How to use a pointer multimeter to accurately measure capacitance
We often use a multimeter to check the quality of capacitors during electrical maintenance. The traditional method is to compare the charging and discharging of capacitors with the same model, which is very inconvenient to operate. Some capacitors cannot be detected with a digital multimeter due to short pins and large capacity. In the long-term maintenance practice, the author has explored a simple and practical detection method, which is now introduced as follows, hoping to bring a little convenience to colleagues.
In electrical measurement, there are two types of ammeters with identical structures. One is the impulse current meter. It is a precision instrument used to measure the electric quantity of pulse current. When the duration of the pulse current flowing through the impulse current meter is much shorter than the free oscillation period of the impulse current meter needle, the maximum deflection amplitude of the needle is proportional to the electric quantity of the pulse current, so that the electric quantity of the pulse current can be measured linearly. Another type is a sensitive ammeter, and the head of a pointer multimeter is a sensitive ammeter. When measuring a capacitor with 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 meter head pointer, the meter head will change from a sensitive ammeter to an impulse ammeter, and the maximum deflection amplitude Am of the pointer is proportional to the amount of charge Q that the pulse current has on the capacitor. And the capacity of the capacitor Q=CE, E is the electromotive force of the battery in this resistance range, which is a constant value. Therefore, Q is proportional to the capacitance C, and the maximum deflection amplitude Am of the pointer is also proportional to the capacitance C. According to this principle, it is possible to measure capacitance using linear readings. The resistance block of the pointer multimeter fully satisfies the above rule when deflected at small angles, so it can accurately measure the capacitance.
Taking the MF500 multimeter as an example, explain the method and use of adding a capacitance scale. The MF500 multimeter dial is shown in the figure. Select the 10 small grids on the left end of the DC uniform scale line as the linear scale for capacitance. This is because it can satisfy the linear condition of small angle deflection and is convenient for 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. Set the R × 1 gear of the 500 type multimeter to zero in ohms. After discharging the capacitor with the tip of the probe, use two probes to touch the two poles of the capacitor and observe the maximum deflection amplitude of the probe. Repeat the above steps in order using R × 10, R × 100, R × 1k, and R × 10k gears, and see which gear has the largest deflection amplitude within the 10 grid range. At the R × 1k gear, the deflection amplitude of the pointer is the largest, which is 3 small grids. Dividing 3.6 μ F by 3 small grids gives the capacitance sensitivity of RX1k gear, which is 1.2F/grid. As long as the capacitance sensitivity of one gear is measured, the sensitivity of other gears can be calculated. The sensitivity of gears with high resistance rate is high, and the sensitivity of gears with low rate is low. The adjacent gears are recursively calculated in a 10 fold relationship. So the capacitance sensitivity of the MF500 multimeter resistor range is as follows: RX1 range -1200F/grid, R × 10 range -1201F/grid, R × 100 range -12F grid. R × 1k gear -1.2F/grid. Rx10k gear ---0.12F (120nF)/grid.
