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A resettable fuse is a kind of over-current electronic protection component. It is made of a high-molecular organic polymer under high pressure, high temperature, and sulfidation reaction. It's mixed with conductive particle materials, and processed by special technology.
Resettable fuses can be divided into 2 types according to materials:
1. Polymer PPTC;
2. Ceramic CPTC.
According to the package form, it can also be divided into 2 kinds:
1. Lead plug;
2. SMD patch.
It can also be divided into resettable fuses of 600V, 250V, 130V, 120V, 72V, 60V, 30V, 24V, 16V, 6V, etc. according to the voltage.
The main advantages of polymer PPTC are:
●zero-power resistance at room temperature can be very small,
●high-current products are only of a few milliohms,
●low power consumption in the circuit is negligible
●relatively small volume.
PPTC can be connected in series in delicate circuits as a resettable thermal fuse for overcurrent protection. The resistance changes fast, in the order of several milliseconds, with small heat capacity, and short recovery time. Also, it has impact resistance, and the cycle protection can reach 8000 times.
Figure 1. Polymer PPTC Resettable Fuse
PTC can be used as a resettable thermal fuse, reflecting the resettable thermal fuse performance and resettable thermal fuse function to a certain extent in the circuit. In this way, over-current protection and over-temperature protection can be achieved in the circuit.
The main advantage of ceramic CPTC is that it is cheap and easy to manufacture. But it has large resistance, large volume, and large in-circuit loss of tens to thousands of ohms, making it more suitable for small current overcurrent protection.
When there is a high temperature and overheating, negative resistance effects are likely to occur (the resistance becomes smaller). Besides, it has a low protection speed of hundreds of ms, a large heat capacity, and a long recovery time.
The application range is relatively narrow. For example, the circuits can not be used for the rapid protection circuit, automobile wiring harness protection, PCB trace protection, etc.. Instead, they are mostly used for heating devices and can be used in some small-signal circuits where loss is not considered.
A resettable fuse is composed of specially treated polymer and carbon black distributed inside.
Under normal operation, the polymer tightly binds conducting particles outside the crystalline structure to form a chain-like conductive path. At this time, the resettable fuse is in a low resistance state, and the heat generated by the current flows through the resettable fuse is small and does not change the crystal structure.
When the circuit is short-circuited or overloaded, the large current flowing through the resettable fuse will cause the polymer to melt, and the volume will increase rapidly to form a high-resistance state. The operating current will decrease rapidly, thereby limiting and protecting the circuit.
When the fault is eliminated, the resettable fuse cools and crystallizes again. The volume shrinks, the conductive particles re-form a conductive path, and the resettable fuse returns to a low resistance state, thus completing the protection of the circuit without manual replacement.
The operating principle of the resettable fuse is a dynamic balance of energy. The current flowing through the resettable fuse generates a certain degree of heat due to the thermal effect of the current (there is a resistance value in the resettable fuse). All or part of the heat generated dissipated to the environment, but not dissipated heat will increase the temperature of the resettable fuse element.
During normal operation, the temperature is low, and the generated and emitted heat reaches a balance. When the resettable fuse is in a low resistance state, it does not operate. And when the current flowing through it increases or the ambient temperature rises, if the generated and dissipated heat reach balance, the fuse remains no action.
If the current or temperature continues to increase at this time, the heat generated will be greater than the heat dissipated, causing the temperature of the resettable fuse to increase sharply. Thus, a small temperature change will cause the resistance to increase significantly, and the resettable fuse element is in a high-impedance protection state, the increase in impedance limits the current, and the current drops sharply in a short time, thereby protecting the circuit from damage. As long as the heat generated by the applied voltage is sufficient for the emitted heat, the resettable fuse in the changing state can always be in action (high resistance).
When the applied voltage disappears, the resettable fuse can recover automatically.
1. Rated Zero Power Resistance
PPTC thermistors should be packaged according to zero power resistance, and the resistance range should be marked on the outer package. After the voltage and current withstanding test, the resistance change rate of each group of samples before itself is extremely poor δ|Ri after-Ri before/Ri before-(Rj after-Rj before)/Rj before |≤100%
2. PTC Effect
To say that a material has the PTC (Positive Temperature Coefficient) effect means that the resistance of the material will increase with the increase of temperature. For example, most metal materials have the PTC effect. In these materials, the PTC effect manifests as a linear increase in resistance with increasing temperature, which is commonly referred to as the linear PTC effect.
3. Non-linear PTC effect
The material that undergoes a phase change will exhibit a phenomenon in which the resistance increases sharply within a narrow temperature range from several to ten orders of magnitude, that is, the nonlinear PTC effect. Many types of conductive polymers exhibit this effect, such as polymer PTC thermistors. These conductive polymers are very useful for making overcurrent protection devices.
4. Minimum Resistance (Rmin)/Maximum Resistance (Rmax)
At a specified ambient temperature, for example, 25°C, before the self-recovery is installed, the resistance value of a specific type of polymer thermistor in the circuit will be within a specified range, that is, between the Rmin and Rmax. This value is listed in the resistance column in the specification.
5. Holding Current Ihold
The holding current is the maximum current that can pass through the polymer PTC resettable fuse when it remains inactive. Under limited environmental conditions, the device can be maintained for an unlimited time without changing from a low resistance state to a high resistance state.
6. Action Current Itrip
It's the minimum steady-state current that enables the resettable fuse series polymer thermistor to operate within a limited time under limited environmental conditions,.
7. Maximum Current Imax (Current Withstand Value)
It's the maximum operating current of the polymer PTC resettable fuse for safe operation in the limited state, that is, the current withstand value of the thermistor. Over this value, the thermistor may be damaged and cannot be recovered. This value is listed in the current withstand column in the specification.
8. Leakage Current Ires
It's current flows through the thermistor when the polymer PTC resettable fuse is locked in its high resistance state.
9. Maximum Operating Current/Normal Operating Current
It's the maximum current flowing through the circuit under normal operating conditions. Under the maximum ambient operating temperature of the circuit, the holding current of the polymer PTC resettable fuse used to protect the circuit is generally greater than the operating current.
10. Action
The polymer PTC resettable fuse changes from low resistance to high resistance when overcurrent occurs or the ambient temperature increases.
11. Action Time
The time from the overcurrent occurrence to the completion of the action. For any specific polymer PTC resettable fuse, the greater the current flowing through the circuit, or the higher the working environment temperature, the shorter the action time.
12. Vmax Maximum Voltage (Voltage Withstand Value)
It's the highest voltage that the polymer PTC resettable fuse can safely withstand under limited conditions, that is, the withstand voltage of the thermistor. Over this value, the thermistor may be broken down and cannot be recovered. This value is usually listed in the withstand voltage column in the specification.
13. Maximum Working Voltage
It's the maximum voltage across both ends of the polymer PTC resettable fuse under normal operating conditions. In many circuits, it is equivalent to the voltage of the power supply in the circuit.
14. Conductive Polymer
Here refers to the conductive composite material made of conductive particles (carbon black, carbon fiber, metal powder, metal oxide, etc.) filled with insulating polymer materials (polyolefin, epoxy resin, etc.).
15. Ambient Temperature
The temperature of still air around a thermistor or a circuit with a thermistor element.
16. Operating Temperature Range
The ambient temperature range where the P element can work safely.
17. Maximum Working Environment Temperature
The highest ambient temperature at which the component is expected to work safely.
18. Power Loss
It's the power consumed by the polymer PTC resettable fuse after the action, which is the product of the leakage current flowing through the thermistor and the voltage across the thermistor.
19. High Temperature and High Humidity Aging
At room temperature, measure the resistance change of the polymer PTC resettable fuse before and after a long time (such as 150 hours) at a higher temperature (such as 85°C) and high humidity (such as 85% humidity).
20. Passive Aging Test
At room temperature, measure the resistance change before and after the polymer PTC resettable fuse at a higher temperature (such as 70°C or 85°C) for a long time (such as 1000 hours).
21. Hot and Cold Impact Test
At room temperature, the test result of the resistance value of the polymer PTC resettable fuse before and after the temperature cycle. (For example, 10 cycles between -55°C and +125°C).
22. PTC Intensity β
The PTC thermistor has enough PTC intensity and cannot show NTC phenomenon. β=lg R140°C/R room temperature≥5 R140°C, which is the rated zero-power resistance value at 140°C and room temperature.
23. Recovery Time
The recovery time after PTC thermistor acts should not exceed 60S.
24. Failure Mode Test
During the failure mode test, the high polymer PTC thermistor may be in a failure state following the test. The allowable failure mode is an open circuit or a high resistance state, but there must be no low resistance state or open flame during the entire test.
1. Determine the following parameters of the circuit:
●Maximum working environment temperature
●Standard working current
●Maximum working voltage (Umax)
●Maximum fault current (Imax)
2. Choose a resettable fuse that can adapt to the circuit's maximum ambient temperature and standard operating current.
Use the table below, and select the temperature that best matches the maximum ambient temperature of the circuit.
Browse this column to find a value equal to or greater than the standard operating current of the circuit.
WH Series | -20℃ | 0℃ | 25℃ | 30℃ | 40℃ | 50℃ | 60℃ | 70℃ | 85℃ |
WH600 | 138% | 119% | 100% | 92% | 83% | 73% | 64% | 55% | 42% |
WH250 | 132% | 117% | 100% | 91% | 85% | 77% | 68% | 61% | 48% |
WH130 | 136% | 119% | 100% | 92% | 81% | 72% | 63% | 54% | 40% |
WH60 | 136% | 119% | 100% | 90% | 81% | 72% | 63% | 54% | 40% |
WH30 | 130% | 115% | 100% | 91% | 83% | 77% | 68% | 61% | 52% |
WH16 | 132% | 120% | 100% | 96% | 88% | 80% | 71% | 61% | 47% |
WH6 | 130% | 115% | 100% | 91% | 83% | 77% | 68% | 61% | 52% |
table of reduction ratio of ambient temperature and current value
3. Compare the maximum electrical rating of the selected component with the maximum operating voltage and fault current of the circuit.
Use electrical characteristics to verify whether the components selected in step 2 will use the maximum operating voltage and fault current of the circuit.
Check the maximum operating voltage and maximum fault current of the device.
Ensure that Umax and Imax are greater than or equal to the maximum operating voltage and maximum fault current of the circuit.
4. Determine the Action Time
The action time is the amount of time it takes to switch this component to a high resistance state when the fault current appears on the entire device.
In order to provide the expected protection function, it is important to clarify the working time of the resettable fuse.
If the component you select moves too fast, abnormal or harmful actions will occur.
If the element moves too slowly, the protected component may be damaged before the element switches to a high resistance state.
Use a typical operating time curve at 25°C to determine whether the operating time of the resettable fuse is too fast or too slow for the circuit.
If yes, go back to step 2 and reselect spare components.
5. Verify the Ambient Working Temperature
Ensure that the minimum and the maximum ambient temperature of the application is within the operating temperature range of the resettable fuse.
The operating temperature range of most resettable fuse is between -40°C and 85°C.
6. Verify the Overall Dimensions of the Resettable Fuse
Use the size chart to compare the size of the resettable fuse you choose with the space conditions of the application.
A fluorescent lamp needs a ballast to generate high voltage and high currents for ignition. The ballast controls the electrical characteristics of the fluorescent lamp.
When the lamp is turned on, the electronic ballast generates a high-voltage impact at both ends of the lamp to make the lamp ignite, and a self-oscillation circuit is formed in the electronic ballast, which is controlled by a transistor.
Many electronic ballasts fail due to the lamp. When the lamp is short-circuited, reaches the service life, or the lamp is removed, an overcurrent situation will occur, which will cause the cathode of the lamp to open.
Due to the power factor, the load resistance becomes lower. During the start-up period, the ballast works more than three times under abnormal operating current and high oscillation frequency; the switching circuit generates overcurrent and causes the ballast to malfunction.
Figure 2. Ballast Fuse
Resettable fuses can provide protection when the lamp reaches the end of its life. Because the ballast often fails when the upper and lower voltage switches of the transistor are turned on at the same time, the fault protection of the transistor is of great significance.
First of all, the resettable fuse has the performance of automatic recovery, which can reduce the number of product repairs and services, thereby reducing costs.
Secondly, because the resettable fuse can operate in a very short time to protect some of the sensitive resistors in the circuit, the reliability and service life of the ballast can be improved.
Third, the power consumption of the resettable fuse is very low, and it will not consume energy due to extreme heating under normal current working conditions. Under the normal operating current, the resistance is very small (usually only a few tenths of ohms) and therefore an oscillating circuit will not form.
Fourth, the self-resetting fuse is small in size and occupies a small space on the circuit board, which is easy to design.
The failure of a power supply device with a transformer is mainly caused by overcurrent, and the cause of overcurrent is usually a short circuit or load reduction; when a failure occurs, the circuit will smoke and catch fire, which will damage the circuit and interface.
The transformer of the lamp body of the low-voltage halogen lamp structure often fails due to a short circuit.
If the installation and connection between the transformer and the lamp body are improper, it is more likely to be damaged.
Since the lamps are used in parallel, the current is particularly large when short-circuited.
The resettable fuse is installed on the secondary winding of the transformer to prevent short circuit and overload faults.
The protection requirements of the horn system are relatively strict.
Ordinary fuses only play a one-time protective role in the horn, which increases the repair rate of the product; in addition, the additional fuse box and wires increase the manufacturer's cost. In addition, the fuse used must also meet the specifications, and the wrong fuse will damage the speaker.
Installing circuit breakers is also a solution; however, they will make noise when they start to disconnect. Therefore, the best choice is a resettable fuse.
The resettable fuse is equivalent to a soft switch in the disconnected state (in a high-impedance state), and will automatically return to the state of a low-impedance path when the fault is eliminated.
(1) Mobile Phone Battery Pack
The key to the mobile phone battery pack lies in its own application characteristics. This battery is contained in a small, light, and narrow box.
NICD, NiMH, and Li-ION, the three main chemical batteries, are all packed in this universal box.
Generally, the working voltage of the battery pack is less than 10V, and the maximum charging voltage is 16V. The working voltage of the latest battery pack is even lower, 3V-4V.
This means that the packaging of battery packs is changing very quickly, from soldering ribbons to mounting components on printed circuit boards.
Battery packs need circuit protection devices, such as VTP210G, which can keep the current at about 1 ampere at 60℃.
The lower the resistance of the protection circuit, the smaller the energy loss, and the larger the space for component selection.
Figure 3. Resettable Battery Strap Fuse
(2) Cordless Phone Battery
The current and voltage of the cordless phone are relatively small. SRP120, LTP070, and LTP100 are all good overcurrent protection components.
(3) Radio Communication Battery
The current used for radio communication is larger than that of mobile phone batteries and smaller than that of laptop computers. The working current of the LR4 series is 7.3 amperes, which is small in size and light in weight, very suitable for this application. SRP or LTP series with large working currents are also applicable.
The application of chemical batteries is becoming wider and wider, and the application of these components will enable battery packs to have a better protection device at a lower cost.
(1) NiCD Battery
NiCD batteries with low impedance and stable chemical characteristics are not as sensitive to overcurrent as NiMH and Li-ION batteries.
But due to low loss, it is still widely used. However, in short-circuit or over-current conditions, their low internal resistance will cause a higher current to pass.
Usually, the cause of the failure of these batteries is overcurrent, not overheating, and they are suitable for products using any battery materials.
(2) NiMH Battery
NiMH batteries have a higher energy density than NiCD batteries.
When it exceeds 90C, these batteries are more prone to degradation.
VTP or LTP is more suitable to protect this kind of battery than SRP/LR4 material.
According to the battery design method, both SRP and LR4 can protect the battery, but the thermal conductivity is stronger when LTP and VTP are used.
(3) Li-ION Battery
Among all chemical batteries, Li-ION batteries have the highest energy density and the most sensitive chemical characteristics.
When using and charging, a circuit protection device is required.
The general protection device is an integrated circuit, but this is not the safest, because the integrated circuit itself may also cause a short circuit or its CMOS startup fails, making the protection device unsafe.
When it exceeds 90℃, the Li-ION battery will also begin to degrade. Because this battery has the highest voltage, the circuit protection requirements are even stricter.
Although LTP, SRP, and other series have been used in this battery for a long time, the most suitable PTC element is VTP; for large-capacity Li-ION batteries, the LR4 series has a shorter operating time and is more suitable than the SRP series.
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