NTC and PTC thermistors, thermostats and thermal cut-outs, thermal
fuses and PTC fuses, platinum sensors and circuit protection devices are
all available from the UK market leaders.
ATC Semitec Products
• NTC Thermistors • PTC Thermistors • Temperature Sensors • Platinum Sensors/Pt100s • Thermostats/Thermal Cutouts • Disc Thermostats • Precision Thermostats • Adjustable Thermostats • Thermal Fuses • PTC Resettable Fuses • Circuit Protection Devices
What is a thermistor? Thermistors are thermal resistors whose resistance changes with temperature. There are two main types;
NTC (Negative Temperature Coefficient) whose resistance drops when the temperature increases
PTC (Positive Temperature Coefficient) whose resistance rises when the temperature increases
NTC Thermistors - These are generally used for temperature measurement as their RT (Resistance - Temperature) characteristic is well defined and can be readily programmed. NTCs are highly stable, low-cost and very accurate hence they are used in a large number of temperature sensors found in the home appliance, automotive and office automation markets.
Some larger NTC disc thermistors are used for current inrush limiting as they offer a neat and cost-effective solution to surge currents arising from switching on certain electronic and electrical products, e.g. S.M.P.S.s (Switch Mode Power Supplies), LED driver circuits or halogen lamps.
PTC Thermistors - These are generally used as solid-state fuses or thermal switches because near their transition point (curie point), the resistance increases rapidly by several orders of magnitude over a small change in temperature. This allows them to be easily incorporated into electronic circuitry and in the case of motor protection thermistors, allows several to be used in series to protect each phase of a 3-phase motor.
What is a thermostat? Thermostats can be either pre-set or adjustable but their prime function is to closely control temperature with a minimum temperature differential (the difference in temperature between the thermostat switching on and switching off).
Pre-set thermostats are calibrated with a specific set-point (determined by mechanical design and their material limits), accuracy (the set-point tolerance) and differential (see above) to suit the intended application. Pre-set thermostats are available in several formats; probe, disc or surface.
Adjustable thermostats have a temperature range e.g. 5~35°C and they can be set by the user to give a suitable comfort level. Adjustable thermostats are generally air-sensing or surface-sensing.
A thermal cut-out is a protection device designed to safely control an over-temperature condition. It tends to have a wide differential (or hysteresis) to stop a fault condition from cycling rapidly and is nowadays available with several reset modes; automatic, manual or self-hold.
The type of reset required depends on the demands of the application and/or safety standard that the product has to meet. They are generally supplied in a probe style or in a disc format. The temperature setting should be chosen carefully to avoid nuisance tripping under normal working conditions.
What is a thermal fuse? A thermal fuse (or thermal cut-off, or t.c.o.) is a one-time safety device designed to open at a pre-set temperature. Thermal fuses cannot be reset or repaired, they have to be replaced only once the fault has been rectified. A typical pellet type axial fuse operates as follows;
When designing with a thermal fuse, you need to consider the operating current, applied voltage and the duty hours of the appliance it will protect. The normal running temperature needs to be a minimum of 25K(25°C) below the fuse opening temperature including any self-heating due to I²R (current) self-heating. The Th (holding temperature) and Tm (maximum temperature) values stated for the fuse should also be taken into account to ensure reliable operation over the life of the product.