PTCSC17

www.vishay.com

Vishay BCcomponents

Revision: 08-Dec-15

3

Document Number: 29017

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

APPLICATION SPECIFIC DATA

Negative Temperature Coefficient (NTC) thermistors are well known for temperature sensing. What is not well known, however,

is that Positive Temperature Coefficient (PTC) thermistors can be used for thermal protection. Although their operating

principles are similar, the applications are very different; whereas NTC thermistors sense and measure temperature over a

defined range, PTC thermistors switch at one particular temperature.

Just like thermostats they protect such equipment and components as motors, transformers, power transistors and thyristors

against over temperature. A PTC thermistor is less expensive than a thermostat, and its switch temperature can be more

accurately specified. It is also smaller and easier to design-in to electronic circuitry.

The PTC thermistor is mounted in thermal contact with the equipment to be protected, and connected into the bridge arm of a

comparator circuit, such as shown in Fig. 1. At normal temperature, the PTC thermistor resistance (

quickly heat up to its trigger or nominal reference temperature

than

APPLICATION EXAMPLES

Fig. 1 - Typical comparator circuit

Fig. 2 - Typical switch characteristic

Fig. 3 - Temperature Protection of 3-phase electric motor

PTC

thermistor

R

p

R

L

R

f

R

s

R1

R2

+



V

O

Trigger

temperature

T

n

V

O

T (°C)

R

(R1 = R2)

R

(R1 = R2)

+

θ

+

θ

+

θ

As soon as one or more of the windings becomes too hot, the motor is switched off.