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232267191005 Datasheet(PDF) 3 Page - Vishay Siliconix |
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232267191005 Datasheet(HTML) 3 Page - Vishay Siliconix |
3 / 5 page 2322 671 91... Vishay BCcomponents PTC Thermistors, For Temperature Protection www.vishay.com For technical questions contact: nlr.europe@vishay.com Document Number: 29074 174 Revision: 10-Oct-03 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. MBD129 θ θ θ Fig.3 Temperature protection of electric motors MBD126 θ θ θ θ A The PTC thermistors located above the fluid level will be heated to a temperature greater than Tn. When immersed they are cooled such that their resistance value is reduced. Fig.4 Liquid-level indication. MBA921 - 2 trigger temperature Tn R p R s R p R s Vo (R1 = R2) (R1 = R2) T ( C) o Fig.2 Typical switch characteristic. MBA920 PTC thermistor R p R f L R R s R1 R2 VO θ Fig.1 .Typical comparator circuit. |
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