How to Use a Pointer Multimeter to Measure Capacitance Accurately
Electronic multimeter, according to the capacitance measurement range, can directly read the capacitance, and can also measure the withstand voltage value.
In the process of electrical maintenance, we often use a multimeter to check whether the capacitor is good or bad. The traditional method is to compare the charge and discharge with the same type of capacitor, which is very inconvenient to operate. Some capacitors cannot be detected by a digital multimeter because of their short pins and large capacity. In the long-term maintenance practice, the author has explored a simple and practical detection method, which is introduced as follows, hoping to bring a little convenience to colleagues.
In electrical measurement, there are two galvanometers with exactly the same structure. One is an impulse galvanometer. It is a precision instrument used to measure the 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 needle of the impulse ammeter, the maximum deflection amplitude of the needle is proportional to the quantity of pulse current, thus The electric quantity of the pulse current can be measured linearly. The other is a sensitive galvanometer, and the head of the pointer multimeter is a sensitive galvanometer. When measuring capacitance with the electric barrier of the 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 pointer, the meter head will change from a sensitive galvanometer to an impact galvanometer. The maximum value of the pointer will be The deflection amplitude Am is proportional to the electric quantity Q charged by the pulse current to the capacitor. The electric quantity of the capacitor is Q=CE, and E is the electromotive force of the battery blocked by the electricity, which is a fixed value, so Q is proportional to the capacitance C, and the maximum deflection range Am of the hands is also proportional to the capacitance C. In this light, it is possible to measure capacitance with a linear readout. The electrical barrier of the pointer multimeter fully satisfies the above rules when it is deflected at a small angle, so the capacitance can be accurately measured.
Now take the MF500 multimeter as an example to illustrate the method and use of adding capacitance scale. The dial of the MF500 multimeter is shown in the figure, and the 10 small divisions at the left end of the DC uniform scale line are selected as the linear scale of the capacitance. This is because it can meet the linear condition of small angle deflection and is convenient for reading. More than 10 divisions, 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 to be 3.61F, and set the R×1 block of the 500-type multimeter to zero the ohm. After discharging the capacitor with the tip of the test pen, touch the two poles of the capacitor with two test leads, and observe the maximum deflection range of the watch needle. Then use the R×10, R×100, R×1k, R×10k gears to repeat the above steps in turn to see which gear has the largest deflection range within 10 small grids. As a result, at the R×1k gear, the deflection range of the watch hands is the largest, which is 3 small divisions. Divide 3.6μF by 3 small divisions, and the capacitance sensitivity of the RX1k gear is 1.2F/division. As long as the capacitance sensitivity of one gear is measured, Then the sensitivity of other gears can be calculated. The sensitivity of the high resistance multiplier is high, and the sensitivity of the low multiplier is low. The relationship between adjacent gears is 10 times. Therefore, the capacitance sensitivity of the MF500 multimeter electric barrier is as follows, RX1 gear-1200F/division, R×10 gear 1201F/division, R×100 gear-12F division. R×1k block——1.2F/block. Rx10k block -----0.12F(120nF)/grid.
It can be seen from the capacitance sensitivity of the 500-type meter above that the maximum measurable capacity is 1200F grid × 10 grid = 12000F, so it can fully meet the requirements of daily maintenance. The author just engraved this group of numbers on the electric blocking knob, which is very convenient to use.
〔Example〕The nominal value of a capacitor to be tested is 10F, try to test whether it is good or not?
1. Gear selection. According to the nominal value of 10F, 1.2F/block should be selected, that is, R1k gear.
2. Ohm zero adjustment, this step must not be ignored, otherwise the reading error will be large.
3. Discharge, measure, and read, use the tip of the meter to short-circuit the two leads of the capacitor under test to discharge. After discharging, use two test leads to contact the two leads of the capacitor respectively (the "+" pole of the electrolytic capacitor is connected to the black test lead, and the "-" pole is connected to the red test lead). At this time, the maximum deflection of the hands can be read, and the actual reading is 8.5 divisions.
4. Calculate the actual capacity by mouth, C=1.2F × 8.5 = 10.2F.
5. Observe that the hands of the watch have returned to zero. Judgment, the capacity is normal, no leakage, it is a good capacitor. Other types of multimeters can add capacitance scales in this way.
