How to measure voltage with a digital multimeter

　　Voltage, also known as potential difference or electric potential difference, is a physical quantity that measures the energy difference generated by the unit charge in the electrostatic field due to the different electric potentials. Its magnitude is equal to the work done by the unit positive charge to move from point A to point B due to the electric field force. The direction of voltage is defined as from high potential to low potential. The following two are well-known voltages.

　　1. AC voltage

　　Electricity whose voltage and direction change over time is called alternating current, such as 220V AC for civilian use and 380V AC for industrial use.

　　The polarity of AC voltage changes continuously over time, that is to say, if one pole is grounded, the voltage of the other pole will change continuously from high to low and from positive to negative. Even if both ends are not grounded, the circuit can still work normally.

　　Generally, the voltage of the DC circuit we come into contact with in daily life is low, below the safety voltage, so there is no danger to personal safety, so the DC circuit does not need to be grounded. Alternating current is the current and voltage with periodic changes in magnitude and direction. The most commonly used is sinusoidal alternating current, such as the daily mains electricity.

　　2. DC voltage

　　DC voltage refers to a voltage whose magnitude and direction do not change with time.

　　In a DC circuit, the voltage applied across the power supply, across a circuit, and across a component is a DC voltage. For example, the voltage across the battery of a flashlight and across the light bulb are both DC voltages. Due to the existence of the series-parallel relationship, the parallel connection of electrical equipment has increased (parallel resistors have a shunt effect). There is a shunt current passing through the parallel branch, and when the shunt current passes through the electrical load, a "shunt voltage" is generated (the shunt voltage is numerically equal to the product of the branch current and the branch resistance). For example, the voltage and current tested in the multimeter are based on the voltage division of the series resistor and the shunt effect of the parallel resistor, which can change the range. The voltage level selected is an extremely complicated matter. In fact, choosing a higher voltage can indeed save a lot of wires and energy, but it will increase the cost of switches or electronic components, and it will not save much money. If we choose 100-120VAC when we start to develop electricity, we will save a lot of money on electrical appliances that directly use rectifier circuits, and it will be safer, and even the interference source of power lines will be reduced a lot.

　　Principle of voltage measurement with multimeter

　　1. DC voltage conversion circuit

　　2. AC voltage conversion circuit

　　How to measure voltage with a multimeter

　　The method of measuring voltage with a multimeter is to first align the range switch to the five ranges marked with V (to test AC voltage, align it to the AC voltage range; to test DC voltage, align it to the DC voltage range). When measuring voltage, connect the meter leads to the circuit under test. Select a suitable range position according to the approximate value of the circuit under test. The maximum value of each dry cell is 1.5V, so it can be placed in the 5V range.

　　At this time, the full-scale reading of 500 on the panel should be read as 5. That is, it is reduced by 100 times. If the needle points to the 300 scale, it reads 3V. Note that the value indicated by the tip of the range switch is the corresponding value of the full-scale reading of the needle on the meter head. When reading the meter, just convert it according to this and you can read the actual value. Except for the resistance range, all range switch ranges are read in this way.

　　In actual measurement, if you are not sure about the approximate value of the measured voltage, you can first turn the switch to the maximum range, and then reduce the range to a suitable position step by step. When measuring DC voltage, you should pay attention to the positive and negative polarity. If the test leads are connected in reverse, the needle will move in the opposite direction. If you do not know the positive and negative polarity of the circuit, you can set the range of the Wan Tian meter to the maximum range, and quickly test it on the circuit to be measured. By observing how the pen needle deflects, you can determine the positive and negative polarity.

　　Measure 220V AC. Turn the range switch to AC 500V. The full scale is 500V, and the reading is read according to the scale of 1:1. Insert the two test leads into the power socket, and the scale where the needle points is the measured voltage value. When measuring AC voltage, the test leads have no positive or negative distinction.

　　1. Measurement of DC voltage, such as battery, Walkman power supply, etc. First, insert the black test lead into the "com" hole and the red test lead into the "V Ω". Turn the knob to a range larger than the estimated value (note: the values ​​on the dial are all the maximum range, "V-" indicates the DC voltage range, "V~" indicates the AC voltage range, and "A" is the current range), then connect the test leads to the power supply or both ends of the battery; keep the contact stable. The value can be read directly from the display. If it shows "1.", it means that the range is too small, so you need to increase the range before measuring. If "-" appears on the left side of the value, it means that the polarity of the test lead is opposite to the actual power polarity, and the red test lead is connected to the negative pole.

　　2. Measurement of AC voltage. The probe jack is the same as the DC voltage measurement, but the knob should be turned to the AC range "V~". There is no positive or negative difference in AC voltage, and the measurement method is the same as before. Whether measuring AC or DC voltage, pay attention to personal safety and do not touch the metal part of the probe with your hands.

　　How to use a multimeter

　　1. Be familiar with the functions of the multimeter before use, and correctly select the gear, range and test lead jack according to the object to be measured.

　　2. When the size of the measured data is unknown, the range switch should be set to the maximum value first, and then switched from the large range to the small range so that the instrument pointer indicates more than 1/2 of the full scale.

　　3. When measuring resistance, after selecting the appropriate magnification, touch the two test leads together to make the pointer point to zero. If the pointer deviates from zero, adjust the "zero adjustment" knob to return the pointer to zero to ensure accurate measurement results. If zero adjustment is not possible or the digital display gives a low voltage alarm, check in time.

　　4. When measuring the resistance of a circuit, the power supply of the circuit under test must be cut off and measurements must not be made while the circuit is powered.

　　5. When using a multimeter for measurement, pay attention to the safety of both the person and the instrument. Do not touch the metal part of the test lead with your hands during the test, and do not switch the gear switch while energized to ensure accurate measurements and avoid accidents such as electric shock and burning of the instrument.

　　Precautions for using a multimeter

　　(1) Before using the multimeter, you should first perform "mechanical zero adjustment", that is, when there is no electrical quantity to be measured, make the multimeter pointer point to the position of zero voltage or zero current.

　　(2) When using a multimeter, do not touch the metal part of the test leads with your hands. This can ensure accurate measurement and personal safety.

　　(3) When measuring a certain amount of electricity, you cannot change gears while measuring, especially when measuring high voltage or high current. Otherwise, the multimeter will be damaged. If you need to change gears, you should disconnect the test leads first and then measure after changing gears.

　　(4) When using the multimeter, it must be placed horizontally to avoid errors. At the same time, attention should also be paid to avoiding the influence of external magnetic fields on the multimeter.

　　(5) After using the multimeter, the conversion switch should be set to the maximum AC voltage. If the multimeter is not used for a long time, the battery inside the multimeter should be removed to prevent the battery from corroding other components in the meter.