0:03

Hey, hey, this is Brian. This is the

0:05

HVAC School Podcast, the podcast

0:08

that helps you remember some things you might

0:10

have forgotten along the way about thermistors and

0:12

thermocouples. But before we talk about

0:14

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All right,

2:15

so let's talk about thermistors. So what is a

2:17

thermistor? Well, a thermistor is a

2:20

resistor that changes its resistance

2:22

based on a change in temperature.

2:25

So in order for a thermistor to do anything, it has to

2:27

be powered. So it actually needs

2:29

a power supply. And then depending on that

2:31

change in resistance, you're going to get a

2:33

change in current and due to the corresponding

2:35

voltage drop across the resistor. So the

2:38

meter can pick that up and the meter tells

2:40

you what the temperature is based on that. Which

2:42

is why when you're testing a thermistor, you actually

2:44

can test it with an ohmmeter at a

2:47

fixed temperature. And generally speaking, the best temperature to

2:49

do that is 77 degrees Fahrenheit, which

2:51

is 27 degrees Celsius. Now again, you can't

2:53

always do that. You can't control something

2:56

to that fixed temperature. So you can take it to

2:58

an ice bath, get ice water

3:01

melted, and put it in an ice bath. Or

3:03

you can warm it up in your hands. And

3:05

those are pretty fixed temperatures. Typical

3:07

temperature inside your hands is going to usually be somewhere

3:09

between 86 and 95 degrees.

3:11

And that may be close enough just

3:14

to tell if a thermistor is in its right range.

3:16

But again, when they rate a thermistor, the

3:18

rating that comes on that thermistor is generally based

3:20

on 25 degrees Celsius, which is

3:22

77 degrees Fahrenheit. So for example,

3:24

you're going to hear sometimes people say a 10k thermistor,

3:28

or a 5k thermistor, or a

3:30

3k thermistor. And generally speaking, what

3:32

they're talking about is a NTC

3:35

thermistor rated at 77 degrees Fahrenheit

3:37

or 25 degrees Celsius. Meaning that's

3:39

what the resistance will be in

3:42

ohms at that temperature. So

3:44

that's how you test a lot of those thermistors. To

3:46

what an NTC means, negative

3:48

temperature coefficient, that means that

3:50

as the temperature decreases, the

3:52

resistance increases. And as the

3:54

temperature increases, the resistance

3:57

decreases. So that is... Inversely

4:00

proportional so opposite

4:02

right temperature goes up resistance goes

4:05

down resistance goes up temperature goes down You

4:08

will find some PTC thermistors at times and

4:10

generally speaking the most common place you'll see

4:12

that in residential HVAC is In

4:15

certain types of hard start kits they'll

4:17

often use a PTC in

4:19

order to Act to

4:22

take the start winding out of the

4:24

circuit or to take a certain pass through out

4:26

of the circuit either way same thing and

4:28

how it does it is because a PTC is

4:30

positive temperature coefficient, which means as the

4:33

thermistor heats up the resistance goes

4:35

up and Once resistance goes up

4:37

so high you essentially get next to no current

4:40

Which is the same as opening the circuit

4:42

effectively. There's still some flow, but it's not

4:44

much So what a PTC thermistor does is

4:47

once that thing has electricity current running through

4:49

it It heats up as it

4:51

heats up the resistance goes up and it

4:53

just creates this cascade effect where it

4:55

ends up that the circuit is Effectively open

4:57

because the resistance is so high and

5:00

again when you're not using a potential relay To

5:03

take a start capacitor out of the start winding

5:05

of the compressor and again That's generally we're talking

5:07

about taking the start capacitor out of the start

5:09

winding the PTC can be

5:11

used in some cases in replacement

5:13

of a Potential

5:15

relay now again PTC takes a

5:17

while to reset and the reason is very simple It's

5:19

just that it has to cool down Before

5:22

that resistance drops to the point that it can restart

5:24

again And that's why you'll see that in some cases

5:26

when you have a short cycle Instance that the hard

5:28

circuit is gonna have a hard time functioning when it

5:30

uses a PTC to take it out of the circuit

5:32

So that's what that's all about the

5:34

next thing that I want to mention are thermocouples So

5:38

thermocouples work based on something called the

5:40

CBAC effect Which is just a fancy

5:42

way of saying that it generates a

5:44

voltage in response to a temperature difference

5:47

Between the two to similar metals, so

5:49

there's actually a voltage Generated

5:53

and you will see thermocouples actually very common.

5:55

There's a lot of thermocouples used in

5:57

temperature sensing a lot of temperature sensing clamps

5:59

use thermocouples thermocouples. A lot

6:01

of times the K-type thermocouples are used

6:03

in a lot of cases in HVAC

6:05

for measuring. And the advantage of thermocouples

6:07

is they measure over a much wider

6:09

range. So they're much more hardy

6:12

in general. Again, there's exceptions

6:14

to anything, but most thermocouples are much

6:16

more hardy. The downside is that they're

6:18

not as accurate. So thermistors tend to be more

6:20

accurate in sort of that middle range where we're

6:23

typically measuring, which is negative

6:25

50 degrees to 250 degrees Celsius is what's

6:27

generally listed. And so again, they're always going

6:29

to be well inside that. That's very cold

6:31

and very hot. We're going to be

6:33

right in there. In almost everything we're going to measure. So

6:35

thermistors tend to be a little bit more accurate. They

6:38

tend to be not quite as hardy

6:40

in general, but in terms of measuring

6:42

very fine distinctions, thermistors tend

6:44

to work better, which is why you'll find thermistors

6:47

in things like thermostats and things where it's

6:49

very important that they are measuring a very

6:52

accurate temperature and things like thermocouples are often

6:54

used for line temperature clamps, things that have

6:56

a much wider range of temperature and don't

6:59

require the level of accuracy. But

7:01

that's it. In the case of the thermocouple, they actually

7:04

generate the voltage. In the case of a thermistor, it

7:06

just changes resistance. Negative temperature

7:08

coefficient is mostly what we see for

7:10

measuring temperature. And PTC

7:13

or positive temperature coefficient is generally what

7:15

we see for things like

7:17

hard search that use a PTC instead of

7:19

a potential relay. And

7:21

the rating that you see is at 77 degrees.

7:24

And if you are at something other than

7:26

77 degrees, then you need to use a

7:28

chart in order to see what that resistance will be in

7:30

order to see if the thermistor is working properly or not.

7:34

So that's that. Thanks for listening. We'll talk

7:36

to you next time on the HVAC

7:38

School Podcast.