How to accurately measure capacitance with an analog multimeter
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.
From the capacitance sensitivity of the 500 type meter above, it can be seen that the maximum measurable capacity is 1200F grids x 10 grids=12000F, which fully meets the requirements of daily maintenance. The author engraved these numbers on the resistance knob, which is very convenient to use.
[Example] The nominal value of a capacitor under test is 10F. Can you test its quality?
1. When selecting a gear, according to the nominal value of 10F, the 1.2F/gear should be chosen, which is R1k gear.
2. Ohm zeroing, this step cannot be ignored, otherwise the reading error will be large.
3. Discharge, measure, and read, short circuit the two leads of the tested capacitor with the tip of a meter to discharge. After discharge, use two probes to respectively contact the two leads of the capacitor (the "+" pole of the electrolytic capacitor is connected to the black probe, and the "-" pole is connected to the red probe). At this point, the maximum deflection of the pointer can be read, and the actual reading is 8.5 cells.
4. Calculate the actual capacity orally, C=1.2F grid x 8.5 grid=10.2F.
5. Observe again that the needle has returned to zero. Judging that the capacity is normal and there is no leakage, it is a good capacitor. Other models of multimeters can add capacitance scales using this method.
