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Polymer PTC thermistor for over-current protection

1.PTC effect
Said a material having a PTC (Positive Temperature Coefficient) effect, positive temperature coefficient effectRefers to the resistance of this material will only increase with increasing temperature. Such as most metals have PTC effect.In these materials, the manifestation of resistance to the PTC effect increased linearly with the temperature increase, which is commonly referred to as linear PTC effect.
2. nonlinear PTC effect
After a phase change material will show a sharp increase in resistance along the narrow temperature range from several to more than 10 orders of magnitude of the phenomenon, namely nonlinear PTC effect, as shown in Figure 1. Considerable experience in many types of conductive polymer exhibit this effect, such as polymer PTC thermistor. These conductive polymer for the manufacture of overcurrent protection devices are very useful.

图1
3. high polymer PTC thermistor for over-current protection
Polymer PTC thermistor is often called a Resettable fuse (below referred to as Thermistors), due to its unique positive temperature coefficient of resistance (the PTC characteristics, as shown in Figure 1), thus very suitable for overcurrent protection devices. Thermistors are used as well as General, and is used in series circuit, as shown in Figure 2.

Figure 2
Dang circuit normal work Shi, thermistor temperature and at room temperature similar, and resistance is small, series in circuit in the not hinder current through; and Dang circuit for fault appeared had current Shi, thermistor due to fever power increased led to temperature rose, dang temperature over switch temperature (Ts, see Figure 3) Shi, resistance moments will surge, loop in the of current quickly reduced to security value. for thermistor on Exchange circuit protection process in the current of changes schematic. Thermistor action, current in a circuit has been greatly reduced, t for the thermistor response time. Due to the design of polymer PTC thermistor, and by changing its own switching temperature%.

Figure 3
4. effect of ambient temperature on polymer PTC thermistor

Polymer PTC thermistor is a directly, step-type thermistor, its electrical resistance change process with its own heating and cooling related, so the current (Ihold), the current (Itrip) and operating time under the influence of the ambient temperature. Figure 4 for the thermistor characteristic of maintaining current and operating current and temperature relationship diagram.When the environment temperature and current when it is in area a, Thermistors is greater than the thermal power of heating power and will; when the ambient temperature and currents in the b district heating power is less than the thermal power, a thermistor will remain in an action state; when the ambient temperature and electric current when it is in area c, thermistor thermal power and heating power close, which may or may not be.Figure 5 is a thermistor response time and current and temperature relationship diagram. Thermistors in the temperature is the same as time as current increases dramatically shortened thermistor ambient temperature is relatively high, have a shorter operating time small and maintain current and operating current.

Figure 4

Figure 5
5. recovery from polymer PTC thermistor characteristic
Polymer PTC thermistor resistance can be restored, which can be used repeatedly. After Figure 6 as a thermistor, resistance diagram of changes over time in the recovery process. Resistance is generally more than 10 seconds to tens of seconds to return to the initial level of around 1.6 times times, thermistor current has returned to nominal value at this time, you can use it again. Generally speaking, relatively rapid recovery of area and thickness of the thermistor and relatively slow recovery area and thickness of thermistors.

Figure 6
6. characteristics of polymer PTC thermistor
 
    Polymer PTC thermistor is a positive temperature coefficient characteristics of conducting polymer materials, traditional fuse the most notable difference is that the former can be used repeatedly. Both of these products are able to provide overcurrent protection function, but with a polymer PTC thermistor that many times to provide such protection, after fuses provide overcurrent protection, it must be replaced with another.
                 Polymer PTC thermistor and bimetal circuit breaker the main difference is that the former has not been ruled out in the accident had been off-State and will not reset, bimetal circuit breaker in the accident are still in place can reset itself, which can lead to electromagnetic waves and reset spark. Meanwhile, reconnect the circuit under a fault condition in the circuit may damage the equipment, so it is not safe. Polymer PTC thermistor can keep high resistance until troubleshooting.
                 
     Polymer PTC thermistor and ceramic PTC thermistor difference components of initial resistance, movement time (on the reaction time of the event) as well as the size of the difference. With the same current of polymer PTC Thermistors as compared to ceramic PTC thermistor, polymer PTC thermistor is smaller, less resistance and more responsive.