Plain old NTC (negative temperature coefficient) thermistors have been around for a very long time. Combined with a suitable resistor network and calibration, they can provide accurate temperature measurement and indication of relative temperature changes. Today, there are small chip component NTCs that mount on a PC board and provide a way to measure the temperature on the board. That is good when the PC board is what you are interested in measuring such as a small board encapsulated inside a smart battery pack or other enclosure. But what happens when you want to measure ambient air temperature or some other fluid separate from the PC board?
Leaded thermistors have found applications where flowing air above the PC board needs to be measured. Thermistor suppliers offer devices with long leads that allow the components to be thru-hole mounted with the raised body located inches above the board for measuring flowing air temperature. This is especially desirable in enclosures with forced fan cooling such as network and telecom equipment and computers. By placing a thermistor off the PC board in the stream of incoming and/or outgoing air, there is the ability to determine quickly whether the equipment is overheating and the effectiveness of the forced air cooling. Failures such as a broken fan, restricted filter, or blocked inlet or outlet can be detected and action taken before any equipment damage occurs.
What if we wanted to perform this same function using a digital temperature sensor with I2C serial interface like the Atmel AT30TSE750? The advantages of a digital temperature sensor are several and include:
- No Calibration Required
- Interface to a Processor Without A/D
- Lower Power Consumption
- Immune to Noisy Analog Measurement
So a digital temperature sensor sounds like a great idea, right? The next problem is that the digital sensor is in a small IC package that mounts on the surface of the PC Board. Atmel offers the AT30TS family of devices in SOIC and WSON 8 pad packages. These are great if you want to know the PC board temperature, but limiting if you must know the temperature of the air flowing above the board.
My thoughts on a proposed solution to this problem is a low thermal mass PC board that can mount the sensor any desired distance from the main PC board. Using thin (0.032″ or less) FR4 material and having no embedded copper plains running the length of the board means minimal thermal coupling between the main board and the thermally isolated sensor. Further steps can be taken to minimize thermal coupling and impedance to airflow by voiding areas of the FR4 and also by making the board as physically small as possible.
A PC Board shaped somewhat like the picture above with a right angle header soldered to the holes in the bottom can become a replacement for a long leaded thermistor in almost any application that includes a processor. Making an intelligent temperature sensing node with an Atmel AT30TS or AT30TSE temperature sensor and an Atmel MCU that can measure ambient temperature is easily realized using this creative PC board approach. And since the PC board is long and narrow, it is possible to yield a high number of boards per panel, lowering the per board cost and making this a feasible solution. Eliminating the factory calibration required with traditional thermistors and the risk of inaccuracy due to uncalibrated sensors makes the Atmel AT30TS sensor an attractive option for thermal management in high availability systems.