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NTE16005-ECG

Polymeric Positive Temperature Coefficient (PTC) Resettable Fuses

NTE Electronics 

RESETTABLE CIRCUIT PROTECTION

When it comes to Polymeric Positive Temperature

Coefficient (PPTC) circuit protection, you now have a

choice.

Polymeric fuses are made from a conductive plastic

formed into thin sheets, with electrodes attached to either

side. The conductive plastic is manufactured from a non–

conductive crystalline polymer and a highly conductive

carbon balck. The electrodes ensure even distribution of

power through the device, and provide a surface for leads

to be attached or for custom mounting.

The phenomenon that allows conductive plastic materi-

als to be used for resettable overcurrent protection de-

vices is that they exhibit a very large non–linear Positive

Temperature Coefficient (PTC) effect when heated. PTC

is a characteristic that many materials exhibit whereby re-

sistance increases with temperature. What makes the

polymeric conductive plastic material unique is the magni-

tude of its resistance increase. At a specific transition tem-

perature, the increase is resistance is so great that it is typ-

ically expressed on a log scale.

107

106

105

104

103

102

101

100

0

20 40 60 80 100 120 140

TEMPERATURE °C

HOW POLYMERIC RESETTABLE

OVERCURRENT PROTECTORS WORK

The conductive carbon black filler material in the poly-

meric device is dispersed in a polymer that has a crystal-

line structure. The crystalline structure densely packs the

carbon particles into its crystalline boundry so they are

close enough together to allow current to flow through the

polymer insulator via these carbon “chains”.

When the conductive plastic material is at normal room

temperature, there are numerous carbon chains forming

conductive paths through the material.

Under fault conditions, excessive current flows through

the polymeric device. I2R heating causes the conductive

plastic material’s temperature to rise. As this self heating

continues, the material’s temperature continues to rise

until it exceeds its phase transformation temperature. As

the material passes through this phase transformation

temperature, the densely packed crystalline polymer ma-

trix changes to an amorphous structure. This phase

change is accompanied by a small expansion. As the con-

ductive particles move apart from each other, most of

them no longer conduct current and the resistance of the

device increases sharply.

The material will stay “hot”, remaining in this high resist-

ance state as long as the power is applied. The device will

remain latched, providing continuous protection, until the

fault is cleared and the power is removed. Reversing the

phase transformation allows the carbon chains to re–form

as the polymer re–crystallizes. The resistance quickly re-

turns to its original value.

PRODUCT SELECTION

To select the correct polymeric circuit protection device,

complete the imformation listed below for application, and

then refer to thwe resettable overcurrent protector data

sheets.

1. Determine the nromal operating current:

__________ amps

2. Determine the maximum circuit voltage (Vmax):

__________ volts

3. Determine the fault current (Imax):

__________ amps

4. Determine the operating temperature range:

Minimum Temperature: __________ °C

Maximum Temperature: __________ °C

5. Select a product family so that the maximum rating for

Vmax and Imax is higher than the maximum circuit volt-

age and fault current in the application.

6. Using the IHold vs. Temperature Table on the product

family data sheet, select the polymeric device at the

maximum operating temperature with an IHold greater

than or equal to the normal operating current.

7. Verify that the selected device will trip under fault con-

ditions by checking in the ITrip table that the fault cur-

rent is greater than ITrip for the selected device, at the

lowest operating temperature.

8. Order samples and test in application.

APPLICATIONS

The benefits of polymeric Resettable Overcurrent Pro-

tectors are being recognized by more and more design

engineers, and new applications are being discovered ev-

ery day.

The use of polymeric types of devices have been widely

accepted in the following applications and industries:

D Personal computers

D Laptop computers

D Personal digital assistants

D Transformers

D Small and medium electric motors

D Audio equipment and speakers

D Test and measurement equipment

D Security and fire alarm systems

D Personal care products

D Point–of–sale equipment

D Industrial controls

D Automotive electronics and harness protection

D Marine electronics

D Battery–operated toys

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