PTCSL datasheet(4/5 Pages) VISHAY

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Document Number: 29012

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Revision: 08-Jun-09

2381 671 911../PTCSL..T....BE

PTC Thermistors, Mini Radial Leaded

for Over-Temperature Protection

Vishay BCcomponents

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 overtemperature. 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.

So how does it work? 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 (Rp) is

lower than Rs (see Fig. 2), so the comparator's output voltage VO will be low. If an equipment overtemperature occurs, the PTC

thermistor will quickly heat up to its trigger or nominal reference temperature Tn, whereupon its resistance will increase to a value

much higher than Rs, causing VO to switch to a high level sufficient to activate an alarm, relay or power shutdown circuit.

APPLICATION EXAMPLES

As soon as one or more of the windings becomes too hot, the

motor is switched off.

Trigger

temperature

T (°C)

p > Rs

(R1 = R2)

p < Rs

(R1 = R2)

PTC

thermistor

Fig. 2 Typical switch characteristic

Fig. 1 Typical comparator circuit

Fig. 3 Temperature protection of electric motors

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