Choosing the Best Type of Temperature Sensor
August 9, 2018
|By: Jim Harrison
Guest Blogger, Lincoln Technology Communications
Determining temperature is one of the most common things an engineer needs to design for. And maybe we don’t do that as much as we should. In an embedded design, we have (almost always) a microcontroller that can easily gather temperature data. So, why not measure the temperature of that very same MCU to aid in improving reliability of your design? If you have a power converter, that temperature would be useful to know – either the controller IC or the switching inductor, or both. If your design has mechanical bits like motors and bearings, we can catch a failure before it happens by monitoring temperature. This can also provide added safety to your design.
There are, of course, times when an engineer needs to precisely measure temperature as part of a process control system and times when he or she needs to measure over an extended range. We’ll consider these cases also.
There are basically four types of temperature sensors for practical application: silicon, thermocouple, thermistor, and RTD. Because they’re usually rock-solid reliable, engineers can count on temperature sensors to watch over their products—making them a hero of analog.
Thermocouple Sensors Cover a Broad Range
There are many types of thermocouples, with the types J, K, and T being the most popular. Each type has a different pairing of two metals. They offer a very broad temperature range – as low as -600°C to as high as 2300°C. Standard accuracy is about ±2°C, but some are as good as ±0.5°C. They are not cheap. Prices vary greatly, but can start as low as $8-$9 for a ¼” diameter probe with a protective tube. They may be the least stable and repeatable sensor type and can be bothered by electrical noise.
Figure 1: A standard K-type thermocouple will measure -60°C to 350°C
The good news is that ICs are available to A/D convert and linearize the output of thermocouples of all types. An example of such a device is the MAX31855 from Maxim, which is available for seven different TC types and has integrated cold-junction compensation. The converter resolves temperatures to 0.25°C. It offers accuracy of ±2°C for temperatures from -200°C to +700°C using a K-type thermocouple and has circuitry to reduce the introduction of noise errors from the thermocouple wires. Data output is signed 14-bit, SPI-compatible, read-only. The IC comes in a 8-pin SO package and works over -40° to 125°C. Supply current is 900µA, typical.
Silicon Sensors: Easy to Use and Accurate
For reasons I do not understand, there are many thousands of available silicon sensors. Many hundreds from each of many manufacturers. You can buy one for as little as $0.20 in 100s. But, you may want to spend a bit more for a more accurate and versatile device.
I chose as an example the Maxim DS18B20U as an excellent all-around device with ±0.5°C accuracy from -10°C to 85°C and a 1-Wire® digital interface. The chip can derive power directly from the data line ("parasite power"). It covers a full -55°C to 125°C range and works from a 3.0V to 5.0V data line. Standby current is 1µA.
Figure 2: DS18B20U block diagram
Each DS18B20U has a unique 64-bit serial code, which allows multiple devices to function on the same 1-Wire bus. The chip is programmable, which is a big deal. First, resolution is selectable from 9 to 12 bits, with resulting conversion time varying from 94ms to 750ms. Next, an alarm function has nonvolatile user-programmable upper and lower trigger points. The IC is available in 8-pin SO (150 mils), 8-pin µSOP, and 3-pin TO-92 packages. Several reference schematics are available.
Maxim also offers the MAX30205 clinical-grade human body temperature sensor with a ±0.1°C (max) accuracy for thermometer applications.
Thermistors: Low Cost and Fast Response Time
Thermistors have some tantalizing qualities. The most common thermistors have a negative temperature coefficient of resistance (NTC). The resistance versus temperature curve of a thermistor is approximately exponential and requires linearization, but that can be done in the microcontroller. ICs are available for sensing and A/D conversion.
Characteristics of thermistors include a moderate temperature range of -55° to 125°C (though some are capable of much higher temperatures), low-to-moderate cost (depending on accuracy), and fast response time. Thermistors are available in probes, standard 0402 and other SMT packages, and in beads with bare leads. You can get thermistors with 1% tolerance over their range and resistance values from 1Ω to 1GΩ. They are priced as low as two pennies a piece in 10,000 quantity ($0.10 in quantity 1). Be careful, though, because some high-performance units are 100s of dollars each. One needs to take care not to heat up the tiny bead with the measurement, but available ICs take care of that.
Figure 3: A 4mm thermistor (Photo courtesty of Nick Ames/Flickr)
The MAX6682 thermistor-to-digital converter is a great example. It produces a 10-bit +sign output code based on the thermistor resistance. It does not linearize the transfer function, but over a limited range with a suitable external fixed resistor it can provide a fairly accurate repeatable measurement without linearization. The IC has a 3-wire SPI-compatible output interface. The chip reduces average thermistor current, minimizing self-heating. Between conversions, supply current is reduced to 21µA (typ). It uses a thermistor with a nominal resistance of 10kΩ at 25°C, comes in an 8-pin µMAX package, and is specified over the -55° to 125°C temperature range. Also available is the MAX6691 four-channel thermistor temperature-to-pulse-width converter.
RTD Sensors Operate Over Large Temperature Range
RTDs are a bit larger than other sensors, at around 0.25” square, but they can operate over a large -200° to 850°C range. The most accurate RTDs are made using platinum, but lower cost RTDs can be made from nickel. These are not as stable or repeatable. Platinum RTDs offer a fairly linear output that is highly accurate (±0.1° to ±1°C) across -200 to 600°C. They do have a slow response time. Prices start at about $1.50 in 100s for a nickel device with ±1.5 degree accuracy. A platinum unit with ±0.3°C accuracy costs about $1.70 in 1,000 quantity.
Figure 4: Block diagram of the MAX31865 RTD-to-digital converter
The MAX31865 RTD-to-digital converter chip is easy to use and has a 15-bit delta-sigma A/D converter. The IC yields a 0.031°C output variation due to RTD nonlinearity and a total accuracy in overall operating conditions of 0.5°C (0.05% of full scale) maximum. It offers a 21ms (max) conversion time and ±45V input protection, and is compatible with 2-, 3-, and 4-wire sensor connections. The IC uses a SPI-compatible output and comes in 20-Pin TQFN and SSOP packages.
The truth is all of these four types do a good job and are cost effective – or at least can be if you’re careful. With the exception of thermocouples for very high temperatures, if I had a PC board-mounted application I’d take a silicon sensor in a 8-pin µSOP SMT package. If I had a spot measurement to do, it would be a bead thermistor (being careful with those delicate leads). It’s the conversion ICs that make the design job pretty easy.