Document Number: 23089

Revision 14-May-02

www.vishay.com

12

307C Overcurrent Thermistors

Vishay Cera-Mite

ceramite.support@vishay.com

How Various Physical Parameters Influence a PTCs:

PARAMETER

VOLTAGE & CURRENT CAPABILITY

HOLD CURRENT & TRIP TIME

Disc Diameter (D)

Increased diameter will increase voltage

Increased diameter will increase

and current ratings.

hold current and lengthen trip time.

Disc Thickness (T)

Increased thickness will increase

Increased thickness will increase

voltage rating; may or may not

hold current and lengthen trip time.

increase current rating.

Curie (Switch) (T

Typically, lower switch temperature

Higher switch temperature

Temperature

materials have higher voltage/

materials increase hold current

current capability.

and lengthen trip time.

Higher resistance will increase

Lower resistance will increase hold

voltage capability.

current and lengthen trip times.

Thermal Loading

Increased thermal loading typically

Increased thermal loading increases

(Heat Sink)

reduces the maximum interrupting current. hold current and lengthens trip times.

Wire Leads

Wire leads added to a PTCR pellet act as Depends on thermal conductivity of

a thermal load resulting in reduced

wire used. Copper will increase

maximum interrupting current.

hold current and trip time.

Coating Material

Applying coating to a leaded PTCR has

Applying coating to a leaded PTCR

minimal effect on voltage/current ratings.

increases hold current/trip time 10-20%.

APPLICATION DATA

TRIPPING ACTION DUE TO OVERCURRENT

During normal operation, the PTCR remains in a low base

resistance state (Fig P- 3, Region 1). However, if current

produce internal self heating. If the magnitude and time of

the overcurrent event develops an energy input in excess of

the device’s ability to dissipate heat, the PTCR temperature

will increase, thus reducing the current and protecting the

circuit.

PTC current limiters are intended for service on telecom

systems, automobiles, or the secondary of control transform-

ers or in similar applications where energy available is limited

by source impedance. They are not intended for application

on AC line voltages where source energy may be high and

source impedance low.

The current required to trip (I

rms conduction current required to guarantee thermistor

switching into a high resistance state (Fig P- 3, Region 2) at

a 25°C ambient temperature.

Ambient temperature influences the ability of the PTCR to

transfer heat via surface radiation and thermal conduction at

the wire leads. At high ambient temperatures, less energy

input (via I

ambients require greater energy input. Approximate derating

effects are shown in Fig P- 2.

Since the tripping operation is due to thermal change, there

is a time-trip curve associated with each device. At relatively

low magnitudes of overcurrent, it may take minutes for the

device to trip. Higher current levels can result in millisecond

response time. Trip time (t) can be calculated as follows

kM(T

Trip Time (t ) =

I

Where:

k = coefficient of heat absorption = 0.603 J/g/ °C

M = mass of PTCR = volume x 5.27x10

R = zero power resistance of PTCR at 25°C

Fig P-3

CERAMIC MATERIALS

The temperature at which the PTCR changes from the

base resistance to high resistance region is determined by

the PTCR ceramic material. Switching temperature (T

described by the boundary between regions 1 & 2 (Fig P- 3),

is the temperature point at which the PTCR has increased

to two times its base resistance at 25°C ambient (R

x R25). Design flexibility is enhanced by Cera-Mite’s wide

selection of ceramic PTCR materials with different switching

temperatures (Fig P- 4).

Fig P-4

Table 2

SELF RESETTING - NON CYCLING - REPEATABLE

After tripping, the PTCR will remain latched in its high

resistance state as long as voltage remains applied and

sufficient trickle current is maintained to keep the device

above the switching temperature. After voltage is removed,

the PTCR resets (cools) back to its low resistance state and

is again ready to provide protection.

PHYSICAL DESIGN CONSIDERATIONS

Diameter (D) - Common diameters range from 4 to 22mm.

Thickness (T) - Typical thickness ranges from 1 to 5mm.

Curie (Switching) Temperature (T

Resistivity (

ρ) -

Determined during sintering process; combined

with pellet geometry results in final resistance

based on:

ρT

Area

PTCR Overcurrent Protection

Vishay Cera-Mite offers

a wide selection of

ceramic PTC materials

providing flexibility for

different ambient

temperatures. Close

protection levels are

possible by designing

resistance and physical

size to meet specific

hold current and trip

current requirements.

Curie Temperature °C (±5°)

100K

10K

1K

100

10

2.0

1.0

0.1

PTC

RESISTANCE

REGION 2

HIGH

RESISTANCE

REGION 1

BASE

RESISTANCE

R vs. T Operating Characteristics

PTC

Temperature

25°C

10

100

1000

10000

100000