The adjustable resistor is an adjustable electronic component. It is composed of a resistor and a rotating or sliding system. When a voltage is applied between the two fixed contacts of the resistance body, the position of the contact on the resistance body can be changed by rotating or sliding the system, and a voltage that has a certain relationship with the position of the moving contact can be obtained between the moving contact and the fixed contact. It is mostly used as a voltage divider. This is because the potentiometer is a four-terminal element. The potentiometer is basically a sliding rheostat, which has several types, and is generally used for adjusting the volume switch of the speaker and the power of the laser head
So, what are the reasons for inaccurate resistivity test? We have summarized the following points for you
Cause 1. Test voltage
Generally, the resistance (rate) of dielectric materials cannot be kept constant in a wide voltage range, that is, Ohm's law does not apply to this. At room temperature, in the lower voltage range, the conductivity current increases linearly with the increase of the applied voltage, and the resistance value of the material remains unchanged. After a certain voltage is exceeded, due to the intensification of ionization, the conductivity current increases much faster than the test voltage, and the resistance value of the material decreases rapidly. It can be seen that the higher the applied test voltage is, the lower the resistance value of the material is, so that the resistance value of the material tested under different voltages may have great differences. It is worth noting that the decisive factor leading to the change of material resistance value is the electric field strength during the test, not the test voltage. For the same test voltage, if the distance between the test electrodes is different, the test results of the material resistivity will also be different. The smaller the distance between the positive and negative electrodes, the smaller the test value.
Cause II. External interference
After adding DC voltage to the high insulation material, the current passing through the sample is very small, which is easily affected by external interference, resulting in large test error. Thermal potential and contact potential are generally small and can be ignored; Electrolytic potential is mainly generated by the contact between wet samples and different metals, which is only about 20mV. In addition, the relative humidity is required to be low in the electrostatic test. When tested in a dry environment, the electrolytic potential can be eliminated. Therefore, the external interference is mainly the coupling of stray current or the potential generated by electrostatic induction. When the test current is less than 10-10A or the measured resistance is more than 1011 Ω; Strict shielding measures shall be taken for the tested sample, test electrode and test system to eliminate the impact of external interference.
Cause 3. Test time
When the measured material is pressurized with a certain DC voltage, the current on the measured material does not reach a stable value instantaneously, but has a decay process. At the same time of pressurization, a large charging current flows through, followed by a relatively long time slowly decreasing absorption current, and finally a relatively stable conductivity current is reached. The higher the measured resistance, the longer the time to reach the balance. Therefore, in order to correctly read the measured resistance value during measurement, read the value after stabilization or take the reading value after 1 minute of pressurization. In addition, the resistance value of high insulation material is also related to its electrification history. In order to accurately evaluate the static electricity performance of materials, when conducting resistance (rate) test on materials, it is necessary to first conduct power dissipation treatment, and let it stand for a certain time, which can be taken as 5 minutes, and then test according to the measurement procedure. Generally speaking, for the test of a material, at least 3-5 samples shall be randomly selected for test, and the average value shall be taken as the test result.
Cause 4. Ambient temperature and humidity
The resistance value of general materials decreases with the increase of ambient temperature and humidity. Relatively speaking, surface resistance (rate) is more sensitive to ambient humidity, while body resistance (rate) is more sensitive to temperature. As the humidity increases, the surface leakage increases, and the body conductance current also increases. As the temperature increases, the carrier movement speed increases, and the absorption current and conductivity current of the dielectric material will increase accordingly. According to relevant data, the resistance value of the dielectric at 70 ℃ is only 10% of that at 20 ℃. Therefore, when measuring the resistance of the material, it is necessary to specify the temperature and humidity at which the sample and the environment reach equilibrium. Cause 5. Leakage of test equipment
During the test, the wiring with low insulation resistance in the line will often be improperly connected in parallel with the tested sample and sampling resistance, which may have a great impact on the measurement results. Therefore, in order to reduce the measurement error, the protection technology should be adopted, and the protective conductor should be installed on the line with large leakage current, so as to basically eliminate the influence of stray current on the test results; Due to surface ionization of high voltage wire, there is certain leakage to the ground, so try to use high voltage wire with high insulation and large wire diameter as high voltage output line and shorten the connection line as far as possible to reduce the tip and prevent corona discharge; Use polyethylene, polytetrafluoroethylene and other insulating materials to make test bench and support body to avoid low test value due to such reasons
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