August 12, 2020

How to use a multimeter

Tip: The type and structure of the multimeter are various. Only when you use the correct method to ensure the accuracy of the test results can you ensure the safety of your personal and equipment.

Tell us about the correct use of the universal meter:

(1) The use of jacks and transfer switches: First, select the position of the jack or transfer switch according to the purpose of the test. Since the voltage, current and resistance are alternately used during use, do not forget to shift gears. It is not possible to measure the current or measure the resistance of the resistance to negotiate the voltage. If a DC current or resistance is used to measure the AC voltage of 220, the multimeter will burn out immediately.

(2) Test table pen use: The multimeter has red, black pen, don't look at it, there are two, it can be used freely in use, it is also very knowledgeable. If the position is reversed, the wrong connection will bring test. The possibility of erroneous or burning the head. Generally, the red pen is "+" and the black pen is "-". When inserting the multimeter jack into the pen, be sure to press the color and positive and negative. When measuring DC voltage or DC current, be sure to pay attention to the positive and negative polarity. When there is no current, the test leads are connected in series with the circuit. When measuring the voltage, the test leads are connected in parallel with the circuit.

(3) How to correctly read: Before using the multimeter, check whether the pointer is on the zero-seven-bit position. If it does not refer to the zero position, adjust the mechanical regulator on the front cover to adjust to the zero position. If the multimeter has multiple rulers, be sure to recognize the corresponding reading scale. You can't save the AC and DC scales arbitrarily, and you can't misread them. The multimeter has multiple ranges for the same measurement item. For example, the DC voltage range has 1V, 10V, 15V, 25V, 100V, 500V, etc. The range selection should be around 2/3 of the full scale of the pointer. When measuring the resistance, the finger should be pointed to the vicinity of the center of the file, so that the measurement can be accurate.

Measurement of commonly used devices:

(1) Measurement of resistance: When using a multimeter with no resistance, firstly short the test leads, turn the zero adjustment potentiometer to zero, and make the pointer on the ohm zero. Moreover, the zero adjustment potentiometer needs to be re-adjusted after each shift. When selecting the ohmic gear position, try to select the resistance value to be measured near the center of the dial to improve the accuracy of the test results; if the resistor is on the circuit board, one of the feet should be welded to test. Otherwise, there are other shunt devices by the resistor, and the reading is not accurate! When measuring the resistance value, do not touch the lead of the test leads and the resistors with the fingers of both hands, so as to reduce the error of the body resistance shunt.

(2) Measuring the resistance value to the ground: the so-called grounding resistance value is the grounding of the multimeter red pen, the black pen is connected to one of the measured components, measuring the resistance of the point to the ground, and the normal resistance The values ​​are compared to determine the extent of the fault. In the measurement, the resistance gear position is set in the R*1k file. When the measured resistance value of the point differs greatly from the normal comparison, it indicates that there is a fault in the part of the circuit, such as filter electric air leakage, open circuit or integration. IC damage, etc. (3) Measurement of the transistor: The range of the multimeter is converted to the ohmic range R*100 or R*1K to measure the diode. Cannot use R*10, R*10K files. Before the two resistors are too small, one resistor is too large, the current through the diode is too large, and the diode is easily damaged. The latter is easy to break down the diode with lower withstand voltage because of the higher internal voltage. If the measured resistance is only a few hundred ohms to several thousand ohms (forward resistance), the red and black test leads should be swapped and retested. If the measured resistance value at this time should be several hundred kilohms (reverse) Resistance), indicating that this diode can be used. When measuring the forward resistance value, the one measured by the red test pen is the negative pole of the diode, and the one measured by the black test pen is the positive pole of the diode ~ (the unidirectional conductive characteristic of the diode). By measuring the forward and reverse resistance values, you can check the quality of the diode. Generally, the reverse resistance is several hundred times greater than the forward resistance. That is to say, the smaller the forward resistance, the better, and the larger the reverse resistance, the better.

(4) Measurement of AC voltage: We can measure the voltage values ​​of turbulence and AC respectively with the DC voltage and AC voltage of the multimeter. Then, connect the multimeter to the circuit under test in parallel. To select the range head pointer to 2/3 of the full scale deflection. If the voltage on the circuit is not estimated, use a large range first, and then use the appropriate range after the measurement is fine. This will prevent the multimeter from being damaged due to excessive voltage. When there is no DC voltage, the red meter of the multimeter should be touched on the positive pole of the circuit under test, and the black pen should be touched on the negative pole of the circuit. Do not reverse it. When measuring the relatively high voltage, you should pay special attention to the two separate Hold the red, black pen to the edge of the measurement, or first fix a test pen at one end, and then touch the test point.

(5) Measurement of the charging transformer: The ohmmeter of the multimeter can be used to estimate the good and bad of the transformer when the transformer is not energized. First select the multimeter in the R*10 file, and measure the DC resistance value of the primary coil of the transformer, generally in the range of a few hundred ohms to several thousand ohms. If the measured value is infinite, it means that the coil has been broken and cannot be used!

(6) Then test the insulation resistance value between the primary coil and the secondary coil, the larger the better. If the resistance is small, the insulation between the primary and secondary is poor, and it cannot be used. If the above measurements are all good, you can connect the transformer to the power supply to measure the output voltage value. For the transformer with the filter circuit, pay attention to the red. The black test pen should be placed correctly on the positive and negative poles of the voltage output. If it is measured. The output voltage is normal, indicating that the performance of the transformer is good. This aspect is usually used on mobile phone chargers.

Precautions:

(1) Before using the multimeter, you should fully understand the functions of each transfer switch, special socket, measuring jack and corresponding accessories, and send out the reading of the dial;

(2) When using the multimeter, the multimeter should generally be placed horizontally without dryness, vibration, or strong magnetic field;

(3) After the measurement is completed, the range selection switch should be adjusted to the maximum voltage level to prevent the multimeter from being burnt out when the measurement is started next time.

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