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Blog #1: Characterising Thermistors - Introduction

Gough Lui

Thanks to element14 and Molex for selecting me as one of the ten chosen to receive the kit of thermistors. In this first blog, I will detail a little bit about myself and why I decided to apply for this "Experimenting With" Design Challenge.

Table of Contents

About Me

My name is Dr. Gough Lui and I am an engineer and electronics hobbyist from Sydney, Australia. image

I currently work as a postdoctoral researcher in the Biomedical Engineering space with Western Sydney University and as a research assistant with Civil and Environmental Engineering in the University of New South Wales. I have an interdisciplinary background, which includes an undergraduate degree commencing in Electrical Engineering, finishing in Solar Photovoltaics Engineering with first-class honours and undertaking a Taste-of-Research in Spatial and Surveying Information Systems. I have a PhD in Civil and Environmental Engineering, specifically in Water Research. In my work, I support the design, assembly, testing and deployment of embedded electronics for applications in health, medicine and humanitarian fields.

Outside of work, I run my own personal blog site at https://goughlui.com where I blog about random things including product reviews, repair logs, computing, networking, retro-technology, radio, television, satellite, random observations and going on holidays. I am a licensed foundation-level radio amateur (VK2FGYL).

I have spent much time over the years at element14 as a Top Member, mostly lurking but occasionally responding to a comment here or there. I have benefited greatly from the RoadTest program which has yielded plenty of useful and interesting equipment for my bench. I have contributed a whopping 32 reviews as of this posting and have learned plenty from the experience. I have participated in a few Project 14 builds as well although not so much as of late. This is my third time entering a Design Challenge, having been selected as the Grand Prize winner for the Experimenting with Thermal Switches Design Challenge and Runner Up for Experimenting with Current Sense Amplifiers which preceded this.

What the Thermistor!?

Thermistors are a relatively low-cost, commonly used sensor for temperature measurement which provides the ability to make relatively accurate measurements without the complexities of dealing with very small voltages developed by thermocouples. Unlike thermal switches, the analog nature of their response provides the ability to produce proportionate responses to changes in temperature which aid systems that need to regulate temperatures.

Despite this, I still find using thermistors to be a little mysterious. From the various Beta values that are quoted on datasheets in different ways, to the limitations of power dissipation and self-heating effects, I feel there is much to be learned from practical experimentation and characterisation.

I applied to this competition, as I needed an excuse to learn more about the parameters which describe a thermistor, to understand just how critical a Beta value is, to experimentally derive the parameters for thermistors to allow them to be calibrated and to try and demystify this for the community. The variety of thermistors on offer and the inclusion of the new ring-type thermistors which solve a key issue of mounting round-beads to flat objects was a key deciding factor as this would be a great assortment for experiments.

My Test Plan

With this in mind, I intend to spend the challenge trying to answer the following questions:

  • What are the types of thermistors available?
  • What does it mean to be point-matched or curve-tracking?
  • What is the Beta value?
  • What about the Steinhart-Hart coefficients?
  • How sensitive are applications to the Beta value?
  • How about self-heating impacts of measurement?
  • How might one characterise thermistors experimentally?
  • How well does a thermistor's measurement match the predicted result from datasheet provided parameters?
  • What happens to thermistors under extreme circumstances?
  • What are they made of?
  • How can they be mounted?
  • How can we create a simple circuit that uses a thermistor for temperature control?
  • How might we interface a thermistor to a microcontroller?
  • What value of resistance would be most suitable for these applications?

In order to do this, I will be plotting several non-destructive and destructive experiments, designing and constructing circuitry and performing a number of calculations, simulations and thought-experiments. This will probably involve a kettle of boiling water, a cup of ice-slush, a modified car fridge and some high-precision measurement equipment. But before that can happen, I'll have to receive and prepare all the thermistors for testing!

Conclusion

I hope this posting has given you an insight into me and my interest in applying for the Experimenting with Thermistors Design Challenge. It has been a bit of a late start, because I've been busy catching-up on work and blogs, so I hope to catch-up over the next few weeks. I will be learning as I go along, so things may not go as smoothly as I may hope, but I’ll be looking at the thermistors, the theory, testing them, torturing them and putting them to practical applications. I am hopeful this exercise will not only teach me about applying and designing with thermistors, but also be an interesting resource for the community.

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