need help with temperature controlled switch

Hello everyone... I know that I am new here, as of about 3 hours ago but I was wondering if anyone could help me out with a circuit I am trying to build.

First a little background info on me. I consider myself to be relatively good with my hands. I took a number of electronic courses right out of high school, but seem to have forgotten a lot of what I learned over the years


I need a circuit to be able to sense a temperature of 420 F and when the temperature reacheds that it needs to energize a relay (normally closed) which will be attached to a light and a buzzer.. so if the temperature is below 420 F the light and buzzer will go on.

I got the idea that I could use a thermistor and have bought several of them with different ratings "all NTC".. some of the circuits I have been able to find range from the really simple sounding
**broken link removed**

to far more complex "for me anyway" circuits
**broken link removed**

I tried the first circuit above with a 12v relay and can get it to sort of work with a 100 ohm thermistor and a pot.... sorry don't remember what rating the pot is, it was just one that was laying around...

The thing is it does not work consistently. Plus I have no idea how to figure out which thermistor/pot combo to use to get it to energize a relay when it reaches 420 F?

I have searched on this site through many a great threads but none really seem to address what I need to have happen in my circuit. Am I on the right track? Can this be done?

It frustrates me because I am sure 11 years ago I would have been able to figure this thing out.... wish I hadn't sold all the books back then! LOL

First, you need to know what the resistance of your thermistor is at 420 deg F. With that you just make a voltage divider using a resistor and thermistor. Have the divider junction go to a comparator input, with the other input of the comparator being your reference voltage. So when ever the temp reaches 420 the thermistor vR drops below the reference, thus the comparator output switches and triggers your relay. Did that make sense?

One more thing, the comparator will most likely need a relay driver on its output, like the one you have.

420 is pretty toastie, not so sure it's a good application for a thermistor, are your thermistors even rated for that temperature? The thermistor and the set point resistor form a voltage divider network which feeds the base of your transistor set up in the darlington config, so you need to tell us what the resistance of the thermistor at 420 degrees is.

Take a look here.
http://www.cpemma.co.uk/rs_thermistors.pdf

Mikebits said:

First, you need to know what the resistance of your thermistor is at 420 deg F. With that you just make a voltage divider using a resistor and thermistor. Have the divider junction go to a comparator input, with the other input of the comparator being your reference voltage. So when ever the temp reaches 420 the thermistor vR drops below the reference, thus the comparator output switches and triggers your relay. Did that make sense?

One more thing, the comparator will most likely need a relay driver on its output, like the one you have.

Click to expand...

I think it makes sense, where can I get this comparator from "part#"? I think I remember seeing it being used in other threads on this board. What is the best way to get the resistance at a said temp? The only thing I can think of is to stick it in the oven at say 200 F and measure the resistance then?

Sceadwian said:

420 is pretty toastie, not so sure it's a good application for a thermistor, are your thermistors even rated for that temperature? The thermistor and the set point resistor form a voltage divider network which feeds the base of your transistor set up in the darlington config, so you need to tell us what the resistance of the thermistor at 420 degrees is.

Click to expand...

Currently no the ones I have are not rated for that high a temp... I have some ordered that should top out at 300C , I just wanted to see if I could get the dang thing to work at all...


Thanks for all the responses by the way!

You need to look at the data sheet for your thermistor. What is the manufacturer and part number?

Mikebits said:

You need to look at the data sheet for your thermistor. What is the manufacturer and part number?

Click to expand...

I bought it from Jameco, the part# is NTC-103-R but jameco does not list the Mfg'er nor do they have a datat sheet. I should probably look for ones to buy where the seller lists the data sheet on them huh?

Jameco Electronics ICs & Semiconductors: VARIOUS : NTC-103-R

Just for the heck of it I am going to stick one of the 10k hm: in the oven at 200 degree and measure it's resistance.....

Does anyone have a part# or place to buy some comparators?

Thanks

For a comparator, check fig 3 in link I provided above. Also check this thread on comparators, https://www.electro-tech-online.com/threads/comparator-circuit.39777/
To calc the R value of the thermistor, Jameco provides the following info. R @ 25C = 10k, B constant = 4100
If you feel like working the math, it is explained here.
Thermistors
I think at 420F your R will be less than 100 ohm. Thermistors are not very linear, so using the oven to calibrate will be inaccurate.

BTW, cool animated gif

I did the math for the thermistor at 215C I got ~47.2 ohms.

Mikebits said:

BTW, cool animated gif

Click to expand...

HAHA thanks!..... was actually surprized it worked "still moving"

Thanks for the information... I have ordered some comparators, when they get here I will have another go at this circuit.

Ordered? Your in Ca, do you have a Fry's nearby? They sell a lot of parts, well at least the one near me does.
**broken link removed**

One warning: Stay away from the tool section, you might run out of money
I always buy tools everytime I go there. lol

You can use a K-Type thermocouple to do your measurements... get one of the ones that are for multimeters and cut off the connector and use it as your temperature sensor.
Up to 1350°C at 41:mu:V/°C

Mikebits said:

Ordered? Your in Ca, do you have a Fry's nearby? They sell a lot of parts, well at least the one near me does.
**broken link removed**

One warning: Stay away from the tool section, you might run out of money
I always buy tools everytime I go there. lol

Click to expand...

Yeah go to fry's all the time! LOL..... but usually for discrete/ IC's they are of no help.... Yes I always buy tools there as well, place is evil!

erosennin said:

You can use a K-Type thermocouple to do your measurements... get one of the ones that are for multimeters and cut off the connector and use it as your temperature sensor.
Up to 1350°C at 41:mu:V/°C

Click to expand...

Hmm never thought of that.... I have one on my Fluke Multimeter I never use.... I might try that out... Thanks!

Don't butcher a Fluke one! They're expensive.... Go get an el-cheapo one to test out hahhaa