How to make an accurate measurement of capacitance using a pointer multimeter
We are in the electrical maintenance process, often use a multimeter to detect the good and bad capacitors. The traditional method is the same type of capacitor and charge/discharge comparison, the operation is very inconvenient, some capacitors due to short pins, capacity is too large, and sometimes can not be detected with a digital multimeter. The author in the long-term maintenance practice, worked out a simple and practical detection method, is described below, I hope to bring a little convenience to colleagues.
In electrical measurements, there are two types of ammeters with identical structure. One is an inrush ammeter. It is used to measure the amount of impulse current precision instrumentation, when the duration of the impulse current flowing through the impact ammeter is much smaller than the free oscillation period of the impact ammeter needle, the needle's maximum deflection amplitude is proportional to the amount of impulse current, so that the amount of impulse current can be measured linearly. Another is a sensitive ammeter, pointer multimeter head is a sensitive ammeter. Measurement of capacitance with a pointer multimeter resistor, will produce a pulse charging current, if the duration of this pulse current is much less than the free oscillation period of the pointer of the head, the head of the sensitive ammeter into an inrush ammeter, and its pointer's maximum deflection amplitude Am is directly proportional to the amount of the pulse current to the capacitor charged by the amount of Q. And the amount of capacitance Q = CE, E is the resistor's battery electromotive force, which is a constant value So Q is in turn proportional to the capacitance C, and the maximum deflection of the meter needle, Am, is proportional to the capacitance C. By this reasoning, it is possible to measure capacitance with a linear reading. Pointer multimeter resistor block in the small angle deflection to meet the above law, so can accurately measure the capacitance.
Now take MF500 multimeter as an example, explain the method and use of adding capacitance scale.MF500 multimeter dial as shown in the figure, select the DC uniform scale line left end of the 10 small cells for the capacitance of the linear scale. This is because it can meet the linear conditions of small angle deflection, but also easy to read. Beyond 10 cells, the scale will gradually become non-linear. Take a new capacitor, such as the nominal value of 3.3F capacitor, with a digital multimeter measured its actual capacity of 3.61F, the 500-type multimeter R × 1 block for ohm zero. After discharging the capacitor with the tip of the pen, contact the poles of the capacitor with two pens and observe the maximum deflection of the needle. Repeat the above steps with the R×10, R×100, R×1k, and R×10k stops in turn to see which stop has the largest deflection within a range of 10 cells. The results in the R × 1k block, the needle deflection amplitude is the largest, for 3 small cells, with 3.6μF divided by 3 small cells, the capacitance of the RX1k block sensitivity of 1.2F / grid, as long as the capacitance of a block of sensitivity can be measured, can be calculated in other blocks of the sensitivity of the resistor multiplication of high sensitivity, multiplication of low sensitivity low, the neighbouring block into a 10-fold recurrence of the relationship. So the capacitance sensitivity of MF500 multimeter resistor block is as follows, RX1 block -1200F/g, R×10 block 1201F/g, R×100 block -12F grid. R×1k block - 1.2F/g. Rx10k block -----0.12F(120nF)/gram.
From the capacitance sensitivity of the 500-type table above, it can be seen that the maximum measurable capacity of 1200F grid × 10 grid = 12000F, therefore, fully adaptable to the requirements of daily maintenance. The author is the set of numbers engraved on the resistance knob, easy to use.
[Example] a capacitor to be tested nominal value of 10F, try to test its good or bad?
1, select the block, according to the nominal value of 10F, should be selected 1.2F / grid block, that is, R1k block.
2, ohm zero, this step must not be ignored, otherwise the reading error is large.
3, discharge, measurement, reading, with the tip of the pen to short-circuit the two leads of the capacitor under test, discharge. After discharge, with two pens respectively contact capacitor two leads (electrolytic capacitor "+" pole connected to the black pen, "-" pole connected to the red pen). At this time you can read the maximum deflection of the needle, the actual reading for the 8.5 frame.
4. Oral calculation of the actual capacity, C = 1.2F x 8.5 = 10.2F.
5. Then observe the needle has returned to zero. Judgement, capacity is normal, no leakage, is a good capacitor. Other models of multimeter can be added according to this method of capacitance scale.
