Temperature sensors like Thermister, IC LM34/35, Thermocoupler, RTD etc are used in various applications to detect temperature variations. These devices can switch off / on circuits when the temperature varies from the normal set level.
LM 34 and LM 35 Integrated Circuits are precision temperature sensors widely used in temperature sensing applications. LM 34 senses temperature in Fahrenheit range while LM 35 senses temperature in Celsius (Centigrade) range. The output voltage of these sensors is linearly proportional to the temperature. LM 34 and LM35 sensors are too sensitive and the static electricity while handling may damage the device.
It is the Fahrenheit sensor and the output voltage is linear to the temperature in the Fahrenheit range. It can operate between -50 to + 300 F range and requires a single supply. The LM 34 has low output impedance, linear output and precise inherent calibration feature. Its draws minimum current of 75 uA during operation and has very low self heating property of less than 0.2 F at normal air temperature. The improved version LM 34 C is designed to sense temperature of -40 to + 230 Fahrenheit. It operates between 5 to 30 Volts DC.
It is the precision temperature sensor with output linearly proportional to the Celsius (Centigrade) temperature. Like LM 34, it also operates in single supply and draws only 60 uA current. Its self heating is as low as 0.1 degree at still air. LM 35 is rated to operate between -55 to +150 Degree Celsius. LM 35 C operates between -40 to +110 Degree Celsius. Supply voltage required for LM 35 ranges between 4 to 30 Volts DC.
Pin Assignment of both LM 34 and 35 are the same.
Pin 1 – Vcc
Pin2 – Output / Adj
Pin 3 – Ground
Check before connecting
Before using the Sensor, test its performance using a digital multi meter. Connect 6 / 9 volt battery to the + and Ground pins. Then connect the Black probe of Multimeter to Ground pin and Red probe to the Output pin of Sensor and set the Meter to 20 volt range. In LM 34 and LM 35, the meter will show 0.25 volts at 25 degree room temperature. Then press the plastic case of the Sensor using fingers. You can see a rise in voltage in the Meter.
TMP 36 is the low power Temperature sensor
This type of resistor varies its resistance based on the temperature. The Thermister was discovered in 1833 by Michael Faraday who reported the semiconducting property of Silver sulphide. First NTC Thermister was developed by Samuel Ruben in 1930. Thermister uses Ceramic or Polymer material and shows higher precision within the range of -90 to 130 degree.
There are two types of Thermister. In NTC (Negative Temperature coefficient) Thermister, the resistance decreases when the temperature increases. In PTC (Positive Temperature Coefficient) Thermister, its resistance increases when the temperature increases. Unlike LDR, it has value ranging from 100 Ohms to 10K. Like LDR, Thermister can be connected either way round.
PTC Thermisters are switching type and their resistance suddenly rises at some critical temperature. The PTC Thermister is made up of doped polycrystalline Ceramic containing Barium titanate and other compounds. The dielectric constant of PTC Thermister varies with the temperature. Below the Curie point temperature, the high dielectric constant prevents the formation of potential barrier between the crystal grains of the material which keeps the Thermister at low resistance. When the Curie point reaches, the dielectric constant drops and a potential barrier develops at the boundary of crystals. This increases the resistance sharply. But if the temperature is too high, sometimes the PTC Thermister reverts to NTC character.
Polymer PTC used as Semifuse and the Silistor (Thermally sensitive Silicon Resistor) are two other forms of PTC Thermister.
The working of NTC Thermister is opposite to the PTC Thermister and its resistance decreases when the temperature increases.
This is another kind of temperature sensor having two conductors made up of metal alloy that produces a voltage proportional to the temperature difference between the conductors. This device can be used to convert a temperature gradient into corresponding electricity. Thermocouplers are available with standard connectors and are self powered.
Resistance Temperature Detector – RTD
These sensors measure the temperature by correlating its resistance with the temperature. It has a fine coiled wire wrapped around a glass or ceramic core. It is kept in a sheathed probe for protection. The material used in RTD is pure material and Platinum RTD is a good quality sensor used in most practical applications. The RTD may be Thin film, Wire wound, and Coiled element types.
Besides these devices, some Germanium diodes like DR25 (Not available now) , IN 34 and Silicon diode IN 4148 etc also exhibits temperature sensing property. Their conductivity increases with the increase in temperature .
Old Germanium diode