# Averaging Thermistors

From time to time someone asks about thermistor sensor averaging. Typical questions are:

• Why do I have to use four?
• Why do they have to be connected in series-parallel?
• What the heck is series parallel?
• Have you ever seen a rash like this before?

Interesting if you are mathematically inclined/deranged. Magical for folks like Patrick…(math hates him, and the feeling is mutual)

And, you should really see a doctor about that….

First, how do we determine temperature using a thermistor?

A thermistor is basically a resistor that varies its resistance in response to changes in temperature that result in changes in voltage drop.

As voltage changes, we (and the control accepting the input) can calculate the resistance of the thermistor and the corresponding temperature.

Ok….

But why do multiple thermistors used for averaging have to be a specific number, and why do they have to be wired in a specific way (series-parallel)?

Because Ohm’s law is pushy and says we have to.

Now for the nuts-and-bolts of why.

Ohm’s law says you add the values of resistors in series.

Resistance one + Resistance two = Total Resistance.

Example

10,000+10,000 = 20,000

Two 10k thermistors in series results in 20k-ohms resistance.

Great!

Except, we need 10k…not 20k

Not to worry, we’ll get to that in a minute.

Resistors in parallel are a little more complicated.

1÷ (1÷ Resistance one) + (1 ÷ Resistance two) = Total Resistance.

Example

1 ÷ (1 ÷10,000) + (1 ÷10,000) = 5,000

Two 10k thermistors in parallel results in 5k ohms resistance.

Once again, Great!

And, once again, we need 10k…not 5k.

This is where the series-parallel thing comes into play.

If connecting thermistors in series adds them, and connecting thermistors in parallel divides them…

We can connect two 10k thermistors in series and get 20k.

Then, we can divide the two sets of 10ks by connecting them in parallel 20÷2=10

10k! Exactly what we are looking for!

There you have it.

All the gory details about why you have to use the series-parallel configuration when averaging temperature sensors, Zen-ified.
Eric R.

Tagged with: , ,
###### 2 comments on “Averaging Thermistors”
1. Taking multiple readings to average out the result helps get slightly better results as well, since you may have noise or fluctuations, we suggest about 5 samples.

2. Olene Lyerla says:

Wonderful post thank you for sharing.