How to measure short circuit, open circuit and open circuit with a multimeter
Whether the multimeter is used to measure the short circuit and open circuit with the on-off gear or the resistance gear
The on-off file is also called the buzzer file. In this gear, if the actual resistance value of the tested circuit is lower than a certain value (I forgot the specific amount, the detailed explanation is in the manual), the buzzer will sound.
Taking a digital multimeter as an example, the buzzer seems to be able to measure a resistance of up to 2,000 ohms.
For example, when measuring a pure line (like a roll of 100-meter wire), the buzzer will beep if the wire is not broken.
Another example is a section of line, which may be connected in series with some resistive elements (such as coils, motor windings) or the line is very long and has many barge interfaces. When measuring at this gear, it may not beep, but it will display a value, the value at this time is the resistance of this line, and it cannot fully explain that this line is open circuit.
For example: You choose a good AC contactor coil at random, and use the buzzer to measure both ends of the coil. It will not beep, but it will display a value (assuming it is 758); The value obtained is still 758, that is, the resistance of this coil is 758 ohms. At this point you cannot say that the coil is an open circuit. If the coil is open, the reading will be zero and there will be no beeping.
Strictly speaking, if there is no beep or display, it still cannot explain that this section of the line is broken. Because as mentioned above, this gear can only measure a maximum resistance of 2 kohms. So it may be that the resistance of this line is higher than 2 kohms. At this time, you can change to a higher resistance level and test again.
In practice, there is generally no need to delve into it so deeply. Like the above-mentioned coil of 100-meter wire, provided that it is not broken, if it does not beep when measured with the buzzer gear, it can basically be judged that the coil is not good enough. Closed.
Another example is knowing that what is to be measured is the winding of the motor. Before the measurement, I know the number in my mind. When measuring it in the buzzer gear, there is no display and no beeping. In order to ensure accuracy, I should change to a larger gear and measure again.
Anyway, I personally think that we should pay attention to: 1. The buzzer can only measure the resistance below 2000 ohms; 2. Only when the actual resistance value is lower than the set value, it will beep. Keep this in mind, and then predict the accuracy of the predicted results according to the actual situation. Or in other words, predict which gear is most suitable for measurement according to the actual situation.
To be honest, I am also used to using the beep file to test the continuity. And I use a digital watch, and what I said above is also explained according to the digital watch. Mechanical watches are rarely used, which means I don’t know much about them.
How to use this multimeter to test whether a section of line is open or broken
point block.
One end of the line under test is directly connected to the ground terminal, the end under test is connected to a test lead, and the other test lead is directly pressed to a nearby reliable ground terminal, the pointer points to zero or close to zero, and the line is basically connected. If the pointer does not change, the circuit breaks. If the digital display meter is zero, it means pass.
If you know that the other line is connected, you can directly short-circuit one end of the line under test with this line, connect the other end of the line under test to the test lead, and connect the other test lead to one end of the line. That's it.
What to do if the multimeter detects the open circuit and short circuit of the line
Use the buzzer file to test at both ends of the line. If there is a sound, it means a short circuit or a path (it should be judged according to the principle, a short circuit is a fault, and the path is normal.), if it should pass, but it doesn’t, it means the circuit is open ( open circuit).
How to use a multimeter to measure the short circuit, open circuit and short circuit of the line
Use the ohm x1 file to measure the two ends of the line. If the resistance is close to zero, it is a short circuit. If there is a certain amount of resistance (depending on the load in the line), it is not a short circuit. When the voltage is constant, the smaller the resistance, the more current flow. The greater the current flowing through the line. Use the ohm 1k or 10k file to measure the two ends of the line. If the resistance is infinite, it is an open circuit.
The basic principle of the multimeter is to use a sensitive magnetoelectric DC ammeter (microampere meter) as the meter head.
When a small current passes through the meter head, there will be a current indication. However, the meter head cannot pass a large current, so some resistors must be connected in parallel or in series on the meter head to shunt or lower the voltage, so as to measure the current, voltage and resistance in the circuit.
The measurement process of the digital multimeter converts the measured value into a DC voltage signal by the conversion circuit, and then converts the voltage analog quantity into a digital quantity by the analog/digital (A/D) converter, then counts through the electronic counter, and finally uses the digital measurement result displayed directly on the display.
The function of the multimeter to measure voltage, current and resistance is realized through the conversion circuit part, and the measurement of current and resistance is based on the measurement of voltage, that is to say, the digital multimeter is expanded on the basis of the digital DC voltmeter.
The A/D converter of the digital DC voltmeter converts the analog voltage quantity that changes continuously with time into a digital quantity, and then the digital quantity is counted by the electronic counter to obtain the measurement result, and then the measurement result is displayed by the decoding display circuit. The logic control circuit controls the coordinated work of the circuit, and completes the entire measurement process in sequence under the action of the clock.
in principle:
1. The reading accuracy of the pointer meter is poor, but the process of the pointer swing is more intuitive, and its swing speed range can sometimes objectively reflect the size of the measured (such as measuring the slight jitter); the reading of the digital meter is intuitive, but the process of digital change looks messy and not easy to watch.
2. There are generally two batteries in the pointer meter, one is low voltage 1.5V, the other is high voltage 9V or 15V, and the black test lead is positive terminal relative to the red test lead. Digital meters usually use a 6V or 9V battery. In the resistance mode, the output current of the test pen of the pointer meter is much larger than that of the digital meter. The loudspeaker can make a loud "da" sound with the R×1Ω gear, and the light-emitting diode (LED) can even be lit with the R×10kΩ gear.
3. In the voltage range, the internal resistance of the pointer meter is relatively small compared with the digital meter, and the measurement accuracy is relatively poor. Some occasions with high voltage and micro current cannot even be measured accurately, because its internal resistance will affect the circuit under test (for example, when measuring the acceleration stage voltage of a TV picture tube, the measured value will be much lower than the actual value). The internal resistance of the voltage range of the digital meter is very large, at least in the megohm level, which has little effect on the circuit under test. However, the extremely high output impedance makes it susceptible to the influence of induced voltage, and the measured data may be false in some occasions with strong electromagnetic interference.
4. In short, pointer meters are suitable for the measurement of analog circuits with relatively high current and high voltage, such as TV sets and audio amplifiers. It is suitable for digital meters in the measurement of low-voltage and low-current digital circuits, such as BP machines, mobile phones, etc. It is not absolute, and pointer tables and digital tables can be selected according to the situation.
