Analog (also known as analog) multimeter has a history of nearly one hundred years of development. Because of its complete functions, simple operation, convenient portability, low price, and easy maintenance, it has long been a required instrument for electronic measurement and maintenance. The following is an elaboration and analysis of several key points of use of the pointer multimeter.
Before using the multimeter, you should be familiar with the functions of each switch knob, special socket, measuring socket and instrument accessories, and understand the measurand corresponding to each scale line. Every time you pick up the test lead and prepare to measure, be sure to check whether the measurement type and range selection switch is in the correct position.
1) The pointer multimeter should generally be placed horizontally when in use, otherwise it will cause tilt errors. If it is found that the watch hands do not point to the mechanical zero point, adjust the screw under the watch head to make the watch hands return to zero and eliminate the zero point error.
2) The multimeter should be used and stored in a dry place with no vibration, no strong magnetic field, and a suitable ambient temperature. Humid environment can reduce the insulation strength of the meter, and can also damage the components in the meter due to moisture. Mechanical vibration and impact can demagnetize the magnetic steel of the meter head, resulting in a decrease in sensitivity. When used near a strong magnetic field, the measurement error will increase. If the ambient temperature is too high or too low, the forward and reverse resistance of the rectifier element will change. Changing the rectification coefficient will affect the sensitivity of the meter head and the resistance values of the shunt resistor and the voltage divider resistor, causing temperature errors.
3) After the measurement is completed, the range selection switch should be set to the highest voltage position to prevent accidental burning of the meter next time. Some multimeters (type 500) have a neutral gear, or after the "OFF" gear is used up, the switch knob should be turned to the proper position to short-circuit the measuring mechanism.
4) If the DC voltage range is used to measure the AC voltage by mistake, the hands of the meter will not move or vibrate slightly; if the AC voltage range is used to measure the DC voltage by mistake, the reading may be twice as high, or it may be zero, which is different from the specific connection method of the multimeter. related.
5) The basic error of the voltage file is expressed by the percentage of full scale, so the closer the measured value is to the full scale value, the smaller the error. Generally speaking, the selected range should enable the deflection of the hands to be above 1/3 to 1/2.
6) When measuring the voltage of a high internal resistance power supply, try to select a higher voltage range to increase the internal resistance of the voltage range, so that the deflection angle of the needle is reduced, but the measurement results obtained can better reflect the real situation. Even so, there will still be a large measurement error.
7) Due to the non-linearity of the rectification element, the error of the multimeter to measure the AC voltage below 1V will also increase, so most multimeters will not scale for AC voltage below 1V.
8) The multimeter cannot directly measure non-sinusoidal voltages such as square waves, rectangular waves, triangle waves, sawtooth waves, and trapezoidal waves. Because the AC voltage range of the multimeter actually measures the average value of the AC current after rectification, but the scale is calibrated according to the effective value of the sinusoidal AC voltage. If the measured voltage is a non-sine wave, the relationship between its average value and effective value will change, so it cannot be read directly.
9) It is strictly forbidden to measure the resistance when the circuit under test is charged, and it is not allowed to check the internal resistance of the battery with a resistance gear. Prevents inaccurate measurements and damage to the multimeter.
10) When monitoring the filter capacitor on the instrument, the positive and negative poles of the electrolytic capacitor should be short-circuited first to prevent the charge accumulated on the large capacitor from being discharged through the multimeter and burning the meter head. It is forbidden not to use the test lead wire instead of the wire to discharge the capacitor, it is easy to burn the core wire. It is advisable to take a 220V, 60V ~ 100V light bulb with a lead wire of the lamp head and connect it to both ends of the capacitor. The light bulb will flash when it is discharged.
11) Do not use the resistance gear to directly measure the internal resistance of the high-sensitivity meter and the galvanometer, so as not to burn the moving coil or bend the needle.
12) When using a multimeter to detect the thermistor, it should be considered that the thermal effect of the current will change the resistance of the thermistor, which is especially obvious in the R×1 range. Thermistors are divided into two types: negative temperature coefficient type (NTC) and positive temperature coefficient type (PTC), and their temperature coefficient of resistance is about 2% to 5% (absolute value). When measuring the thermistor, do not pinch the surface of the resistor with your hands, otherwise the temperature of the human body will also change the resistance value.
13) In order to expand the range of the resistance file, some multimeters (such as 500A type) have added a DΨ file, which is specially used to measure the resistance of low resistance value. The principle structure of this resistance gear is quite special, the ohm zero point is located at the left end of the scale, which is just opposite to the reading habits of ordinary ohmmeters, so it is also called an inverted ohmmeter. The inverted ohmmeter has the largest needle deflection angle when the test pen is open, and consumes the most power, while it consumes less power during normal measurement. Therefore, once it is set to this position, it will consume power even if it is left unused.
Once the multimeter breaks down, first check whether the core of the test lead is broken, whether the fuse tube is burned, whether the battery is invalid or not installed, and then check whether the meter head is damaged. In the case of confirming that the meter head is intact, the fault can be searched against the circuit diagram. There are many faults caused by blown shunt resistors, poor contacts in transfer switches or open leads.
This article is excerpted from Cai Liying's "Pointer Multimeter Measuring Skills and Key Points of Use", please read the full text for more content.
