TEMPERATURE PROTECTION - Siemens Semiconductors

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stable over a long life

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wide range of protection temperatures

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no need to reset supply after overtemperature trip

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excellent thermal coupling

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leaded chips and naked chips (supplied in hermetically-

sealed plastic bags) available.

Just like a thermostat, a PTC thermistor can be used

for temperature protection of equipment such as motors,

transformers, thyristors and power transistors. However, a

PTC thermistor is less expensive than a thermostat, and its

trip temperature can be more accurately speciﬁed.

It’s 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 that shown in

Fig.6. At normal temperature, the PTC thermistor’s

resistance (Rp) is lower than Rs(Fig.7), so the

comparator’s output voltage Vout will be low.

If an equipment overtemperature occurs, the PTC

thermistor will quickly heat up above its nominal working

temperature Tn(Table 4), whereupon its resistance will

switch to a value much higher than Rs(Fig.7), causing

Vout to increase to a level sufficient to activate a trip or

alarm.

Philips Components manufactures a wide range of

temperature protection PTC thermistors (Table 4),

including naked chips and leaded devices.

Applications

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industrial electronics

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power supplies

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electronic data processing.

Features and beneﬁts

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well-deﬁned protection temperature levels

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very fast reaction time

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accurate resistance for ease of circuit design

PTC

thermistor

MBA920

trigger

temperature

RpRs

(R1 = R2)

T ( C)

MBA921-2

TEMPERATURE PROTECTION

Fig.7 Comparator output signal.

Fig.6 Typical comparator circuit.