NTC and PTC Thermistors for Temperature Measurements

There are plenty of applications that require ambient temperature measurements as part of data acquisition or control applications. When you need to gather these measurements electrically, your primary options are:

• Infrared measurements
• Thermocouple measurements
• Measurements with an RTD probe
• NTC and PTC thermistors

Each of these components has their place in the temperature measurement landscape. If you’re building a compact system that needs to gather sensitive ambient temperature measurements over a limited temperature range, an NTC or PTC thermistor is the ideal choice. There are a number of PCB-mountable components available on the market, and these components provide highly accurate measurements within specific temperature ranges.

Which Temperature Measurement is Best for My Application?

Among the various temperature measurement techniques, thermistors find their place in gathering highly accurate measurements within limited temperature ranges. The table below shows a comparison of the capabilities and applications of these components.

| | Thermistors | RTD Probes | Thermocouples | IR Sensors | | ---------- | ---------- | ---------- | ---------- | ---------- | | Typical Range | -100 to ~300 °C | Up to ~700 °C | Up to ~2000 °C | Up to ~3000 °C | | Accuracy | 10-2 to 1 °C | 10-1 to 1 °C | 10-1 to 10 °C | 10-3 to 10 °C | | Active or Passive | Active (const. voltage or current source) | Active (const. voltage or current source) | Passive (outputs a voltage) | Battery-powered | | Linearity | Nonlinear | Somewhat linear | Nonlinear | Highly linear | | Noise Susceptibility | Low | Low | High | None | | Long-term Stability | High | Very high | Varies | Varies |

The long-term stability of thermistors is very high compared to other devices, and is second only to the stability of RTD probes. These components are also best used for ambient temperature measurements, although they can be used with a probe housing to gather point measurements in certain applications. The components themselves are highly resistant to noise, although the measurement circuitry may still be susceptible to EMI.

Perhaps the most important aspect of thermistors that makes them ideal for compact devices, such as IoT products, is their small footprint. These components have the same footprints as capacitors or resistors, and they are available as through-hole or surface mount components. As we can see from the above table, if you need to gather an accurate ambient temperature measurement within a limited range using a small component, then a thermistor may be the ideal choice.

Types of Thermistors and Measurements

There are two types of thermistors: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). These two types of thermistors have different temperature responses. When the thermistor heats up, its resistance will change. The resistance of an NTC thermistor will decrease as temperature increases, and vice versa for a PTC thermistor.

The resistance of these components needs to be measured as the temperature is determined from a measurement of the voltage drop across the thermistor, or the current through the thermistor. This measurement is then converted into a temperature change. This requires some calibration when designing the device, and the sensitivity in the measured voltage/current changes will depend on the circuit used for measurement. A typical measurement technique is to use a Wheatstone bridge circuit or voltage divider circuit. The conversion to a temperature value can then be performed using a simple processor, such as a microcontroller.

NTC and PTC Thermistors

Vishay, NTCS0603E3473FHT

The NTCS0603E3473FHT from Vishay is part of the NTCS0603E3 family of thermistors. These thermistors come in a variety of nominal resistance measurements and have NTC values. This particular surface-mount component is a glass-encapsulated component, making it ideal for industrial or automotive applications.

The image below shows an LTSpice model for the NTCS0603E3473FHT thermistor in a voltage divider circuit for temperature measurements. Here, the NTCS0603E3473FHT thermistor has nominal resistance of 47 kOhms, and the resistors in this voltage divider are set to 4.7 kOhms. In this type of simulation, the values of the resistors R1 and R2 need to be carefully chosen to ensure the measured voltage drop across the thermistor/R2 produces high sensitivity in the desired temperature range. Be sure to carefully select the other resistors used in these circuits and in Wheatstone bridge circuits.

LTSpice simulation model for a voltage divider circuit and simulation results showing voltage drop across R2 with changes in temperature. From the NTCS0603E3473FHT application note.

EPCOS, B57237S109M

The B57237S109M from EPCOS is an NTC thermistor that also provides inrush current protection. This particular through-hole thermistor has a long response time of 90 seconds due to capacitance at the leads. Although it has a long response time, it provides protection against sudden large temperature changes that may be found in industrial applications.

Inrush current limiting in the B57237S109M NTC thermistor. From the B57237S109M datasheet.

EPCOS, B59980C80A70

The B59980C80A70 through-hole PTC thermistor from EPCOS has a fast response time, and it can operate with DC voltages reaching 63 V. The response time of this component is a function of the current in the component during switching, as shown in the graph below. The response time can vary from ~100 seconds down to ~1 ms at high driving current. As the current in these components is also a function of their resistance and temperature, the response time will vary with ambient temperature. EPCOS has provided significant data to design these systems and ensure consistent response time within your desired operating range, but this should be kept in mind when working with this type of component.

Response time at 80 °C and rated current in the B59980C80A70 PTC thermistor. From the B59980C80A70 datasheet.

When you need to select NTC and PTC thermistors, you’ll find plenty of options on Octopart. You’ll also find many options for supporting surface-mount or through-hole components for your next PCB.

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