Circuit Diagram 18-Analysis of the Principle of Power Supply Overcurrent Insurance Circuit

tiankai001

Circuit Diagram 18-Analysis of the Principle of Power Supply Overcurrent Insurance Circuit [Copy link]

The overcurrent insurance circuit in the power supply circuit plays an overcurrent protection role, that is, when the current flowing through the circuit reaches a certain level, the fuse (i.e., fuse) or fuse resistor in the circuit automatically blows, cutting off the current loop and preventing the large current from further damaging other components in the circuit. Fuses, also known as fuses, are mainly used in overcurrent insurance circuits. In addition, there is a component called a fuse resistor that also has an overcurrent protection function.

The fuse circuits mainly include the following types:

AC high voltage circuit fuse circuit, AC low voltage circuit fuse circuit, AC high voltage and low voltage circuit dual fuse circuit, DC circuit fuse circuit, AC DC circuit dual fuse circuit, fuse resistor overcurrent protection circuit.

1. AC high voltage circuit fuse circuit

The figure below shows the AC high voltage fuse circuit in the power circuit. In the circuit, T1 is the power transformer, S1 is the power switch, and F1 is the fuse. 3447461.1、Circuit analysis The fuse is represented by "F" in the circuit diagram. The 1 in F1 indicates that it is a fuse in the circuit. There may be multiple fuses in the power supply circuit. The working principle of the AC high-voltage circuit fuse circuit is analyzed as follows: 34)]1) When the switch S1 is turned on, the AC mains current flows through the primary winding of T1 through S1 and F1. S1, F1 and the primary winding of T1 are a series circuit, so the current flowing through F1 is equal to the current flowing through the primary winding of T1.

2) When there is a current fault in the circuit, the current flowing through the primary winding of T1 will increase. The more serious the overcurrent fault is, the greater the current flowing through the primary winding of T1 and the current flowing through F1. When the current flowing through F1 reaches a certain level, that is, exceeds the fuse current of F1 (the fuse current of F1 in this circuit is 1A), F1 automatically fuses and cuts off the power supply. There is no power in the primary winding circuit of the power transformer, that is, there is no power in the entire circuit and it stops working. 2. Circuit Fault Analysis 1) The failure rate of the fuse itself is very low. In some cases, it may cause poor contact and open circuit failure due to quality problems. 2) Fuse F1 is a one-time protection element, that is, once it is blown, it can never return to normal after power failure and must be replaced with a new fuse.

3) Since the fuse is set in the AC high-voltage circuit, after it blows, the entire circuit after the power transformer has no working voltage.

4) The fuse of the high-voltage circuit is in the 220V AC circuit. Be sure to turn off the power before replacing it. Generally, the fuse will be installed in a tube sleeve, as shown in the figure below, a) is the fuse, b) is the fuse tube sleeve.

2. AC low-voltage circuit fuse circuit

The figure below shows the AC low-voltage circuit fuse circuit in the power supply circuit. In the circuit, T1 is the power transformer, S1 is the power switch, and F1 is the fuse.

1. Circuit analysis

The fuse F1 in this circuit is set in the secondary winding loop of the power transformer. There is no fuse in the primary winding loop of T1. Therefore, F1 protects the circuit after the secondary winding of T1 and has no protective effect on the current fault caused by the fault of the primary winding of T1 itself.

When the current flowing through the secondary winding of T1 is greater than the fuse current of F1, F1 automatically blows and disconnects the secondary winding loop of T1, thus protecting the circuit after the secondary winding.

2. Circuit Fault Analysis

The fault analysis of the AC low-voltage circuit fuse circuit is as follows:

1) After the AC low-voltage circuit fuse is blown, the load circuit of the secondary winding will have no working voltage. If the power transformer has only one set of secondary windings, the whole circuit will have no DC working voltage; if the power transformer has several sets of secondary windings, it will only affect the normal operation of the load circuit of the secondary winding, and will not affect the operation of other secondary winding load circuits.

2) After the fuse S1 is blown, the primary winding of the power transformer still has current flowing, but this current is very small, which is the no-load current of the transformer, that is, the current when there is no load. 3) When there is a short circuit fault in the primary winding circuit of the power transformer, the fuse F1 in the circuit cannot play the role of overcurrent protection. This is the shortcoming of the low-voltage circuit fuse circuit. III. AC high-voltage and low-voltage circuit dual fuse circuit34)]The AC high-voltage and low-voltage circuit dual fuse circuit has the following two characteristics

1) This circuit can simultaneously protect the high-voltage and low-voltage circuits from overcurrent, and its overcurrent protection effect is much greater than the previous protection circuit.

2) The fuses used in the AC high-voltage and low-voltage circuits have different fusing currents and different installation methods, and the installation operation of the AC circuit fuse is complicated.

The figure below shows the AC high-voltage and low-voltage circuit dual fuse circuit in the power supply circuit. In the circuit, T1 is the power transformer, F1 is the fuse in the AC high-voltage circuit, and F2 is the fuse in the AC low-voltage circuit.

1. Circuit analysis

There are two fuses in this circuit, which are respectively set in the primary winding and secondary winding circuit of the power transformer. It is a double fuse circuit. Many power circuits use this method. Its overcurrent protection ability is stronger than that of a circuit with a single fuse.

The working principle of the double fuse circuit of AC high-voltage and low-voltage circuits is analyzed as follows:

1) In this circuit, when the primary winding itself fails, F1 will blow, but F2 will not. When a short circuit occurs in the secondary winding load circuit, F2 or F1 may blow first, or both F1 and F2 may blow at the same time.

2) The fuses F1 and F2 in the primary and secondary winding circuits of the power transformer have different breaking currents. The fuse F1 in the primary winding circuit has a small breaking current, while the fuse F2 in the secondary winding circuit has a large breaking current. 3) In the power step-down transformer, because the current of the primary winding is smaller than the current of the secondary winding, the F1 fusing current is required to be smaller than the F2 fusing current. 2. Circuit fault analysis Since there are multiple fuse circuits in the power circuit, the fuse fusing conditions will be different depending on the overcurrent. In case of severe overcurrent, F1 and F2 will be blown at the same time. In other cases, F2 will be blown or F1 will be blown.

1) When a short circuit occurs in the circuit after F2, F2 usually blows first, but F1 may also be disconnected (indicating that the F1 fusing current is set too small), or both F1 and F2 may blow at the same time.

2) When a short circuit occurs in the circuit between F2 and F1, F2 will not blow, only F1 will blow. 3) The logical fault location can be judged from the different fuse combinations of F1 and F2. 4. DC circuit fuse circuit The DC circuit fuse circuit is a fuse circuit set in the circuit after the output end of the power circuit. The current flowing through this fuse is a DC current.

The figure below shows the DC low-voltage circuit fuse circuit in the power supply circuit. In the circuit, F1 is the fuse in the DC circuit, and Uo is the DC output voltage.

1. Circuit Analysis

Fuse F1 is located after the rectification and filtering circuits, that is, in the DC current circuit, so it is called a DC circuit fuse. The fuse itself does not distinguish between DC and AC, and its role and function are the same.

Analysis of the working principle of DC circuit fuse circuit

1) The fuse F1 in this circuit only protects the DC power load after F1, and has no overcurrent protection effect on the filter circuit, rectifier circuit and step-down circuit before F1. 2) When there is an overcurrent fault in the circuit after F1, F1 automatically blows to prevent further damage to the circuit before F1 due to excessive current. After F1 is disconnected, there is no DC working voltage in the circuit after F1 and it does not work, but the filter circuit, rectifier circuit and step-down circuit before F1 are still in working state. 2. Circuit Fault Analysis 1) When there is a fault in the circuit after fuse F1, F1 blows, and when the components in the step-down circuit, rectifier circuit and filter circuit are short-circuited, F1 will not blow.

The fault analysis method of DC circuit fuse circuit is the same as that of AC circuit fuse circuit.

V. AC/DC circuit dual fuse circuit 1. AC/DC circuit dual fuse circuit

In many practical power supply circuits, multiple overcurrent fuse circuits are set in the DC and AC circuits. There are mainly the following combinations.

Double fuse circuit for DC circuit and AC circuit; double fuse circuit for DC circuit and AC low voltage circuit; double fuse circuit for AC low voltage circuit and AC high voltage circuit; when there are multiple groups of DC output circuits, fuse circuits can be set in each group of output circuits or in several main DC output circuits.

2. Fuse tube installation method

The 220V AC mains circuit fuse tube is installed on the back casing of the electronic equipment. There is an insulated knob. When the knob is turned down, the fuse tube is exposed. This installation method ensures safety. The fuse is not exposed outside and cannot be touched by people.

The figure below shows the installation method of the DC circuit fuse tube. The fuse tube is installed inside the machine. Since the working voltage of the fuse in the DC circuit is generally within the safe voltage range, it can be exposed to the outside and fixed with a bracket. The bracket is welded on the circuit board. The bracket is used as the lead wire of the fuse tube and connected to other components in the circuit.

3. Circuit analysis

When analyzing the fuse circuit, it should be understood that the fuse only protects the subsequent circuit and has no protective effect on the previous circuit. It only protects against overcurrent faults and has no protective effect on open circuit faults in the circuit (which reduces the working current).

1) Overcurrent protection only protects components in the circuit that have not been damaged, in order to prevent the overcurrent fault from further expanding the damage surface of the components in the circuit.

2) If the working current of a component in the circuit increases abnormally, if the circuit working current is not cut off in time, the overcurrent for a long time will damage the component. After the fuse circuit is set, when an overcurrent fault occurs in the circuit, the fuse will melt in the first time, causing the circuit to lose the working voltage, thereby protecting the component. 3) There can be multiple fuses in the power supply circuit. The closer the circuit is to the front, the smaller the fuse's current is, because the working voltage of the front circuit is high and the working current is small. 4. Circuit failure analysis The fuse will automatically blow after an overcurrent. Depending on the size of the overcurrent, there are three different situations after the fuse blows.

1) The fuse tube is not black, and the two shiny broken ends of the fuse can be clearly seen after it is blown. This means that the overcurrent is not large. It is probably due to poor quality of the fuse or occasional surge current. At this time, you can try to replace a fuse.

2) The fuse tube is black, and the blackening is not serious. The glass of the fuse tube is not broken, indicating that the overcurrent is larger than the previous case.

The fuse tube is seriously black, burnt or the glass tube is broken, which means that the overcurrent is large.

34)]1) The fuse tube is not black, and the two shiny broken ends of the fuse can be clearly seen after it is blown, which means that the overcurrent is not large. It is probably due to poor quality of the fuse or occasional surge current. At this time, you can try to replace a fuse.

2) The fuse tube is black, and the blackening is not serious. The glass of the fuse tube is not broken, which means that the overcurrent is larger than the previous case.

The fuse tube is seriously blackened, burnt or the glass tube is broken, which means that the overcurrent is large.

34)]1) The fuse tube is not black, and the two shiny broken ends of the fuse can be clearly seen after it is blown, which means that the overcurrent is not large. It is probably due to poor quality of the fuse or occasional surge current. At this time, you can try to replace a fuse.

2) The fuse tube is black, and the blackening is not serious. The glass of the fuse tube is not broken, which means that the overcurrent is larger than the previous case.

The fuse tube is seriously blackened, burnt or the glass tube is broken, which means that the overcurrent is large.