The need to control energy usage in many applications — from handheld devices to industrial systems and residential design—has given rise to an increasing variety of thermal sensors. So it should not be surprising that the range of thermal sensors available today is broader than ever before. Traditional thermal sensors, such as bimetallic strips, thermocouples, thermistors, and resistance-temperature devices (RTDs — see the TechZone article “Selecting a Thermistor or RTD”) are being joined by IR detectors, and MEM-based devices to offer more options, and thus, open more potential applications. In this article, we’ll take a closer look at some of these new developments.
One of the most common applications for thermal sensors is simply keeping track of heat in electronic systems. By measuring internal temperature, a thermal sensor can signal when cooling is needed, and thus, cause a fan or some other cooling device, such as a Peltier module, to be activated. Having the sensor for control means the cooling device doesn’t have to be constantly on, which not only cuts down power consumption but also, in the case of a fan, allows the system to operate more quietly. Further, the sensor also allows users to create more sophisticated thermal/power management schemes, such as reducing power to some areas of the system that are not as critical to current operations — in effect deliberately creating controlled system “brown outs” to save energy.
Thermal control can be achieved by measuring the temperature of individual ICs in a system, which can be done by monitoring a diode junction, such as a substrate PNP of a microprocessor, or a diode-connected transistor such as the 2N3904 (NPN type) or 2N3906 (PNP type). This works because, at low values of forward current, a change in junction temperature produces a corresponding change in junction forward voltage.
There are several off-the-shelf ICs designed for temperature sensing by junction monitoring; a recent example is the SA56004X from NXP (Figure 1). An SMBus-compatible 11-bit remote/local digital temperature sensor, the device provides a channel for monitoring a remote diode with an accuracy of ±1°C, and it also measures temperature locally, on chip.
Figure 1: NXP’s SA56004X remote/local digital temperature sensor can accept an input from a diode junction to determine an IC’s temperature, allowing the systems to react in several ways to control temperature (Courtesy of NXP).
SA56004X users can program under- and over-temperature limits and thresholds to provide outputs indicating when the on-chip or remote temperature is at issue. When the limit value is reached, it will cause an alert output that may be used as a system interrupt or SMBus alert. The threshold output, T_CRIT, is activated when the on-chip or remote temperature measurement rises above the programmed T_CRIT threshold register value. This output may be used to activate a cooling fan, send a warning, or trigger a system shutdown. To further enhance system reliability, the SA56004X employs an SMBus time-out protocol.
The SA56004X is available in the SO8, TSSOP8, and HVSON8 packages, has eight factory-programmed device address options, and is pin-compatible with the LM86, MAX6657/8, and ADM1032.
A new type of device also worth considering for such applications, because it can provide more opportunities in system design, is the TMP006 Infrared Thermopile Sensor from Texas Instruments. Housed in a small 1.6 x 1.6-mm wafer/chip-scale package (WCSP), this part can measure the temperature of an object without the need to make contact with it. This MEMS-based sensor uses a thermopile to absorb IR energy emitted from the object being measured: the change in thermopile voltage corresponds to the temperature of the object (Figure 2).
Figure 2: The 1.6 x 1.6-mm TI TMP006, which uses a MEMS-based thermopile to sense temperature remotely, opens new opportunities to designers (Courtesy of Texas Instruments).
The sensor’s voltage range (specified from –40° to 125°C), low power consumption (240 μA from a 2.2-V source), and low package height (0.625 mm, maximum) allow it to be used in a wide range of applications. Since it is suitable for battery-powered applications, it can be used in handheld devices to measure case temperature for cooling purposes, as well as for making other temperature measurements on the go; also, the chip-scale format makes it possible to use the standard high-volume assembly methods employed for handheld devices. TI also suggests that the sensor can be used for measurement of comfort index and motor-case temperature, as well as for server-farm power management.
With the cost of heating and cooling facilities at an all-time high, monitoring temperature throughout factories and office buildings has become even more critical than before. Fortunately, technology now exists to put sophisticated monitoring in place and in a cost-effective manner.
One device that is proving useful in such applications is the DS18S20 High-Precision 1-Wire Digital Thermometer from Maxim (Figure 3). The DS18S20 digital thermometer provides 9-bit Celsius temperature measurements, which it communicates over a 1-wire bus pin to a central microprocessor. The device’s other two pins are for ground and input power, but the sensor can actually derive power directly from the data line (so-called “parasite power”), eliminating the need for an external power supply. Since each device has a unique 64-bit serial code, multiple devices can function on the same 1-wire bus, allowing one microprocessor to control many sensors distributed over a large area.
Figure 3: For monitoring temperatures throughout facilities, the Maxim DS18S20 digital thermometer’s 1-wire bus makes setting up a distributed measurement system much easier (Courtesy of Maxim).
The compact sensor also provides an alarm function with user-programmable upper and lower trigger points that are stored in nonvolatile memory. The DS18S20 is accurate to ±0.5°C over the range of –10°C to +85°C and has a full operating temperature range of –55°C to 125°C. The sensor’s attributes recommend it for use in HVAC environmental controls, temperature monitoring systems inside buildings, equipment, or machinery, and process monitoring and control systems.
The various techniques for thermal sensing discussed here allow any engineer to at least consider the advantages temperature measurement could add to a product design. And by combining thermal data with inputs from other sensors, systems can be designed to perform with greater levels of sophistication and intelligence than ever before. Considering the reasonable price of most thermal sensors, the functionality they add can bring a considerable and rapid payback. For more information on the products mentioned in this article, use the links provided to access product pages and datasheets on the Digi-Key website.
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