EYE on NPI - KEMET TRS and OHD Thermal Sensors #EYEonNPI @DigiKey @KEMETCapacitors @adafruit
Channel: Adafruit Industries
Date: 2022-09-01

This week's EYE ON NPI is 'curing' your thermal interlock woes with a huge collection of different thermal monitoring sensors from KEMET (https://www.digikey.com/en/product-highlight/k/kemet/trs-series-and-ohd-series-thermal-sensors). These thermal relays are reliable and easy to spec and use in your design for either Make or Break on temperature-crossing. Use them as a simple temperature control system, or as a last-line-of-defense interlock.

KEMET’s TRS and OHD thermal sensors (https://www.digikey.com/en/product-highlight/k/kemet/trs-series-and-ohd-series-thermal-sensors) are simple and reliable sensors that work using the Curie point property  (https://en.wikipedia.org/wiki/Curie_temperature) - which we learned about in high school and had totally forgotten about until now. Basically, there are some magnetic materials where, when the temperature hits a certain point, the crystalline alignment of the material breaks down completely and the material become a-magnetic.

We can take advantage of the Curie Point property to create a special kind of reed switch (https://www.digikey.com/en/products/filter/magnetic-reed-switches/193). Normally reed switches are activated by an external magnetic field that is brought closer to the switch (https://www.youtube.com/watch?v=H85MyhbcC0U) which pulls two two ferrous elements inside together to make contact. With these thermal sensors, the magnet is wrapped on the outside with interleaved Thermorite (https://www.digikey.my/en/articles/kemet-thermorite-temperature-sensor-built-in-on-off-reed-switch) tuned-Curie-point rings that either channel or break the magnetic field. So, say normally the reed switch inside is connected because there's a magnetic ring of Thermorite around it. When the temperature rises, and hits the Curie Point, the magnetic ring suddenly turns amagnetic, and the metal springs are no longer attracted to each other, opening the connection. A similar technique occurs for the Make type but with a clever interleaved Thermorite-sandwich instead.

By tweaking the Thermorite contents, the switches can be tuned to different temperature ratings with 15 degree C binning (https://www.digikey.com/en/product-highlight/k/kemet/trs-series-and-ohd-series-thermal-sensors). KEMET also makes them in a variety of shapes with and without leads, and encased with UL listing. 

You can get started by picking up a KEMET Thermal Sensor Engineering Kit (https://www.digikey.com/short/3tnpf2bw) to get a collection of various Make and Break sensors. Whatever version you need, Digi-Key has a wide range of KEMET’s TRS and OHD thermal sensors (https://www.digikey.com/en/product-highlight/k/kemet/trs-series-and-ohd-series-thermal-sensors) in stock for immediate delivery. Order today and you'll be Curie'ing by tomorrow afternoon.