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 /  PTC Thermistor  /  NTC Thermistor  /  Pressure Sensors

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
Temperature range
Resistance range
Measuring accuracy
Environment (surrounding medium)
Response time
Dimensional requirements.

One of the circuits suitable for temperature measurement is a Wheatstone bridge with an NTC thermistor used as one bridge leg.

Wheatstone bridge circuit with NTC thermistor for temperature measurement

Temperature sensing bridge with op-amp which acts as differential amplifier.

Temperature sensing bridge with op-amp which acts as differential amplifier

Simple Thermostat Circuit

Simple Thermostat Circuit

Circuit diagram of a typical heat detector using a matched pair of NTC thermistors.

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.

circuit with NTC thermistor and microcontroller

Example of NTC thermistor circuit protection in PC cooling fan
NTC thermistor circuit protection sensor position
NTC thermistor protection circuit in cooling fan

NTC thermistor as part of a smart charging control unit for Battery Pack

battery pack charging control unit using NTC thermistor as temperature sensors

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 C up to 45 C for slow charging, and 5 C ... 10 C up to 45 C 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 A 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

Refrigerator thermostat using an NTC temperature sensor

date: 2015-9-22

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