# Yet Another Thermistor Discussion

Quote of the Day

Vision is the art of seeing things invisible.

— Jonathan Swift, Thoughts on Various Subjects, 1711.

## Introduction

I received a question yesterday on a variation on my simple thermistor linearizer discussion in this blog post. In that post, I wanted to accomplish two things:

• linearize the output voltage from a thermistor/resistor divider circuit about a specific temperature.
• The circuit needed an output voltage that increased with increasing temperature.

The question I received yesterday was closely related in that they wanted to:

• linearize the output voltage from a thermistor/resistor divider circuit about a specific temperature.
• The circuit needs a decreasing output voltage with increasing temperature.

I thought enough people may want to see this slight variation that it would be worth putting into a separate post. It turns out the optimal linearization value for RS is exactly the same as for my original post. This result makes sense if you think about it a bit because a nearly linear output across one resistor in the circuit implies a linear voltage across the other. You can derive this result rigorously using the same approach I used in the Appendix of my original post.

For those interested in a slightly more sophisticated circuit, see the short note I published in EDN magazine.

## Background

All the background material from the original post is true here. Please see the original post for background details.

## Analysis

My discussion here will be brief because I am just expanding on my previous post.

### Circuit

Figure 1 shows the circuit in question. The thermistor and the resistor positions have been swapped.

Figure 1: Thermistor Circuit with Decreasing Voltage Versus Temperature Characteristic.

### Example

Figure 2 is a rework of my example from my previous post.

Figure 2: Worked Example.

## Conclusion

Just a quick post to answer a specific question that I thought others may be interested in.

## Appendix A: Derivation of Optimal Value of RS

Figure 3 shows an abbreviated derivation -- I did not include much detail.

Figure 3: Derivation of Optimal Resistor Value.

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