How to accurately measure capacitance with a pointer multimeter
We often use a multimeter to check the quality of capacitors during electrical maintenance. The traditional method is to charge and discharge capacitors of the same model, which is very inconvenient to operate. Some capacitors, due to their short pins and large capacity, sometimes cannot be tested with a digital multimeter. In my long-term maintenance practice, the author has developed a simple and practical testing method. Here is an introduction, hoping to bring some convenience to colleagues.
In electrical measurement, there are two types of ammeters with identical structures. One type is an impact ammeter. It is a precision instrument used to measure the electrical quantity of pulse current. When the duration of the pulse current flowing through the impact ammeter is much shorter than the free oscillation period of the impact ammeter needle, the maximum deflection amplitude of the needle is directly proportional to the electrical quantity of the pulse current, thus linearly measuring the electrical quantity of the pulse current. Another type is a sensitive ammeter, and the head of a pointer multimeter is a sensitive ammeter. When measuring capacitance using the resistance range of a pointer multimeter, a pulse charging current will be generated. If the duration of this pulse current is much shorter than the free oscillation period of the pointer on the meter head, the meter head will change from a sensitive ammeter to an impact ammeter, and the maximum deflection amplitude of the pointer, Am, is proportional to the amount of electricity Q charged by the pulse current to the capacitor. And the electric quantity of the capacitor Q=CE, E is the battery electromotive force of the resistance range, which is a constant value, so Q is directly proportional to the capacitance C, and the maximum deflection amplitude of the meter needle Am is also directly proportional to the capacitance C. Based on this principle, it is possible to measure capacitance using linear readings. The resistance of the pointer multimeter meets the above rules when deflected at a small angle, so it can accurately measure capacitance.
Taking the MF500 multimeter as an example, this article explains the method and use of adding capacitance scale. The dial of the MF500 multimeter is shown in the figure. Select the 10 small cells on the left end of the DC uniform scale line as the linear scale for capacitance. This is because it can meet the linear condition of small angle deflection and facilitate reading. Beyond 10 grids, the scale will gradually become non-linear. Take a new capacitor, such as a capacitor with a nominal value of 3.3F, and use a digital multimeter to measure its actual capacity of 3.61F. Measure the R of the 500 type multimeter × Zero the ohm in first gear. After discharging the capacitor with the tip of the probe, use two probes to contact the two poles of the capacitor and observe the maximum deflection amplitude of the probe. Reuse R × 10. R × 100, R × 1k, R × Repeat the above steps for 10k gear in order to see which gear has the maximum deflection within the 10 grid range. Result in R × At 1k gear, the maximum deflection of the watch needle is 3 small grids, using 3.6 μ Dividing F by 3 small grids yields a capacitance sensitivity of 1.2F/grid for RX1k gear. As long as the capacitance sensitivity of one gear is measured, the sensitivity of other gears can be calculated. The sensitivity of high resistance ratio is high, while the sensitivity of low ratio is low, and the relationship between adjacent gears is recursively 10 times. So the capacitance sensitivity of the resistance range of the MF500 multimeter is as follows: RX1 range -1200F/grid, R × 10 gears 1201F/grid, R × 100 gears -12F grid. R × 1k gear -1.2F/grid. Rx10k gear -0.12F (120nF)/grid.
