HVAC Thermistor Training Project

This training project is designed to show how bead thermistors and tube thermistors work, along with the difference in sensitivity between the two.

Why does the Thermistor Matter?

Thermistors are critical to the efficient operation of modern inverter systems. They are the devices used by the system to monitor temperatures, at various locations, across the system. The electrical resistance of each thermistor (shown on each display) changes as the temperature of thermistor changes. Simply place a finger on each thermistor to watch the electrical resistance lower. The bead thermistor has a faster sensitivity to temperature changes, whereas the tube thermistor is slower to change. To learn more about the thermistors, watch this video: https://youtu.be/V0cyVnTTMBs and to learn more about inverter mini splits, check out our book: https://www.acservicetech.com/mini-split-book

Building the Display

A thermistor training display can be built in a number of ways, using a variety of different materials. For this example, the tools, parts, and steps are listed below:

Tools Required

• Standard Drill

• 1/4" Hex Bit

• 1/4" Zip Screws

• Wire Cutter/Stripper

• Hand Saw

Parts Required

• 3/4" x 7" x 9" backing board (plywood or vinyl work well)

• 3/4" x 3/4" x 9" backing board (plywood or vinyl work well)

• 8 - AA Batteries

• 1 - AA Battery holder case 1.5V x 8 for 12V: https://amzn.to/3JCDiLj

• 6 - Command strips to secure items: https://amzn.to/422OOqy

• 2 - 2K Resistance Display, 12V powered: https://amzn.to/3YLgJsc

• ¼” nylon cable clamps to hold wiring: https://amzn.to/3LhSuil

• 10K NTC Bead Thermistor Pack: https://amzn.to/3TgS8dV

• 10K NTC Tube Thermistor Pack: https://amzn.to/3YK4CvG

• 1 - 16-22 gauge wire nuts: https://amzn.to/3ZP1gZI

Step #1

Cut a 3/4" x 8" board to a length of 9" to make the backing board. This will be the base for building the remainder of the project. Additionally, cut an extra 3/4" length from the 3/4" x 8" board to secure onto the back of the base, so there is room for the wiring.

Step #2

At this point, its a good idea to do a rough layout of the parts to ensure everything fits and that the thermistors, wires, battery bank, and resistance meter displays are all in the proper locations. When lining up the parts, keep in mind, all of the wiring will be connected on the underside of the display base.

Step #3

After making sure everything will fit with enough clearance for testing, mount the AA battery holder case using the command strips. Make sure the wires are accessible for connection to the resistance meters on the underside of the display.

Step #4

Next, each thermistor should be mounted to the board. This can be done by using the nylon cable clamps and 1/4" sheet metal screws to hold each thermistor in place. The tube thermistor should fit securely within the clamp, while the clamp will loosely fit over the wiring for the bead thermistor. The wiring should run beside the battery holder, to the underside of the display.

Step #5

Once the thermistors are mounted in place, the resistance meters should be dry fit into place again. Before mounting the resistance meter displays with the command strips, it is a good idea to connect all of the wiring using the wire nuts. Be sure to verify the wiring according to the resistance meter used. For the particular meter displays used in our example, the red and black wires connect to the 12V power supply and the yellow and blue wires connect to the thermistor wires.

Step #6

Once the wiring is complete, the resistance meters can be mounted to the board. One of the simplest ways to mount the resistance meter is by using the command strips. The resistance meters can sit over their own wiring, as well as the wiring from the thermistors if a gap is left between the command strips where the wires can fit.

Step #7

To create space for the wiring to sit, the 3/4" piece of backing board can be mounted to the underside of the base near the top using screws. Notches can also be cut to let the wiring rest safely under the base.

Using the Display

The display should now be complete and useful for teaching! Make sure to turn the power on at the power supply switch on its side. It's important to communicate to students that the measurement on the display is showing electrical resistance in K-ohms. Feel free to add additional clamps or clips to secure the wiring.

If you are interested in learning more about thermistors, check out the video below!

If you want to know more about Inverter Mini Splits, check out our Book!

• Check out our free Quizzes to test your knowledge here!

• Check out our Free Calculators here!

• If you want to learn about inverter mini split systems, check out our "Inverter Mini Split Operation and Service Procedures" book!

• If you want to learn about refrigerants and how they work in a system, check out our “Refrigerant Charging and Service Procedures for Air Conditioning” book . Test your knowledge with our 1,000 question workbook along with the answer key! We also have quick reference cards for use out in the field! Bundle Packs are a great way to save and get faster shipping! Check out www.acservicetech.com/store

• Tools that we use: www.amazon.com/shop/acservicetech

• Tools that we use: https://www.trutechtools.com/?affid=8 6% off coupon code "acservicetech"

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Published: 3/29/2023

Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel.