This week’s EYE ON NPI (video) is ready to talk turkey – because it’s Thanksgiving in the US and we love to cook a bird to celebrate fall! Most important, besides the basting and brining, is to make sure the inside temperature gets to 165°F so you know it’s moist and safe.
You need to stick some sort of thermometer into the gobbler and you want something that can withstand the high temperatures, while still giving you good consistent results. That’s why this weeks’ NPI is these miniature RTD sensors from Littelfuse, they are just the tiniest little temperature sensors, with great precision.
Measuring temperature is incredibly common for electronics. It’s used for feedback systems, thermal monitoring, IoT sensors, sensor and device de-rating. Its likely every product or project you work on will have some sort of temperature sensing built in. There’s a few common ways to measure temperature. The easiest and cheapest is using a basic diode – we know from the ideal diode law that ID = IS(eVD/VT−1) where VT = kT/q – that kT/q is still burned into my skull. Suffice to say, if you have a current source, the voltage across the diode can be be used to solve for T – the temperature. This works great for a penny or two, but is not super precise. We don’t want to under-cook or over-cook a turkey!
Next best is the bandgap temperature sensor, which is not as dependent on exact currents, you can use it to measure the changes between two diodes, the comparison makes it a little less fussy. Wonderful, but most circuitry conks out at over about 125 °C and our oven be up to 450 °F (about 230°C) and even though the inside of the turkey may be 165, the outside could definitely hit those high temperatures.
OK, so moving up, we could go with a K type (or other) thermocouple. These take advantage of a very tiny voltage differential between two metals when they are heated. They can definitely survive our oven, easily working at up to 500°C or higher, where the temperature probe is in the oven and then the voltage difference is measured some distance away. That’s what is used in many meat thermometers – but there is some temperature variation, these are not very precise sensors (although they will do the job quite well for most purposes.)
We can engineer better! For high temperature range AND precision, we can use an RTD – a temperature dependent resistor. Made out of platinum, they are cut to size to have an exact resistance at 25 degrees C. Any variation in temperature will nearly-linearly change the resistance in a predictable, universal manner. Then you just need an analog circuit to convert that resistance to a digital value and index it against a table to get the temperature. These are the ‘gold standard’ (‘platinum standard’??) of temperature sensors. They are not as inexpensive as diode or as common as thermocouple based sensors, but their high quality and stability makes them a great choice for high temperature precision temperature monitoring.
These little RTD’s from Littelfuse come with a ceramic backing for mechanical support and are so tiny they can fit anywhere. They’re available in both PT100 – 100 ohm and PT1000 – 1000 ohm varieties and that will match with many amplifier boards and specification notes for RTD usage. May we recommend one of our RTD amplifier breakouts? They work like a candied-yam treat!
Digi-Key short link https://www.digikey.com/short/zv7523