Applications Case
Product center / Products
Contact

TEL:86-755-84012223

FAX:86-755-84012280

E-mail:sales@ampron.com

China · Shenzhen · Headquarter

Address:Floor2-4, Building 65,43#, FuKang Road, FuMin Industrial Park, PingHu, LongGang, ShenZhen, China

Application principle and related circuit of PTC thermistor for overcurrent protection

Date: 2018-03-27
Viewed: 32
案例名称: Application principle and related circuit of PTC thermistor for overcurrent protection


Application principle and related circuit of PTC thermistor for overcurrent protection


product description


PTC thermistor for overcurrent protection is a kind of protection element for automatic protection and automatic recovery of abnormal temperature and abnormal current. It is commonly known as 'self-resetting fuse' and '10,000 times fuse'. It replaces the traditional fuse and can be widely used for overcurrent protection of motors, transformers, switching power supplies, electronic circuits, etc. PTC thermistors for overcurrent protection limit the residual current value by limiting the consumption in the entire line by sudden changes in resistance. . The traditional fuse cannot recover itself after the line is blown, and the PTC thermistor for overcurrent protection can be restored to the pre-protected state after the fault is removed. When the fault occurs again, the overcurrent protection function can be realized.


Select the overcurrent protection PTC thermistor as the overcurrent protection element, first confirm the maximum normal operating current of the line (that is, the non-operating current of the PTC thermistor for overcurrent protection) and the installation position of the PTC thermistor for overcurrent protection ( During normal operation, the maximum ambient temperature, followed by the protection current (the operating current of the PTC thermistor for overcurrent protection), the maximum operating voltage, and the rated zero-power resistance, should also take into account factors such as the external dimensions of the components.


As shown in the figure below: the relationship between ambient temperature, non-operating current and operating current.

过流保护用PTC热敏电阻应用原理及相关电路

Application principle


When the circuit is in a normal state, the current through the overcurrent protection PTC thermistor is less than the rated current, and the overcurrent protection PTC thermistor is in a normal state, the resistance value is small, and does not affect the normal operation of the protected circuit. When the circuit fails and the current greatly exceeds the rated current, the PTC thermistor for overcurrent protection suddenly heats up and is in a high-resistance state, so that the circuit is in a relatively 'off' state, thereby protecting the circuit from damage. When the fault is removed, the PTC thermistor for overcurrent protection also automatically returns to the low resistance state, and the circuit resumes normal operation.

过流保护用PTC热敏电阻应用原理及相关电路

Figure 2 is a schematic diagram of the volt-ampere characteristic curve and load curve of the circuit during normal operation. From point A to point B, the voltage applied to the PTC thermistor is gradually increased, and the current flowing through the PTC thermistor also linearly increases. It indicates that the resistance value of the PTC thermistor is basically unchanged, that is, it remains in a low resistance state; from point B to point E, the voltage is gradually increased, and the resistance of the PTC thermistor increases rapidly due to heat generation, flowing through the PTC thermistor The current also drops rapidly, indicating that the PTC thermistor is in a protected state. If the normal load curve is lower than point B, the PTC thermistor will not enter the protection state.


There are generally three types of overcurrent protection:


1. Current overload (Fig. 3): RL1 is the load curve during normal operation. When the load resistance decreases, such as transformer line short circuit, the load curve changes from RL1 to RL2. When it exceeds point B, the PTC thermistor enters the protection state;

过流保护用PTC热敏电阻应用原理及相关电路

2, voltage overload (Figure 4): the power supply voltage increases, such as 220V power line suddenly rises to 380V, the load curve changes from RL1 to RL2, beyond point B, the PTC thermistor enters the protection state;

过流保护用PTC热敏电阻应用原理及相关电路

3. Temperature overheating (Fig. 5): When the ambient temperature rises above a certain limit, the volt-ampere characteristic curve of the PTC thermistor changes from ABE to A-B1-F, the load curve RL exceeds B1 point, and the PTC thermistor Enter the protection state;

过流保护用PTC热敏电阻应用原理及相关电路

Overcurrent protection circuit diagram

过流保护用PTC热敏电阻应用原理及相关电路

Model parameter

过流保护用PTC热敏电阻应用原理及相关电路

Overcurrent Protection PTC Thermistor Selection Guide


Maximum operating voltage


The PTC thermistor is connected in series in the circuit. Only a small part of the voltage is kept on the PTC thermistor during normal operation. When the PTC thermistor starts to be in a high-impedance state, it must withstand almost all the power supply voltage, so choose For PTC thermistors, it is necessary to have a sufficiently high maximum operating voltage, taking into account possible fluctuations in the supply voltage.


2. No action current and action current


In order to obtain a reliable switching function, the operating current must be at least twice the operating current.


Since the ambient temperature has a great influence on the non-operating current and the operating current (see the figure below), the worst case should be taken into account. For the non-operating current, the value at the highest allowable ambient temperature is selected. For operating current, choose the value to apply at lower ambient temperatures.

过流保护用PTC热敏电阻应用原理及相关电路

3. Maximum current allowed at maximum operating voltage


When the PTC thermistor is required to perform the protection function, it is necessary to check whether there is a condition in the circuit that exceeds the maximum allowable current, which generally means that the user has a possibility of generating a short circuit. The specification has given the maximum current value, which can cause damage or early failure of the PTC thermistor when used above this value.


4. Switching temperature (Curie temperature)


We can provide Curie temperature 80 °C, 100 °C, 120 °C, 140


Overcurrent protection components of °C, on the one hand, the non-operating current depends on the Curie temperature and the diameter of the PTC thermistor chip, from the perspective of cost reduction, high Curie temperature and small size components should be used; The PTC thermistor thus selected will have a higher surface temperature and will cause undesirable side effects in the line. In general, the Curie temperature exceeds the maximum ambient temperature of 20


~ 40 °C.


5. The impact of the use environment


When contacting chemical reagents or when using infusion materials or fillers, special care must be taken to reduce the PTC thermistor effect caused by the reduction of barium titanate ceramics, and the change in thermal conduction conditions due to perfusion may cause local overheating of the PTC thermistor. And damaged.


Attachment: Power transformer overcurrent protection PTC thermistor selection example


It is known that a power transformer has a primary voltage of 220V, a secondary voltage of 16V, a secondary current of 1.5A, and a primary current of about 300mA during a secondary abnormality. It should enter a protection state within 10 minutes. The working temperature of the transformer is -10 ~ 40 °C, and it works normally. When the temperature rises by 15 ~ 20 °C, the PTC thermistor is installed close to the transformer. Please select a PTC thermistor for primary protection.


1. Determine the maximum working voltage


It is known that the working voltage of the transformer is 220V. Considering the factors of power supply fluctuation, the maximum working voltage should reach 220V × (1+20%) = 264V.


The maximum operating voltage of the PTC thermistor is 265V.


2. Determine the non-operating current


After calculation and actual measurement, the primary current is 125mA when the transformer is working normally. Considering that the ambient temperature of the installation position of the PTC thermistor can reach 60 °C, it can be determined that the non-operating current should be 130~140mA at 60 °C.


3. Determine the operating current


Considering that the ambient temperature of the PTC thermistor is at least -10 °C or 25 °C, it can be determined that the operating current should be 340~350mA at -10 °C or 25 °C, and the operating time is about 5 minutes.


4. Determine the rated zero power resistor R25


The PTC thermistor is connected in series in the primary, the voltage drop generated should be as small as possible, and the heating power of the PTC thermistor should be as small as possible. Generally, the voltage drop of the PTC thermistor should be less than 1% of the total power supply, R25 Calculated:


220V × 1% ÷0.125A=17.6 Ω


5. Determine the maximum current


According to the actual measurement, when the secondary of the transformer is short-circuited, the primary current can reach 500mA. If more current is passed when the primary coil is partially short-circuited, the maximum current of the PTC thermistor is determined to be above 1A.


6. Determine the Curie temperature and dimensions


Considering that the ambient temperature of the PTC thermistor is up to 60 °C, the Curie temperature is increased by 40 °C, and the Curie temperature is 100 °C, but considering the low cost, and the PTC thermistor The device is not installed in the transformer cable package, and its high surface temperature will not cause adverse effects on the transformer. Therefore, the Curie temperature can be selected to be 120 °C, so that the diameter of the PTC thermistor can be reduced by one step, and the cost can be reduced.


7. Determine the PTC thermistor model


According to the above requirements, consult our company's specification sheet.


Namely: Maximum operating voltage 265V, rated zero power resistance value 15Ω± 20%, non-operating current 150 mA, operating current 300 mA, maximum current 1.2A, Curie temperature 120 °C, maximum size ø11.0mm.


PTC failure mode


There are two main indicators for measuring the reliability of a PTC thermistor:


A. Withstand voltage capability----Exceeding the specified voltage can cause short-circuit breakdown of PTC thermistor. Applying high voltage can eliminate the product with low withstand voltage and ensure that the PTC thermistor is below the maximum working voltage (Vmax). safe;


B. Current withstand capability ---- Exceeding the specified current or number of switches can cause the PTC thermistor to fail in an unrecoverable high-resistance state. The cycle-on-off test cannot completely eliminate the products that failed early.


Under the specified conditions of use, the PTC exhibits a high resistance state after failure. The long-term (generally greater than 1000 hours) voltage applied to the PTC thermistor results in a very small increase in the resistance of the room temperature resistance, and the PTC heating element having a Curie temperature exceeding 200 ° C is relatively obvious. In addition to PTC heating elements, the main cause of PTC failure is stress cracking due to the center of the ceramic body during switching operations. During the operation of the PTC thermistor, the uneven distribution of temperature, resistivity, electric field, and power density in the PTC ceramic causes a large central stress and delamination.


Precautions for use


1, welding


Care should be taken during soldering that the PTC thermistor cannot be damaged by excessive heating. The following maximum temperatures, the longest time and the minimum distance must be observed:


Dip soldering


Solution bath temperature max. 260 °C max. 360°C


Brazing time max. 10s max. 5 s


Minimum distance from PTC thermistor min. 6mm min. 6mm


Under the harsh brazing conditions, it will cause a change in the resistance value.


2, coating and perfusion


When coating and infusion are applied to the PTC thermistor, mechanical stresses are not allowed due to different thermal expansions during curing and subsequent processing. Use caution with perfusion materials or fillers. It is not allowed to exceed the upper limit temperature of the PTC thermistor during curing. In addition, it should be noted that the infusion material must be chemically neutral. Reduction of titanate ceramics in PTC thermistors may result in reduced resistance and loss of electrical performance; changes in thermal dissipation conditions due to perfusion may cause local overheating on the PTC thermistor causing it to be destruction.


3, cleaning


Mild cleaning agents such as Freon, trichloroethane or tetrachloroethylene are suitable for cleaning. Ultrasonic cleaning can also be used, but some cleaning agents may damage the performance of the thermistor. It is best to test or come to me before cleaning. Company consulting.


4, storage conditions and deadlines


If properly stored, there is no deadline for the storage period of the PTC thermistor. In order to maintain the solderability of the PTC thermistor, it should be stored in an aggressive atmosphere, taking care of the air humidity, temperature and container material. The components should be stored in the original packaging as much as possible. Contact with a metal coating of an unwelded PTC thermistor may result in reduced solderability. Exposure to excessive or excessive temperatures may change the performance of some specifications, such as the solderability of tin-lead, but it can be stored for a long time under normal electrical component storage conditions.


5, matters needing attention


In order to avoid accidents such as failure/short circuit/burnout of the PTC thermistor, special attention should be paid to the following when using (testing) the PTC thermistor:


Do not use (test) PTC thermistors in oil or water or in flammable or explosive gases;


Do not use (test) the PTC thermistor above the 'maximum operating current' or 'maximum operating voltage' conditions.


Related to the case Case / More >
2018 - 03 - 27
点击次数: 18
Portable electronics are growing rapidly, from mobile phones and MP3 players to PDAs, personal DVD players, and more traditional laptops, and people are revisiting many aspects of consumer and profess...
2018 - 03 - 27
点击次数: 32
Application principle and related circuit of PTC thermistor for overcurrent protectionproduct descriptionPTC thermistor for overcurrent protection is a kind of protection element for automatic protect...
2018 - 03 - 27
点击次数: 21
Overcurrent protection circuit works When the circuit is in a normal state, the current through the overcurrent protection PTC thermistor is less than the rated current, and the overcurrent prote...
2018 - 03 - 27
点击次数: 22
Detailed description of the composition and principle of the switching power supply circuitFirs, the circuit composition of the switching power supplyThe main circuit of the switching power supply is ...
link:
  • Scan the qr code
    Focus on the official WeChat
  • Scan the qr code
    Check the mobile version
Copyright © 2018 深圳安培龙科技股份有限公司
Rhino cloud provides cloud computing services
犀牛云提供云计算服务