In the field of electrical engineering, thermal fuse and fuse are two common overload and short circuit protection components, although they bear the heavy responsibility to protect the circuit from abnormal current damage, but in the working principle, application, and protection mechanism are different. INFINITECH will elaborate on the differences between the two and provide detailed answers to user concerns.

The content of this article

> Definition and working principle of thermal fuse and fuse

> What is the difference between a thermal fuse and a fuse

> How to choose the right thermal fuse or fuse

> Detailed replacement procedures for hot fuses and fuses

> The cause of fuse failure and the effect of fuse material on the fuse characteristics

Hot fuse

Thermal fuse, also known as temperature fuse or thermal fuse, is a non-resettable one-time thermal protection device. It is mainly used for overheating protection of circuits or equipment, and works by monitoring temperature changes in the surrounding environment or inside the equipment. When the temperature of the device abnormally rises to the preset safety protection value due to abnormal operation or other reasons, the low-melting alloy or organic chemical materials contained in the thermal fuse will melt or change the state, thus cutting off the circuit and preventing equipment damage and fire caused by further temperature rise.

How does a hot fuse work?

The thermal fuse contains one or more low-melting alloy elements, which are very sensitive to temperature changes. In normal operation, the heat generated by the current is absorbed by the surrounding medium, and the element remains solid. Once the current exceeds the design threshold, the additional heat cannot be dissipated in time, causing the alloy temperature to rise sharply until it melts, thus interrupting the circuit. This "thermal inertia" design allows the thermal fuse to tolerate a slight current overload for a short period of time without misoperation.

Fuse wire

A fuse, also known as a current fuse or fuse, is an electrical component installed in a circuit whose main function is to provide overload protection. According to the IEC127 standard, a fuse is defined as a "fuse." The design of the fuse is based on the principle that when the current flowing in the circuit exceeds the predetermined safety value, the fuse will quickly fuse due to its internal conductor (fuse) heating to the melting point, thereby cutting off the circuit and preventing other parts in the circuit from being damaged by excessive current or preventing fire accidents that may be caused.

The history of fuses dates back more than a hundred years and was originally invented by Thomas Edison to protect the then expensive incandescent light bulbs. With the development of technology, the application range of fuses has expanded to almost all electrical and electronic equipment, including home, industrial, automotive and aerospace fields. There are many types of fuses, according to different shape, size, rated current and voltage, and fusing characteristics (such as fast or delayed fuses) are distinguished, common forms are filament, sheet, glass tube, ceramic tube, insert sheet (commonly used in automobiles), surface mount (SMD) type and plastic shell type.

What are the working principles and types of fuses?

The core of the fuse is the wire or sheet inside it, which works based on the Joule thermal effect of the current, which is the heat generated when the current passes through the conductor. According to the different fuse characteristics, fuses can be divided into:

• Fast fusing type: Suitable for circuits that require an immediate response, such as precision electronic equipment, which can respond to high currents in a very short time.

• Delayed fusing type: Designed for situations that can tolerate a certain overload, such as the peak current when the motor is started, by using special materials or structures to extend the fusing time.

What is the difference between a thermal fuse and a fuse?

Working principle difference

• The hot fuse works mainly according to the thermal effect, when the current through exceeds its rating, the component itself will heat up, and after reaching a certain temperature, the low-melting point alloy inside will melt, thus cutting off the circuit. This process relies on thermal energy accumulation, so the response time is relatively slow.
• The fuse is directly fused through the thermal effect of the current, and the internal wire is designed to quickly heat up to the melting point under the condition of overcurrent, so as to achieve immediate disconnection of the circuit. The reaction speed of the fuse is fast, and it can deal with the instantaneous large current impact more effectively.

Structure and material

• The fuse usually contains a melt (such as lead-antimony alloy), electrodes and brackets, and the structure is relatively simple.
• Thermal fuses may contain more complex structures to improve the accuracy of temperature sensing, such as the use of temperature-sensitive wire materials, and are sometimes equipped with arc extinguishing devices.

Trigger mechanism

• Fuse: mainly designed to detect the current in the circuit. When the current in the circuit exceeds the rating of the fuse, the fuse in the fuse will be blown due to overheating, thus cutting off the circuit and preventing damage caused by circuit overload or short circuit. The working principle of a fuse depends on the amount of current.
• Hot fuse: mainly for temperature monitoring. It will disconnect the circuit when the environment or equipment reaches or exceeds a predetermined temperature threshold, and its role is not related to whether there is abnormal current in the circuit, but directly responds to temperature changes. The thermal fuse is more used to prevent equipment damage or fire caused by excessive temperature.

Application situation

• Because of its slow response characteristics, the hot fuse is more suitable for occasions that are sensitive to temperature or require a certain overload tolerance, such as short-term overload protection at the start of the motor, and temperature control protection of household appliances.
• Fuses are widely used in circuit protection that requires a fast response, such as electronic equipment, power supplies and automotive electrical systems, to prevent fire or equipment damage caused by short circuits.
• Protection mechanism

The hot fuse provides more overheat protection, which is suitable for long-term overload monitoring; The fuse is a short circuit protection for the instantaneous large current to ensure the immediate safety of the circuit.

How to choose the right thermal fuse or fuse

Choosing the right hot fuse (hot fuse) or fuse requires consideration of a number of factors to ensure that they will function correctly under predetermined conditions and protect the circuit from overload or short circuit hazards. Here are the key steps and points to consider when choosing:

1. Determine the normal operating current of the circuit:

   • First, determine the maximum normal operating current in the circuit, which is usually the rated current of the appliance or device.

   • The rated current of the fuse should be slightly greater than this value, but not too large to avoid misblowing during normal operation.

2. Consider ambient temperature and temperature reduction:

   • If the operating environment temperature is higher than the standard room temperature, the bearing capacity of the fuse will decrease. At this time, it is necessary to refer to the temperature reduction factor provided by the manufacturer (generally known as the temperature compensation factor K) to calculate the actual current that can be carried (usually the rated current multiplied by the temperature reduction factor).

3. Evaluate short circuit current and fusing characteristics:

   • Determine the maximum short circuit current that can occur in the circuit, and choose a fuse that can blow quickly when a short circuit occurs to avoid damage to other circuit components.

   • Fast fuses are suitable for situations where the current needs to be cut off immediately, while delay fuses are suitable for loads with starting current shock, such as motors.

4. Select the fuse type:

   • Choose the appropriate fuse type depending on the application area, for example, semiconductor protection may require a fast response fuse, while lighting or electric heating equipment may use a standard response fuse.

   • Consider the physical size, shape (such as cylindrical, sheet, surface mount, etc.) and installation space of the fuse.

5. Rated voltage:

   • The rated voltage of the fuse should not be lower than the rated operating voltage of the circuit. For direct current circuits, special attention should be paid to the manufacturer's recommendation and derating may be required.

6. Certification and safety standards:

   • Ensure that the selected fuse complies with the safety standards and certification requirements of the region or industry, such as UL and CE.

7. Special application requirements:

   • If there are special requirements in the application, such as surge resistance, mechanical vibration resistance, etc., you need to choose a fuse with the corresponding characteristics.

8. Testing and verification:

   • Before the actual application, it is recommended to test and verify to ensure that the fuse operates correctly under the expected conditions, while taking into account current fluctuations and transient current shocks in actual operation.

Detailed replacement procedures for hot fuses and fuses

Although the steps to replace the hot fuse (hot fuse) and the fuse have similarities, there are also some differences, mainly because of their application environment and specific design differences. The following are the general replacement steps for the two components:

Replacement steps of hot fuse:

1. Power off: First, ensure that the equipment is completely powered off, for electric fans, air conditioners and other electrical appliances, unplug the power cord from the socket. If it is part of the built-in electrical or industrial equipment, the corresponding circuit breaker or isolation switch needs to be turned off to ensure safety.

2. Discharge: For devices containing capacitors (such as some electric fans), you need to wait for a period of time after power failure to discharge the internal capacitor to avoid the risk of electric shock.

3. Disassembly: Use a suitable tool (such as a screwdriver) to remove the electrical housing, exposing the internal structure and hot fuse. Be careful not to damage other parts.

4. Confirmation and replacement: Find the hot fuse and use a multimeter to confirm its status (if not sure whether it has been blown). Once confirmed, carefully remove the hot fuse, keeping in mind its specifications or models.

5. Install a new thermal fuse: purchase a new thermal fuse of the same specification to replace it, ensure that it is properly installed and connected to the original location.

6. Assembly: Reassemble the electrical enclosure to ensure that all screws are tight and not loose.

7. Test: Reconnect the power supply to test whether the equipment works normally.

Fuse replacement steps (take car fuses as an example) :

1. Locate the fuse box: Locate the vehicle's fuse box, usually located under the hood or in the cab.

2. Consult the manual: Consult the vehicle manual to determine the specific location and specifications of the fuse that needs to be replaced.

3. Power outage: While replacing a car fuse does not require a power outage of the entire vehicle, be sure not to operate any equipment that could activate that circuit.

4. Open the fuse box: Gently open the cover of the fuse box and use the fuse extractor or special tool provided in the box to pull out the suspected faulty fuse.

5. Check and confirm: Check whether the fuse is blown (usually manifested as a broken or discolored metal strip in the middle).

6. Replace with a new fuse: Purchase a new fuse of the same specifications as the original fuse and replace it to ensure that it is properly installed.

7. Reset and test: cover the fuse box cover, try to start the previously affected functions, such as lights, audio, etc., to confirm whether it is back to normal.

When performing the preceding operations, observe the safety regulations to ensure personal safety and device safety. If you are unsure or experiencing difficulties, it is best to seek professional help.

What are the causes of blown fuses?

A blown fuse is usually caused by the following reasons:

1. Overload: When the current in the circuit exceeds the rated current of the fuse, long-term overload will cause the fuse to heat up until it is blown. This often occurs when high-power appliances or multiple appliances are used at the same time, exceeding the load capacity of the circuit design.

2. Short circuit: When a short circuit occurs in the circuit, the current will suddenly increase sharply, because the current finds a very low resistance path to flow directly back to the negative terminal of the power supply, in this case the fuse will blow in a very short time to prevent circuit damage or fire.

3. Ground fault: The line or electrical appliance behind the fuse and the body (in the case of automobiles) or the ground wire, forming an abnormal low-impedance loop, resulting in too much current and blowing the fuse.

4. Electrical failure: The internal failure of the electrical appliance after the fuse leads to an abnormal increase in power consumption, which exceeds the withstand range of the fuse.

5. Frequent starting: Especially motor appliances, frequent starting and stopping will produce a high starting current, and repeated this process may cause the fuse to blow due to overheating.

6. Improper modification: Privately modified high-power lights, sound systems, etc., beyond the capacity of the original car line and fuse design, easy to lead to fuses.

7. Poor contact: Poor fuse contact points or connections in the circuit may cause local overheating, and eventually lead to fuses.

8. Environmental factors: such as in a car, flushing the engine compartment with water may cause a short circuit, resulting in a blown fuse.

9. Aging: The fuse is used for a long time, aging itself, increasing resistance, overheating and fusing.

10. Pulse current: When the circuit starts or the power supply is unstable, the instantaneous large current pulse may also cause the fuse to blow.

Effect of fuse material on fusing characteristics

The material inside the fuse is critical to its fusing characteristics. For example, silver or copper have a low melting point and are suitable for rapid fusing applications; The addition of zinc, lead and other elements can adjust the melting point to achieve a delayed fusing effect. The purity, cross-sectional area and length of the material also affect the fusing speed and fusing characteristics, which need to be calculated precisely at design time to ensure the best protection of the circuit.

Overview of this paper

Although there are similarities between thermal fuse and fuse, their unique design and working principle make them play an irreplaceable role in different application scenarios. The correct selection and application of these protective elements is very important to ensure the safe operation of electrical equipment.