Thermistors are the most frequently used sensing elements in the Building Automation and Controls industries. In this series of posts, we’ll answer the most common questions about thermistors and why they’ve become so popular.
What is a thermistor?
The word thermistor is a portmanteau of “thermal” and “resistor”. They are thermally sensitive resistors whose resistance varies predictably based on temperature. They are typically made of a metal oxide and then formed into a small bead, ribbon, chip, or other shape. The specific materials used in the thermistor determine the resistance curve or output. Once manufactured, the materials used within the thermistor don’t change, so it follows that the resistance output will be stable, reliable, and predictable.
What is a resistance curve?
The resistance curve is the relationship between resistance and temperature. Because thermistors are non-linear, when graphed the line is curved and not straight.
Do all thermistors have the same curve?
No, there are two categories of thermistors, NTC and PTC (see below). Within these categories, there are dozens if not hundreds of different resistance curves. Different curves are just that–different, and not necessarily better or worse than another. Choosing the correct curve is most often a matter of system compatibility. Most thermistors are named based on their resistance at 77˚F / 25˚C.
What do PTC and NTC mean?
PTC (positive temperature coefficient) and NTC (negative temperature coefficient) describe the thermistor’s relationship between temperature and resistance. PTC means the resistance will increase with a temperature increase and NTC means the resistance will decrease with a temperature increase. NTC thermistors are by far the most commonly used–all of the thermistors listed below and all ACI thermistors are NTC.
What are some common thermistor curves?
There are a few commonly used thermistors in the BAS and controls industries. Below is a partial list.
- 10K type II(ACI part number A/CP): commonly used in Carrier, Trane, Alerton, Distech, and JCI systems.
- 10K type III(ACI part number A/AN): commonly used in Schneider Electric, Carrier, Delta Controls, KMC Controls, and Reliable Controls Systems.
- 10KS: commonly used in Siemens systems
- 10K-E1: commonly used in Carel USA systems
- 3K: commonly used in ASI and Alerton systems
- 20K: commonly used in Honeywell systems
- 1.8K: commonly used in Schneider Electric systems
Check out Part 2 of this series where we answer questions about the advantages and disadvantages of using thermistors and troubleshooting common problems.
Check out ACI's Thermistor Curve Calculator »
