Resettable Fuse

RESETTABLE  FUSE 

      

INTRODUCTION 

 

A resettable fuse or polymeric positive temperature coefficient device is a passive electronic component used to protect against overcurrent faults in electronic circuits. The device is also known as a multi-fuse / poly fuse/poly switch. It is made of a high-molecular organic polymer under high pressure and high temp. and sulfidation reaction. It is mixed with conductive particle materials, and processed by special technology. These devices were first discovered and described by Gerald Peason at Bell Labs in 1939.

CLASSIFICATION 

According to materials, a resettable fuse is divided into two types:

• Polymer PPTC
• Ceramic CPTC

According to the package form, a resettable fuse is divided into two parts:

• Lead plug
• SMD patch

Poly fuses are packaged in radial, axial, surface mount, chip, or washer form. These are available in voltage ratings of 30 to 250 volts and current ratings of 20mA to 100A.

WORKING  PRINCIPLE

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 path, and the resettable fuse returns to a low resistance state, thus completing the protection of the circuit without manual replacement. 

ADVANTAGES              

• Unlike fuses, PTCs do not need to be replaced.
• Transient faults will not shut down the system.
• Fewer service calls mean lower warranty costs.
• Fewer service calls mean better product quality. 
• Avoids possible damage to the circuit by incorrect fuse replacement.

APPLICATION

fig: Resettable Battery strap fuse

                 

• Using a poly fuse in series with the speaker will protect it from overcurrent and overheating damage. Choosing a correct trip current-rated poly fuse is important to match the power level of the speaker.
• The equipment powered by a transformer gets overheated due to excessive current or short circuits. A poly fuse on the secondary side of the transformer will protect the equipment against overload.
• Batteries are constantly charged and discharged over their lifecycle. Overcharge results in an increase in the temperature of the electrolyte. This could cause either a fire or an explosion. Polyfuse can play a vital role in the batteries' charging and discharging cycle.

Besides these, resettable fuses are also used in automobiles, computers and peripherals, industrial controls, electronic modules, medical electronics, etc.