Use Of Electric Meters

Ammeters and voltmeters are the two most basic and most important measuring instruments in the junior high school electrical experiment. The two meters are also the instruments that are frequently tested in the middle school exams. Some students lose points because they have not mastered the correct use of them, so they teach students to master the ammeter and The method of using the voltmeter is very necessary.

1. The ammeter must be connected in series in the circuit to be tested, the "+" pole of the ammeter must be extremely close to the "+" of the power supply, the "?" Pole of the ammeter must be extremely close to the "?" Of the power supply, and the voltmeter must be connected in parallel Measure the two ends of the circuit, pay attention to the positive and negative poles can not be reversed. When using an ammeter, its two terminals must not be directly connected to the poles of the power supply, so as not to burn the ammeter due to excessive current.

2. The range ammeter and voltmeter that will be selected have three terminals. See how the wires are connected. For example, if the ammeter is connected to the two terminals "+" and "0.6", the range is 0.6 amps. Read the lower one on the dial Number of groups; for the voltmeter, if it is connected to the two terminals "+" and "15", the range is 15 volts, then the upper group number on the dial should be read. Before the experiment, if you do not know how to connect, you can first estimate the current intensity and voltage value of the circuit. If the estimated current intensity is less than 0.6 amperes, select the range of 0 to 0.6 amps. If the estimated current intensity is greater than 0.6 amperes and less than 3 amps, then select the 0 to 3 amps range. If you cannot estimate, you can use the method of trial touch ( Fix a terminal, and use the other end of the circuit to quickly test the terminal of the maximum range) Select the appropriate range according to the test data. For the voltmeter, if the estimated voltage is less than 3 volts, then select a range of 0 to 3 volts. If the estimated voltage is greater than 3 volts, then the range of 0 to 15 volts should be selected. If it cannot be estimated, the test method should also be used.

3. You will try to connect the circuit according to the circuit diagram. After you have connected the physical picture, you must test the circuit and carefully observe the deflection of the pointers of the two watches. If the pointers do not deflect, it means that the circuit is broken somewhere, and the positions of the two watches may be wrong. If the pointer deflects in the opposite direction, it means that the positive and negative terminals are reversed. If the pointer deflection is too large in the required direction, it means that the range is selected to be small; if the range is selected, it is not accurate enough. If the pointer deflects in the required direction If it is too small, it means that the range is too large. According to the actual situation of trial connection observation, make corresponding adjustments, and then the experiment can be carried out.

4. Reading the meter is very important for the third grade students. The question types in the middle school exams give the position of the two dials and the pointer. Let the corresponding readings be read. Based on the readings, many physical quantities such as resistance, electric power, electric power, and electric heat are calculated. The first step If you make a mistake, most of the questions below will lose points. First figure out how much each large grid is and how much each small grid is. Each large grid represents 0.2 amperes. The position is 1 large grid and 8 small grids, then the reading should be 0.36 amps.

Each large grid represents 5 volts, and each small grid represents 0.5 volts. At this time, it can be accurate to 0.5 volts. Then look at the position of the pointer. Now the pointer is pointing at 2 large grids and 4 small grids. It should be 12 volts.

5. It will distinguish the similarities and differences between the two watches: ①The positive poles of the two watches are close to the positive pole of the power supply. ② Both tables must pay attention to the measurement range. Different points: ① The ammeter must be connected in series in the circuit under test, and the voltmeter must be connected in parallel across the circuit under test. ②The ammeter cannot be directly connected to the two poles of the power supply, the voltmeter can be directly connected to the two poles of the power supply.

Resistance measurement

(1) Measurement steps

First insert the red test lead into the VΩ hole and the black test lead into the COM hole

Turn the range knob to the appropriate position of "Ω" range

Connect the metal parts at both ends of the resistor with red and black test leads respectively

Read the data displayed on the display

(2) Note

Range selection and conversion. If the range is smaller, the display will show "1." At this time, the larger range should be used; otherwise, if the range is selected, the display will show a number close to "0". Use a smaller range.

How to read? The number displayed on the display plus the unit selected by the side gear is its reading. It should be reminded that the unit is "Ω" in the "200" file, "kΩ" in the "2k ~ 200k" file, and "M" in the "2M ~ 2000M" file.

If the measured resistance value exceeds the maximum value of the selected range, the overrange "1" will be displayed. A higher range should be selected. For resistances greater than 1MΩ or higher, the reading will stabilize after a few seconds, which is normal .

When there is no connection, such as an open circuit, the meter displays "1"

When checking the impedance of the circuit under test, make sure to remove all power sources in the circuit under test and discharge all capacitors. If the power source and energy storage components in the circuit under test will affect the accuracy of the circuit impedance test.

The 200MΩ range of the multimeter has 10 words when short-circuited. When measuring a resistance, these 10 words should be subtracted from the measurement reading. For example, when measuring a resistance, it is displayed as 101.0, and 10 words should be subtracted from 101.0. The actual resistance of the device under test is 100.0, that is, 100MΩ