NTC thermistor for temperature measurement and control selecting reference information
The high sensitivity of an NTC thermistor makes it an ideal candidate for temperature sensing applications.
These low-cost NTC sensors are normally used for a temperature range of -40C to +300C.
Selection criteria for NTC thermistors are
Environment (surrounding medium)
One of the circuits suitable for temperature measurement is a Wheatstone bridge with an NTC
thermistor used as one bridge leg.
Temperature sensing bridge with op-amp which acts as differential amplifier.
Simple Thermostat Circuit
Circuit diagram of a typical heat detector using a matched pair of NTC thermistors.
An example of a circuit including an NTC thermistor and microcontroller is as following.
Example of NTC thermistor circuit protection in PC cooling fan
NTC thermistor as part of a smart charging control unit for Battery Pack
Schematic drawing of the charging control unit of a battery pack using NTC thermistors as temperature sensors
All rechargeable batteries and lithium ion batteries in particular must be controlled and protected
by smart charging circuits, as the mobile phone drawing power from the batteries must operate in
a variety of environments, including low and high-temperature operation.
As preferred temperature detection devices NTC thermistors are used in the protective circuitry.
NTC thermistors can detect the ambient temperature for different purposes, depending on the
battery system. Especially for quick charging the ambient temperature has to be measured, as not
all batteries allow the charging in the hot and cold temperature region. Usually charging temperatures
of 0 ¡ãC up to 45 ¡ãC for slow charging, and 5 ¡ãC ... 10 ¡ãC up to 45 ¡ãC for quick charging are
recommended by the battery pack manufacturers depending on the battery chemistry.
The NTC thermistor is part of a smart charging control unit, which assures that the
ambient temperature is in the range allowing quick charging. During charging the NTC thermistor
repeatedly measures the temperature all 5 to 10 seconds and can detect a rise in the battery
cell's temperature at the end of the charging cycle or precipitated from abnormal charging conditions.
During discharging NTC thermistors also perform temperature compensation for the voltage
measurement, which helps to measure the remaining charge in the battery.
NTC Thermistor Temperature Sensors Used as a Thermal Switch
A common use of an NTC thermistor temperature sensor is in one of the bridge arms of a thermal switch circuit using an operational amplifier such as the ¦ÌA 741. Following figure shows a typical thermal switch circuit for a refrigerator thermostat.
The circuit consists of a 10 VDC zener diode stabilized power supply, a wheatstone bridge (containing the NTC thermistor temperature sensor) and an integrated comparator circuit
controlling a triac. The circuit is designed to switch a maximum load current of 2A off at -5C and on at +5C.
Refrigerator thermostat using an NTC temperature sensor