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Controlling Power On/Off with a thermistor ?

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Smiley13

New Member
Okay, guys, Im a complete noob to electronics in general. Im working on a pet project for my boss and Im not near as smart as I thought I was.

I've got a small heating coil, that heats up air passing over it. I need to be able to cut power to the coil when the temperature of the air passes a particular threshold and put power back on when it falls back below.

I figure I need to use an IC controller (am thinking Arduino) with a simple IF statement
IF X >= 425
No Power to relay

Else
Power to relay

With the relay closing the circuit to my heating coil.

I'm completely in the dark about how to go about finding the right thermistor (or other sensor?) and how to wire it to my arduino in order to make this work. Can anyone give me some pointers? Or point me in the right direction?
 

Smiley13

New Member
Temps can get as high as 700 degrees farenheit, but I want power cut off when it goes over 425 degrees farenheit. This is for a very miniature application. Im needing a probe or something I can put inside the airflow which is only about .25" diameter
 

tcmtech

Banned
Most Helpful Member
A thermocouple with a simple analog comparator would work if high accuracy is not needed.
Ideally using a dedicated temperature controller would be best.

What is this for, how much power, and how accurate does it need to be?
 

MikeMl

Well-Known Member
Most Helpful Member
Temps can get as high as 700 degrees farenheit, but I want power cut off when it goes over 425 degrees farenheit. This is for a very miniature application. Im needing a probe or something I can put inside the airflow which is only about .25" diameter
A thermistor will not stand 700degF. Use a type-K or type-J thermocouple followed by high-gain amplifier, like an ic instrumentation amplifier to boost the voltage level. A simple ic comparator (or an opamp) followed by a PowerFet will switch an SSR or a relay.


Is the heater powered with the AC line?
 

Smiley13

New Member
The heater is powered by 7.5-8 volts DC.
I'm thinking since I want all power to the heating element to be cut off at 425 degrees, im thinking one that goes up to 570 F will work, since, it should never get that hot. With a lack of data from the sensor, if it were damaged, the arduino could just kill the circuit.
 

tcmtech

Banned
Most Helpful Member
An IR detector/monitoring circuit would likely work at that temperature as well. Plus the IR sensor would not need to be directly in the heat.
 

georgetwo

Member
a thermistor will do just fine. if you think the temperature is too much, cover the sensor with something to reduse the effect of heat on it.

just one smart opamp can do the switching. with a potentiometer you can very the switching temperature and it will permanently remain in that temperature
 

MikeMl

Well-Known Member
Most Helpful Member
a thermistor will do just fine. ...
Epoxy encapsulated thermistors are only good to 150degC, while glass encapsulated ones go to 300degC. The OP's requirement of 570degF puts a glass one on the hairy edge.
 

MikeMl

Well-Known Member
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