Is The Pseudo Soldering A Short Circuit Or An Open Circuit? _ How To Use A Multimeter To Detect It?
What is short circuit and open circuit
If the short circuit is used, all the current can pass. At this time, the electric appliance can be regarded as a wire. There is no resistance. The ammeter can be regarded as a short circuit when it is connected to the circuit. Therefore, the ammeter cannot be connected in parallel with the electric appliance, and the electric appliance is short-circuited. Then, the number of currents connected in series becomes larger, and the number of voltages connected with the electric appliance is reduced.
The open circuit means that all currents cannot pass, and even the branch of the voltmeter can be regarded as an open circuit, and no current flows. The current of the branch on the open circuit is represented by 0, and the voltage indicates a large number.
Short circuit judgment method
Method 1: According to Ohm's law I=U/R, since the resistance of the wire is small, the current on the circuit will be very large when the power supply is short-circuited. Therefore, the ammeter is used to measure the current in the current. If it is too large, it is short-circuited.
Method 2: When the short circuit occurs, the current is directly passed through the wire. The resistance of the circuit part of the short circuit is very small. The resistance is measured with an ohmmeter. If it is very small, it is a short circuit.
(2) Open circuit: When the circuit does not close the switch, or the wire is not connected well, or the appliance is burned out or not installed (such as connecting the voltmeter in the circuit), the whole circuit is disconnected somewhere. The circuit in this state is called an open circuit (also called an open circuit).
Open circuit judgment method
Method 1: The circuit breaker is equivalent to an infinite resistor, which shares all the voltages. Therefore, the voltage between the two points is gradually detected by a voltmeter. If it is the power supply voltage, the two points are disconnected. Then gradually narrow the detection range.
Method 2: Test method, one end of the ammeter is connected to the negative pole of the power supply, one end is taken in the hand and the test is started from the negative pole. If there is no reading when connecting, the test is continued until there is a reading, and the open circuit is at the last test point and the previous test point. between.
Is the short welding a short circuit or an open circuit?
The Pseudo weld is an open circuit.
The Pseudo soldering is generally caused by oxidation or impurities at the solder joints and poor soldering temperature due to improper methods. The essence is that there is an isolation layer between the solder and the pin. They are not completely in contact. The state is generally invisible to the naked eye. However, its electrical characteristics are not conductive or poorly conductive, affecting circuit characteristics. So in general, the Pseudo weld is judged to be an open circuit.
How to use Pseudo multimeter to detect
Even if the Pseudo soldering is an open circuit, then the method of detecting the open circuit with the multimeter is the same, and then follow the small series to learn more about it:
1. Overhaul of open circuit fault
A circuit open circuit fault is a fault in which a certain circuit of the circuit is abnormally disconnected so that current cannot flow in the circuit.
1.1. The phenomenon and harm of open circuit failure
The most basic form of open circuit fault is that the loop is not available. Such as broken wires, poor electrical contact, etc., in some cases, the circuit will also cause voltage changes, the arc generated by the circuit breaker may also cause electrical fires and explosions.
1.1.1, the circuit must form a loop to work properly. The disconnection of one of the circuits in the circuit often results in the loss of some or all of the functions of the electrical device (not working).
1.1.2. In a three-phase circuit, if a phase failure occurs, the motor may be burnt due to phase loss operation; the three-phase circuit may be asymmetrical, the voltage of each phase changes, and the voltage of one of the phases rises. , causing a malfunction. In a three-phase circuit, if the neutral line (neutral line) is open, the effect on the single-phase load is greater.
1.2, the reason for the cause of the open circuit failure
To repair the open circuit fault, first determine the approximate range of the open circuit fault, that is, in which line segments, under which circumstances the open circuit fault is prone to occur.
1.2.1. The electrical contact point is the multiple point of the open circuit fault: in the circuit, in addition to the electrical contact point of the switch contact, the electrical contact point is easy to cause an open circuit fault, and other electrical contact points in the circuit are also prone to open circuit failure.
1.Wire connection points: Whether it is a connection point of any connection method such as splicing, crimping, welding, bolting, etc., it is a frequent occurrence of open circuit failure;
2. Stress point of the wire: Under the action of external force or repeated force, it is easy to break the circuit; c. Copper-aluminum transition point: Under electrochemical corrosion, it is most likely to cause poor contact and open circuit failure.
1.2.2, Pseudo contact and Pseudo solder joint cause open circuit fault: the connection point that is similar to the contact that is not actually touched is called Pseudo contact point, and if it is a solder joint, it is a Pseudo solder joint. Solder joints soldered with soldering iron may cause a solder joint if the soldering iron temperature is low, the wire is not completely melted, or the rosin is too much and is not completely melted. Such Pseudo joints and Pseudo solder joints cannot be distinguished by the naked eye and can only be detected by borrowing instruments.
1.2.3, dust can also cause open circuit fault: the electrical pickup of a contactor coil is very normal, but it can not be connected to the circuit. After checking, the contactor is stained with a layer of dust, resulting in poor contact contact. Circuit breaker failures due to dust, oil, rust, etc. are also common.
1.3. Methods for repairing open circuit faults
Firstly, it should be judged to be an open circuit fault according to the fault phenomenon, and then determine the range of the open circuit fault and the short circuit according to the part where the open circuit fault may occur, and then use the detection tool to find the short circuit point.
1.3.1. Voltage method: The circuit is disconnected, no current flows through the circuit, and there is no voltage drop in the various step-down components in the circuit. The power supply voltage drops all at both ends of the trip point. Therefore, the disconnection fault point can be judged by measuring the voltage of the trip point.
In the simple circuit shown in Fig. 1, the power supply voltage is 100 V DC, and the electromagnetic coil Y is controlled by the normally open contact QF1 and the normally closed contacts QF2, QF3, and QF4. The instrumentation is a universal multimeter with a DC voltage of 250 V (greater than or equal to the 100 V gear). Assume that the circuit has an open circuit fault point at A. When the normally open contact QF1 is manually closed (or shorted by a wire), the electromagnetic coil Y still does not work. Connect the multimeter red test lead to the "+" pole of the power supply, the black test lead to the power supply "-" pole, the multimeter indication should be 100 V, and then move the black test lead, in turn with the endpoints 1, 2, 3, 4, 5, 6, 7, 8 connected, if the multimeter indication is also 100 V, it means that these points to the power "-" pole circuit without open circuit failure. When the black test pen moves to endpoint 9, the multimeter indicates zero, and the open circuit fault is between 8-9. At this time, if the voltage between 8-9 is measured again, it must be equal to the power supply voltage, and then it can be judged that the circuit has only one open circuit fault point at A.
1.3.2, resistance method: After the circuit has an open circuit fault, the resistance at both ends of the breaking point is infinite, while the resistance of the other segments is approximately zero, and the resistance at both ends of the load is a certain value. Therefore, the circuit breaker point can be found by measuring the resistance value of each line segment of the circuit. The detection resistance value is generally measured by a multimeter ohm (Ω). Taking Figure 2 as an example, suppose the circuit has an open circuit fault at point B. The search step: disconnect the power supply. Place the multimeter in ohms and generally select R × 10 Ω or R × 1 Ω, instead of selecting a high barrier of R × 1 kΩ or more to avoid errors. Connect a meter of the multimeter to the L point in the circuit, hold the other test lead, and connect it to 1 point. Since the power supply L and 1 are a normally open contact, you should manually close it and then disconnect it. The header indicates to check if this contact is normal. Then short-circuit the normally open contact QF1, and then connect the test leads to 2~8. At 7 o'clock, the multimeter indicates that the resistance is the resistance RY of the coil Y, that is, R1-7=RY. At 8 o'clock, the multimeter indicates that the resistance is ∞, and the open circuit fault occurs at the connection line between 7-8.
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