Common sense of recoverable fuse

What is the working principle of a polymer resettable fuse?
The polymer resettable fuse consists of a polymer matrix and carbon black particles that make it conductive. Since the polymer resettable fuse is a conductor, current will flow through it. When an overcurrent flows through the polymer resettable fuse, the heat generated (I2R) will cause it to expand. As a result, the carbon black particles will separate and the resistance of the polymer resettable fuse will increase. This will promote the polymer resettable fuse to generate heat faster, expand more, and further increase the resistance. When the temperature reaches 125°C, the resistance changes significantly, so that the current is significantly reduced. At this time, the small current flowing through the polymer resettable fuse is enough to keep it at this temperature and in a high resistance state. When the fault is cleared, the polymer resettable fuse shrinks to its original shape to reconnect the carbon black particles, thereby reducing the resistance to the level of the specified holding current. The above process can be cycled many times.

What is the difference between Rmin, Rmax and R1max?
Rmin is the minimum resistance specified for the polymer resettable fuse provided by ANDU. This resistance determines the lowest operating current of the polymer resettable fuse. Rmax is the highest resistance specified for the polymer resettable fuse provided by ANDU. R1max is the maximum resistance that the polymer resettable fuse should reach after its action, and its resistance determines the maximum holding current of the polymer resettable fuse. When the polymer resettable fuse is activated, the resistance of the resistor provided by ANDU (greater than or equal to Rmin and less than or equal to Rmax) will rise to less than or equal to R1max

How much voltage drop will there be on a polymer resettable fuse?
This depends on the specific circuit. Generally speaking, if the resistance and the current in the equilibrium state are known, the voltage drop can be calculated. The maximum voltage drop of the polymer resettable fuse is calculated using the resistance R1max; the typical voltage drop can be the resistance Rmax or the average value of Rmin and R1max if Rmax is not provided. If Iop is the normal working current,  Rps is the resistance of the polymer resettable fuse (R1max, ( Rmax or (Rmin + R1max)/2)), then the polymer resettable fuse in the circuit The voltage drop is:Vdrop = Iop x Rps

Can polymer resettable fuses be used in series?
This has no practical meaning. Because there is always one that will act first, and the others will not protect the circuit.

How to calculate the resistance of polymer resettable fuse operating state?
The resistance of the polymer resettable fuse operating state depends on the specific type and the voltage and power. It can be calculated by the following formula：Rt = V2/Pd?

在How many times can the polymer resettable fuse operate under the maximum voltage and impulse current?
Each polymer resettable fuse has a specific working voltage and withstands a specific impulse current. UL stipulates that the polymer resettable fuse must still show the PTC effect after 6,000 operations. The SN/SF polymer resettable fuse used in communication equipment stipulates that its various performance parameters are still within the original range after at least a dozen or as many as hundreds of actions under the maximum voltage. Designers should realize this: The polymer resettable fuse is used for protection, not for the occasion that its non-stop action is regarded as a normal working state.

How soon can the polymer resettable fuse be restored after it is activated?
The time it takes for a polymer resettable fuse to return to its low resistance state after its action is affected by the following factors: the type of polymer resettable fuse; how it is mounted or fixed; the ambient temperature; Generally speaking, although many will recover within a few seconds, most polymer resettable fuses will recover within a few minutes.

How long can the polymer resettable fuse stay in the operating state without being damaged?
UL stipulates that the polymer resettable fuse must stay at the maximum voltage for 1000 hours without losing its PTC characteristics. The longer the time that the polymer resettable fuse stays in the operating state, the more likely it is that its resistance value cannot be restored, and thus may not meet its initial definition. The time that each polymer resettable fuse can stay varies with the fault event and type.

Can polymer resettable fuses be classified according to resistance value?
Some of our polymer resettable fuses are classified according to resistance value and then provided to users. Polymer resettable fuses mainly used in the communication field, such as SF250, SD250 and SF600.

What is the impact of the encapsulation of polymer resettable fuses?
Generally speaking, even though some customers have successfully encapsulated our products, we still do not advocate. When packaging, we must pay attention to the selection of materials and the method of bending packaging. If the packaging material is too hard, the polymer resettable fuse will not be allowed to expand according to the design requirements, so that it cannot work according to the design requirements. Even if the packaging material is soft, the heat transfer characteristics of the polymer resettable fuse will be affected, making the polymer resettable fuse behave differently from the design requirements.

What is the effect of pressure on polymer resettable fuses?
Pressure will affect the electrical performance of polymer resettable fuses. If the pressure during the operation is too large to limit the expansion of the polymer resettable fuse, the polymer resettable fuse will not operate as specified.

How to determine the type of polymer resettable fuse from the appearance of a sample?
Most polymer resettable fuses will be printed with trademarks and models. Various standard polymer resettable fuse product models are listed in the company's product manual. In addition, the company also customizes products for users to meet their special requirements.

What is the maximum ambient temperature for polymer resettable fuses?
The polymer resettable fuse under working condition depends on the product type. For most of our products, this range can reach 85°C, some can be as high as 125°C (such as SN/SF), and some are as low as 70°C (LP-CW). Some polymer resettable fuses in non-working state can withstand reflow soldering temperature for a short period of time(LP-SM, LP-MSM, SD)。

Can a polymer resettable fuse reset itself? How to recover? How fast is it?
Yes, once the fault is cleared and the polymer resettable fuse has a chance to cool down, it will reset itself. Cooling allows the carbon black particles to contact and reconnect, thereby reducing electrical resistance. Generally, the method to cool the polymer resettable fuse is to cut off the energy supply of the protected equipment and cut off the fault current so as to allow the polymer resettable fuse to cool. Polymer resettable fuses should be distinguished from bimetal devices that are also capable of resetting. Even if the negligent event is not cleared, a typical bimetallic device will self-recover, thereby switching between the negligent event and a protected state that may damage the equipment. The polymer resettable fuse will remain in a high resistance state until the fault event is cleared. The time required for a polymer resettable fuse to reset to a low resistance state depends on a large number of factors: the type of polymer resettable fuse; how it is mounted or fixed; the ambient temperature; the internal cause and duration of the action. Generally speaking, although Many will recover within a few seconds, but most polymer resettable fuses will recover within a few minutes.

Can polymer resettable fuses perform state transitions? How can I keep the state unchanged?
When the fault event is not eliminated, the polymer resettable fuse will not change between normal and operating states. When the polymer resettable fuse operates, its resistance changes from low to high. In the high-impedance state, a small amount of fault current still exists. This small fault current is sufficient to keep it in a high-impedance state. When the fault is cleared, the polymer resettable fuse can be cooled back to a low resistance state.

What is the difference between IH and IT? Why is it different?
IH is the highest current in still air that does not trigger a sudden resistance jump (from 20°C to 25°C depending on the temperature of the product), that is, the highest operating current at room temperature. IT is the minimum current when the polymer resettable fuse operates in still air (from 20°C to 25°C depending on the temperature of the product), that is, the minimum fault current at room temperature. For most of our products, the ratio of IT to IH is 2:1, for some products it may be as low as 1.7:1, and for some products it may be as high as 3:1. The difference in materials and production methods and the change in resistance after the action will determine this ratio.

Under what circumstances does a polymer resettable fuse reset?
The resetting of polymer resettable fuses is a function of current, voltage and temperature. Polymer resettable fuses often start to reset when the temperature is below 90°C (it can be said that the polymer resettable fuse has resettable below 80°C)

What is the difference between polymer resettable fuses and ordinary fuses and other circuit protection devices? How does a polymer resettable fuse protect the circuit together with a loadable overvoltage device?
The most obvious difference between a polymer resettable fuse and an ordinary fuse is its resettable characteristics. Although both can provide over-current protection, polymer resettable fuses can provide over-current protection many times. Once the ordinary fuse is blown, it must be replaced to make the circuit work properly. The performance of the polymer resettable fuse is somewhat similar to the time delay fuse, both of which need to take their own heat dissipation into account, but the polymer resettable fuse does not dissipate heat according to I2t like the time delay fuse, because the polymer resettable fuse is in the initial stage Did not work. The difference between a polymer resettable fuse and a bimetallic fuse is not that it can be resettable. The bimetallic fuse can reset itself when the fault still exists. When it operates, it generates a larger voltage and reconnects to a fault that may damage the equipment. The polymer resettable fuse will remain in a high resistance state until the fault is eliminated. The difference between polymer resettable fuses and ceramic resettable fuses lies in their initial resistance, response time to failure and size. Both are self-resetting, but compared with the ceramic resetting fuse with the same holding current, the polymer resetting fuse is faster due to its smaller size. The combined use of polymer resettable fuses and load-bearing overvoltage devices is usually used in the communications field. For many fault events, thyristors, gas discharge tubes or diodes, etc. can carry overvoltage devices to provide protection. Polymer resettable fuses can protect these overvoltage protection devices in certain fault events. Of course, polymer resettable fuses can also provide overcurrent protection.

The polymer resettable fuse will swell during operation, will it return to its original state when resetting?
The polymer resettable fuse in the operating state will expand and will return to its original size and shape after cooling and resetting. Although its resistance value will not return to its original value, it will return to a value that meets its definition.

What temperature can the polymer resettable fuse reach?
The maximum surface temperature of the polymer resettable fuse can reach 150°C, but the typical surface temperature is 110°C.