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307C Overcurrent Thermistors

Vishay Cera-Mite

Document Number: 23089

Revision 14-May-02

ceramite.support@vishay.com

PTCR Overcurrent Protection

FEATURES:

Sizes For Your Application - Hold currents from 5 mA to 1.5 A are available

in sizes from 4 to 22mm.

Better Protection, Maintenance Free - PTCRs reset after an overcurrent

situation. Protection levels may be set lower than possible with fuses, without

worrying about nuisance trips.

Resetting, Non Cycling - Functioning as a manual reset device, PTCR

overcurrent protectors remain latched in the tripped state and automatically

reset only after voltage has been removed. This prevents continuous cycling,

and protects against reclosing into a fault condition.

Simplified Mounting - PTCRs may be mounted directly inside end use

equipment. Unlike fuses, no bulky fuseholder or access for user replacement

is required.

Ceramic Material Selection - Various curie materials are available to tailor

hold and trip current operating points.

Repeatable, No Hysteresis - After resetting, ceramic PTCRs return to the

initial resistance value, providing repeatable, consistent protection levels.

Unlike polymer type PTCRs, Vishay Cera-Mite devices exhibit no resistance

hysteresis application problems.

Telecom Line Balance - In telecom circuits matched pairs are used to

maintain line balance. Unlike polymer PTCRs, ceramic devices maintain

balance after resetting.

A NEW DIMENSION

The Positive Temperature Coefficient

Resistor’s (PTC thermistor) unique property

of dramatically increasing its resistance

above the curie temperature makes it

an excellent candidate for overcurrent

protection applications. Overcurrent

situations in electronic devices occur

due to voltage fluctuations, changes in

load impedance, or problems with system

wiring. PTC thermistors monitor current in

series connected loads, trip in the event

of excess current, and reset after the

overload situation is removed, creating a

new dimension of flexibility for designers.

APPLICATIONS:

In a typical current limiter application, the PTC device is connected in series

with a load impedance (Fig P-1). When current (I) flows, internal I

attempt to increase the PTCR’s temperature. To maintain the low resistance

“on” state, stabilization must occur below the switching temperature, where

the heat generated (I

conduction.

Hold current (I

be maintained in a low resistance “on” state while operating at rated ambient

temperature (typ 25°C). To prevent nuisance tripping, choose the rated hold

current to be greater than the normal current expected.

Since heat dissipated by the device is proportional to the ambient temperature,

hold current must be derated for ambients higher than 25°C according to

the following relationship:

Hold Current (I

D( T

R

PTC

Where:

D

= Dissipation Constant

(varies based on disc

size, wire type, &

coating material)

T

Switching (Curie)

Temperature of

PTCR Material

T

A

= Ambient Temperature

R

Resistance of PTCR

at 25°C

Fig P-1

Typical PTC Current Limiter Application

This relationship is shown in Fig P-2,

which provides hold current (I

estimates for ambient temperatures in

excess of 25°C. Five curie materials

illustrate the design flexibility offered by

ceramic PTCR’s.

Fig P-2

PTC Thermistor Overcurrent Protectors

Ambient Temperature Derating of Hold and

Trip Currents

LOAD

PTC

V

AC

or

DC

Ambient Temperature (°C)

-10 0

10

20 30

40

50 60

70 80

90 100 110 120

70°C Curie Material

80°C Curie Material

90°C Curie Material

105°C Curie Material

120°C Curie Material

140%

120%

100%

80%

60%

40%

20%

0%

APPLICATION DATA