PTCSGM3 Datasheet, PDF (3/4 Pages) Vishay Siliconix – Well-defined protection temperature levels

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PTCSGM3
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 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.
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 (R_p)
is lower than R_s (see Fig. 2), so the comparator’s output voltage V_O will be low. If an equipment over temperature occurs, the
PTC thermistor will quickly heat up to its trigger or nominal reference temperature T_n, whereupon its resistance will increase to
a value much higher than R_s, causing V_O to switch to a high level sufficient to activate an alarm, relay or power shutdown circuit.
APPLICATION EXAMPLES
R1
R_s
R_L
V_O
R_f
R2
+
PTC
thermistor
R_p
Fig. 1 - Typical comparator circuit
V_O
R_p < R_s
(R1 = R2)
R_p > R_s
(R1 = R2)
Trigger
temperature
T_n
T (°C)
Fig. 2 - Typical switch characteristic
Revision: 15-Dec-15
3
Document Number: 29018
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