PTC Thermistor Limit Temperature Sensor
PTC Thermistor Limit Temperature Sensor is ceramic PTC thermistor disk in radial resin bead or metal lug construction, applied for temperature sensing and temperature protection due it switches at one particular temperature.
PTC thermistors with their very steep curves are very suitable for the monitoring of temperature limits and consequently for switching on a fan when a particular temperature is reached.
Making use of the characteristics that the resistance of ceramic PTC thermistors increases sharply above curie temperature, when ambient temperature increases abnormally, protective circuit installed PTC thermistor will turn on or cut off loop through resistance changing, achieving thermal protection purpose.
PTC thermistors 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. Therefore PTC thermistors can be ideal devices for temperature detection, monitoring, indicating, limiting and over (excessive) temperature protection.
PTC Thermistor Limit Temperature Sensor RT Graph
A further advantage of the PTC temperature characteristic is that PTC thermistors can be connected in series and thus, in their function as temperature sensors, can very easily monitor several hot spots.
As soon as one of these PTC sensors in a series connection exceeds the specified limit temperature, the circuit switches to the high-ohmic state.
With PTC thermistors as temperature sensors, only the steep region of the R/T characteristic is used.
PTC thermistor is only useful as a temperature measuring device over a relatively short range of temperatures near the switch temperature.
The resistance of the PTC thermistor is to be regarded as a function of the ambient temperature [RPTC =f(TA)].
The precondition for this relationship between resistance and ambient temperature is that self-heating and/or the varistor effect are excluded. This means that these PTC thermistors must be operated in the lowest possible field strengths.
To enable a fast response, thermistor sensors have especially small dimensions.
High control accuracy is achieved by using materials with an extra steep resistance/temperature characteristic.
When the PTC is being used as a temperature sensor, the amount of current through the PTC must be small so as not to self-heat the thermistor and cause errors.
Normally, it is not possible to use a PTC thermistor as a temperature sensor when it is in a self-heated mode of operation.
·Limit temperature measurement of Industrial electronics application.
·Power supplies, switch power supply.
·Home appliances, such as dish washers, washing machines, ironing machines, electric cookers, etc.
·Electronic data processing and communications engineering (DC/DC converters).
·Motor windings overheating Protection.
·Semiconductor heatsinks, enclosure panels, temperature-limited transistors.
A PTC thermistor can be used effectively to detect whether a temperature limit in industrial equipment, liquids, etc, is exceeded.
The PTC thermistor is mounted in thermal contact with the medium that has to be protected from overheating.
When the rated temperature limit of the medium is reached, the resistance of PTC thermistors increases abruptly.
In connection with a control unit, this signal can be used to automatically switch the power supply of a load.
PTC thermistor for temperature protection works in this way, 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 over temperature 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.
PTC Thermistor in Comparator Circuit
Typical Switch Characteristic
PTC Thermistor Temperature Protection of 3 phase electric motor
Fig (1) shows a basic circuit for temperature compensation.
In biasing the transistor, the resistance of PTC thermistor is used. If the transistor overheats, the PTC thermistor will heat up as well.
As the PTC thermistor passes the switch temperature, it will go high resistance and unbiased the circuit, and turning off the transistor. In case of Fig. (2) and (3), it also can be used as an over heat sensor as well.
When PTC thermistor is used for temperature compensation, it does not change the input impedance, unlike a negative temperature coefficient Thermistor (NTC Thermistor), because the PTC thermistor is not connected to input circuit in parallel.
Therefore, the PTC thermistor is suitable for circuits which do not need a change of input impedance as a pulse circuits, regional amplifier, and measurement equipment.
PTC Thermistor for Transistor temperature compensation and protection circuit
More than two pieces of PTC thermistors can cover multi hot spots working with a comparator.
Following shows basic circuit idea to connect multiple PTC thermistors in series.
When One PTC thermistor detects overheat at least, a comparator can work by the sharp temperature resistance characteristic. It easily allows changing a number of PTC thermistor or sensing temperature in the same basic circuit design.
Circuit with 2 PTC sensors for 2 hot spots limit temperatures monitoring
Circuit with two series-connected PTC thermistor sensors for the monitoring of limit temperatures, When the limit temperature is exceeded a fan can be switched on.
A PTC thermistor can be used for the overheat sensing of motors, transformer winding, bearings, power transistors, and other machinery.
(1) shows an example of overheat sensing for motor using a relay. In case of small regular operating current, the circuit can be directly stopped by PTC thermistor.
In case of larger regular operating currents, the circuit can be stopped by using the PTC thermistor with a relay or a thyristor.
In either case, PTC thermistor is a very easy to use device, because it’s small, light, and a two terminals device.
PTC Thermistors temperature protection circuit example for motor transistor transformer
Following shows most simple example of a temperature indicator. Temperature is sensed by PTC thermistor. If the target temperature is exceeded, a neon tube lights up.
PTC Thermistors for temperature indication circuit
PTC Thermistor Overheat Protection Circuit
PTC Thermistor Overheat Sensing Circuit