TEMPERATURE PROTECTION 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 industrial electronics power supplies electronic data processing. Features and benefits well-defined protection temperature levels very fast reaction time accurate resistance for ease of circuit design stable over a long life wide range of protection temperatures no need to reset supply after overtemperature trip excellent thermal coupling leaded chips and naked chips (supplied in hermeticallysealed plastic bags) available. R1 RL Rs VO Rf R2 PTC thermistor Rp MBA920 Fig.6 Typical comparator circuit. Vo Rp R s (R1 = R2) MBA921-2 Rp R s (R1 = R2) trigger temperature Tn Fig.7 Comparator output signal. T ( oC)