Can the TMP75, mounted above a heated surface, be used to detect when the surface becomes heated? The TMP75 plastic case and pins will all be heated by the rising hot air. Does the sensor only measure the difference in the temperature of the case and pins?
Thank you, Travis Metcalf
metcalfengineering@cox.net
Hi Travis,
The TMP75 actually reads out the temperature of its own die by measuring the change in the voltage drop across an on-chip diode. The main thermally conductive pathway to the die is the pins of the IC so you can consider the part to be thermally coupled to its local area of the PCB. Because this pathway is far more thermally conductive than the radiative or convective pathway from an adjacent surface, the local temperature of the PCB is really going to dominate the measurement. I believe the TMP006 would be a better candidate for your application as it is intended specifically for contactless measurements like the one you describe.
John Caldwell
Analog Applications Engineer
PA Linear Apps
Thank you for your quick response, and suggestion to use the TMP006. I think because of the added difficulty in handling the TMP006, I will try the TMP75 first.
Travis Metcalf
I understand your concerns, the wafer chip-scale package does require additional care during assembly. In terms of using a local temp sensor to determine the temp of an adjacent surface, one process that we have recommended in the past is to set up the system to determine a "transfer function" between the adjacent surface and the temp sensor. This can be done by leaving the surface unheated and allowing the measured temperature of the TMP75 to stabilize, then heating the adjacent surface and comparing its actual temperature (measure with an RTD or thermocouple for prototyping purposes) to the TMP75's measured temperature. By taking data at several temperatures, you should be able to develop a function to calculate the temperature of the surface from the measured temperature at the TMP75. It is important to remember that air currents, other sources of heat, and many other variables will affect the overall accuracy of this approach.
Thank you for your suggestion. Since accuracy is not a big issue for my application, I think the TMP75 should work fine.