that may cause inaccurate resistance testing.
Test Voltage
Resistivity values of dielectric materials generally do not remain constant over a wide voltage range, meaning Ohm's Law does not apply here. Under normal temperature conditions, within a lower voltage range, the conductive current increases linearly with the applied voltage, while the material's resistivity remains constant. Beyond a certain voltage, due to intensified ionization movement, the increase in conductive current is much faster than the increase in test voltage, causing the material's resistivity to rapidly decrease. Thus, the higher the applied test voltage, the lower the material's resistivity, resulting in potentially significant differences in material resistivity values obtained at different voltages. It is worth noting that the determining factor for the change in material resistivity is the electric field intensity during testing, not the test voltage. For the same test voltage, if the distance between the test electrodes varies, the test results for the material's resistivity will also differ; the smaller the distance between the positive and negative electrodes, the lower the test value.
External Interference
High insulation materials, when subjected to DC voltage, exhibit very small currents through the sample, making them susceptible to external interference, which can lead to significant test errors. Thermoelectric potential and contact potential are generally very small and can be neglected; electrolytic potential is primarily caused by moisture in the sample contacting different metals, typically around 20mV. Moreover, in electrostatic tests, a low relative humidity is required, and in dry environments, electrolytic potential can be eliminated. Therefore, external interference is mainly due to the coupling of stray currents or the potential generated by electrostatic induction. When the test current is less than 10-10A or the measured resistance exceeds 1011 ohms, strict shielding measures should be taken for the tested sample, test electrodes, and test system to eliminate the impact of external interference.
III. Testing Time
When applying a certain direct voltage to the material under test, the current on the material does not instantly reach a steady value but rather decays over a process. Simultaneously with the application of voltage, a large charging current flows, followed by a relatively long, gradual decrease in the absorbing current, eventually reaching a more stable conductance current. The higher the resistance value of the material under test, the longer it takes to reach equilibrium. Therefore, to correctly read the resistance value of the material under test, it should be read after it has stabilized or after one minute of application of voltage. Additionally, the resistance value of high-insulating materials is also related to their charging history. To accurately evaluate the electrostatic properties of the material, it should first be decharged and allowed to rest for a certain period, which can be taken as 5 minutes. Then, proceed with the measurement procedure. Generally, for testing a material, at least 3 to 5 samples should be randomly selected and tested, with their average value being used as the test result.
IV. Environmental Temperature and Humidity
Typically, the resistance of general materials decreases with an increase in environmental temperature and humidity. In comparison, surface resistance is more sensitive to changes in humidity, while volume resistance is more responsive to temperature changes. As humidity rises, surface leakage increases, and so does the volume conductive current. With higher temperatures, the movement rate of charge carriers accelerates, leading to a corresponding increase in the absorption and conductive currents of the dielectric material. Reports indicate that the resistance of a general dielectric at 70°C is only 10% of its resistance at 20°C. Therefore, when measuring the resistance of materials, it is crucial to specify the temperature and humidity at which the sample has reached equilibrium with the environment.
Section V: Leakage of Test Equipment
During testing, connections with low insulation resistance in the circuit often improperly parallel with the test samples and sampling resistors, which may significantly affect the measurement results. To minimize measurement errors, protective techniques should be employed, installing protective conductors on lines with high leakage currents to essentially eliminate the impact of stray currents on the test results; High-voltage lines, due to surface ionization, have some leakage to the ground, so it is advisable to use high-insulation, large-diameter high-voltage conductors as high-voltage output lines and to shorten the connections, reduce sharp tips, and prevent corona discharge; Test benches and supports should be made from insulating materials such as polyethylene and polytetrafluoroethylene to avoid low test values due to such reasons.
